Older Fallers and Non-fallers’ Neuromuscular and Kinematic Alterations in Reactive Balance Control: Indicators of Balance Decline or Compensation?

Older Fallers and Non-fallers’ Neuromuscular and Kinematic Alterations in Reactive Balance Control: Indicators of Balance Decline or Compensation? | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Older Fallers and Non-fallers’ Neuromuscular and Kinematic Alterations in Reactive Balance Control: Indicators of Balance Decline or Compensation? Ringo Tang-Long Zhu, Timmi Tim Mei Hung, Freddy Man Hin Lam, Jun-Zhe Li, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4422750/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Background : Falls and fall consequences in older adults are global health issues. Previous studies have compared postural sways or stepping strategies between older adults with and without fall histories, to identify the associated factors of falls. However, more in-depth neuromuscular/kinematic mechanisms have remained unclear. This study therefore aimed to comprehensively investigate and compare the muscle activities and joint kinematics during reactive balance control in older adults with different fall histories. Methods : This pilot observational study recruited six community-dwelling older fallers (≥1 fall in past one year) and six non-fallers, who received unexpected translational balance perturbations in randomized directions and intensities during natural standing. The whole-body center-of-mass (COM) displacements, eight dominant-leg joint motions and muscle electrical activities were collected, and analyzed using the temporal and amplitude parameters. Four-way ANOVA and post hoc analyses were conducted to examine the effects of fall history, perturbation direction, perturbation intensity, and postural sway/joint/muscle on each parameter. Results : Post hoc analyses revealed that compared to older non-fallers, fallers had significantly: (a) smaller activation rate in ankle dorsiflexor, delayed activation in hip flexor/extensor, larger activation rate in knee flexor, and smaller agonist-antagonist co-contraction in lower-limb muscles; (b) larger knee/hip flexion angles, longer ankle dorsiflexion duration, and delayed timing of recovery in joint motions; and (c) earlier downward COM displacements and larger anteroposterior overshooting COM displacements following unexpected perturbations ( p < 0.05). Conclusion and Implication : Compared to older non-fallers, fallers tended to use more suspensory strategies to maintain reactive standing balance. Such strategies could enable older fallers to compensate for their inadequate initiation of ankle/hip strategies, but led to prolonged and overacted balance recovery among them. This study’s comprehensive neuromuscular/kinematic analyses and controlled balance perturbation preliminarily uncovered some specific declines and ineffective strategies in fall-prone older adults during reactive balance control, which can potentially enhance the instrumented assessments for early identification of fall-prone older adults and facilitate the targeted training to prevent their falls. Further longitudinal studies are still needed to examine diagnostic accuracies of these identified neuromuscular/kinematic factors in differentiating fall risks of older people. Community-dwelling electromyographic (EMG) neuromuscular kinematics postural sways reactive balance falls older adults muscle activation. Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. Background Falls and fall consequences in older adults burden the society heavily and are global health issues. 1 Annually, around one in three older adults falls, one in ten older adults has fall-related injuries, and 684,000 fall-related deaths happen worldwide. 1 , 2 However, even the multi-factorial fall-prevention management has shown relatively limited success in fall reduction, especially in older adults with fall histories, i.e., fallers. 3 Given that balance and gait disorders are the second leading causes of falls except accidents, 1 some in-depth physiological alterations of balance control in older fallers that have remained unidentified may be modifiable to prevent older adults’ future falls more effectively. Several balance control strategies with the involvement of lower limbs have been proposed based on the analyses of kinematics (i.e., postural sways, joint motions) and neuromuscular activities (i.e., electromyographic [EMG] signals). The feet-in-place strategy is commonly employed to keep the whole-body center of mass (COM) within the base of support (BOS) when external perturbations are not large, which comprises a single or a combination of the ankle strategy, hip strategy, and suspensory strategy (bending knees to lower COM for stability). 4 The stepping strategy is used to establish a new BOS when the feet-in-place strategy is not enough to overcome the increasing perturbation intensity. 4 , 5 Compared to young adults, older adults tended to rely more on the proximal lower-limb joint motions and muscles than the distal ones, and may use the stepping strategy for reactive/compensatory/automatic balance control following unexpected perturbations. 6 Apart from the age-related changes in the responses of multiple muscles/joints, prior studies have also shown the interaction effects of age with the perturbation direction and perturbation intensity on the balance control strategies. 6 – 8 Nevertheless, the identified age-related kinematic and neuromuscular alterations underlying the reactive balance control may not be directly indicative of fall risks, due to the potential existence of the confounding factor of age. Specific investigations and comparisons of the older adults with and without fall histories (i.e., fallers vs. non-fallers, and excluding the confounding factor of age) are therefore warranted to identify further balance control alterations in older individuals who are prone to falls, and to identify the fall-related factors. Previous studies have intensively analyzed the stepping strategies and whole-body postural sways to compare the reactive balance control between fallers vs non-fallers, 9 – 15 while the differences in specific joint motions or muscle activities were less focused. 16 – 18 Firstly, lower-limb muscle activities during reactive balance control have primarily been examined within a restricted number of lower-limb muscles, i.e., the ankle dorsiflexor/plantarflexor, 16–18 knee flexor/extensor, 16,18 and hip abductor. 18 The difference on hip adductor and hip flexor/extensor activation across fallers and non-fallers remained unknown. Secondly, prior investigations of lower-limb muscle activities have examined only one single EMG parameter in each study. 16 – 18 Fallers were reported to exhibit longer EMG onset latency of ankle dorsiflexor following anterior translational perturbations during standing, 17 longer EMG onset latencies of hip abductor and knee flexor in the weight-bearing leg following lateral shoulder-impact perturbations during standing, 18 and no significantly different agonist-antagonist co-contraction index (CCI) of ankle dorsiflexor-plantarflexor or knee flexor-extensor following optical flow perturbations during walking as compared to non-fallers. 16 The existing analysis of timing and amplitude characteristics of EMG signals may have been insufficient, since only the delayed muscular reaction was identified to differentiate fallers from non-fallers. 17 , 18 Thirdly, regarding joint kinematics, interestingly, no prior studies seemed to have compared them in fallers vs. non-fallers during reactive balance control to the best of authors’ knowledge. Although fallers exhibited decreased range of motion in lower-limb joints than non-fallers, 19 it has been unclear whether the lower-limb joint motions during reactive balance control differ between fallers and non-fallers. More comprehensive analyses of lower-limb muscle activities and joint kinematics are needed to facilitate the understanding of older fallers’ balance control strategies. Reactive balance control strategies are influenced by both the perturbation direction and perturbation intensity, 5 , 20 while there is still insufficient evidence to determine whether fallers and non-fallers respond differently to diverse directions or intensities of balance perturbations. Regarding the perturbation intensity, a previous study reported that fallers and non-fallers’ difference in stepping strategy was more pronounced following a higher intensity of mediolateral perturbation, 10 whereas another study did not observe an interaction effect of fall history and perturbation intensity on the reactive stepping strategy following unexpected anterior perturbations. 21 Regarding the perturbation direction, prior studies also reported inconsistent differences in postural sway between fallers and non-fallers when responding to unexpected anteroposterior 21 , 22 or mediolateral perturbations. 18 , 22 – 24 The underlying reasons for these inconsistent findings have not been thoroughly understood/explained. Analyzing neuromuscular responses and joint kinematics during reactive balance control can potentially help better explain how the fall-prone older adults respond to varied levels of threats of suddenly losing balance, which may also provide useful insights for clinical assessments of reactive balance. The main aim of this study was therefore to explore the older fallers’ neuromuscular and kinematic alterations of lower limbs during reactive balance control as compared to non-fallers. Specifically, this study had the research question of how EMG/angle signals varied among the eight different dominant-leg muscles/joint motions, different fall histories, directions, and intensities of unexpected translational perturbations. In addition, how the COM displacements varied among the six different postural sway directions (i.e., forward/backward, medial/lateral, and upward/downward), different fall histories, perturbation directions, and perturbation intensities were investigated. The timing parameters including onset latency, time to peak, and burst duration and the amplitude parameters including the rate of rise, peak amplitude, and/or agonist-antagonist CCI were analyzed for these signals. We hypothesized that the analyzed parameters during reactive balance control would be affected by the interaction of fall history, muscle/joint motion/postural sway direction, perturbation direction, and perturbation intensity. Further for the simple main effects of fall history, based on the previously available findings related to ageing 4 , 6 , 7 , 16 , 19 , 25 and fall histories, 16 – 18 we extrapolated that fallers would have the delayed timing and larger amplitudes of proximal muscles’ activation/joint motions than non-fallers following a high intensity of unexpected anterior or lateral balance perturbation. 2. Methods 2.1 Study Design and Subjects This study was a pilot observational cross-sectional study. Subjects were recruited through convenience sampling. Inclusion criteria were: 1) aged 65 years old or over, 2) living in the community independently and been able to walk for 400 m without any assistance, and 3) fallers (with at least one fall within the past one year) or non-fallers (with no fall within the past one year) in matched age and gender. Exclusion criteria were: 1) being hospitalized or living in nursing homes for more than six months in the past year; 2) experienced fall(s) due to traffic or occupational accidents; 3) diagnosed with cognitive impairment or severe systemic disease (e.g., neuromuscular, renal, hepatic, orthopedic, vestibular, or cardiopulmonary disorders) that impacts or limits physical activities; and 4) participated in any structured exercise training or strengthening exercises within the past 1 year. A total of twelve older participants were finally eligible for this study. Before being tested, each subject has read and signed an informed consent to participate in this study (Ethics approval agency: Institutional Review Board, The Hong Kong Polytechnic University; Ethical reference number: HSEARS20201230002). Each subject participated in the experiment once, involving subjective assessments and perturbation trials. 2.2 Subjective Assessments The collection of demographic data (e.g., age, gender, height, body mass), medical history, and fall history was first conducted, followed by the assessments using questionnaires/scale. A fall is defined as an event coming to rest inadvertently on the ground or floor or other lower level and not resulting from an intrinsic or overwhelming hazard. 26 The short Falls Efficacy Scale-International (FES-I) and the Chinese Version of the Physical Activity Scale for the Elderly (PASE-C) were introduced to the subject for the measurement of their fear of falling and physical activity level. 27 , 28 The Mini-Balance Evaluation System Test (Mini-BEST) was used to assess the subject’s functional balance performance including the anticipatory postural control, reactive postural control, sensory orientation, and dynamic gait. 29 Then the subject’s dominant leg was determined for the placement of EMG sensors. 5 All subjective assessments were conducted by the same examiner. 2.3 Perturbation Trials 2.3.1 Experimental Set-Up A moving-platform perturbation system was used to induce the unexpected translational perturbations (Fig. 1 ), with technical details reported in a previous study. 5 Generally, the platform can move horizontally at a random starting time, with random moving direction and random moving distance/velocity/acceleration (related to different intensity) to constitute an unexpected balance perturbation to the subject standing on it. The whole-body kinematics were collected using an 8-camera motion capture system (Nexus 2.11, Vicon Motion Systems Ltd., Yarnton, UK) that sampled at 250 Hz. An eight-channel Trigno Wireless Biofeedback System (Delsys Inc, Natick, MA, USA) that sampled at 2000 Hz was used to record the muscular electrical activities. The data collection was synchronized for the three systems. 5 2.3.2 Protocol of Perturbation Trials The procedure of perturbation trials was briefed to the subject first. Subjects were informed in advance to wear their daily footwear, except impractical shoes such as sandals, high heels, ballet shoes and slippers. Each subject was given an identical type of tight shirt and shorts, to optimize the Vicon motion capture and the placement of retroreflective markers and EMG sensors. Before the perturbation trials, EMG sensors and retroreflective markers were placed on the subject. The eight wireless surface EMG sensors were placed on the eight dominant-leg muscles according to the recommendation of Surface ElectroMyoGraphy for the Non-Invasive Assessment of Muscles (SENIAM) project ( Appendix 1 ). 30 The major muscles relevant to ankle, knee and hip joint motions were selected, including tibialis anterior (TA), gastrocnemius medialis (GM), rectus femoris (RF), long head of bicep femoris (BF), sartorius (SA), gluteus maximus (GMax), gluteus medius (GMed), and adductor maximus (AM). A standard skin preparation procedure included shaving, cleaning and slightly abraded with alcohol wipes before adhering the EMG electrodes. The sensors were applied on the skin with double-sided tapes (Trigno Sensor Adhesive Interface, Delsys, Boston, MA) with medical tapes to enhance fixation. Then a set of 39 retroreflective markers were attached to the bony landmarks of the head, torso, left and right upper limbs, pelvis and left and right lower limbs. 31 All placements were conducted by the same examiner. The subject was then instructed to stand with two feet wearing shoes and shoulder-width apart on the middle of platform, and hold a light rod at waist level and close to the trunk to keep the arms from blocking the reflective markers. The subject was told to stand naturally and look forward at the beginning, try the best to maintain balance if feeling the perturbation, and then return to the original foot position marked by the dark-colored tapes as quickly as possible if they have moved the foot. A safety harness system (PG-360, Physio Gait Dynamic Unweighting System, Healthcare International Ltd., Langley, WA, USA) was equipped on each subject as a safety measure during the perturbation. Each subject then experienced four trials (each consisted of 12 random perturbations) covering a total of 48 unexpected balance perturbations (4 directions × 4 intensities × 3 repetitions), with 5 minutes of rest after each trial. The platform moved horizontally in a pre-determined direction and intensity first, then remained stationary for 12 seconds, and was finally pulled back to its original position. The triggering time, directions (anterior, posterior, medial, lateral), and intensities (highest, high, low, lowest) were randomized and blinded to the subject. Based on the human’s limits of stability in different directions and our pilot study results in young adults, 5 the highest intensity for the anterior, posterior, medial, and lateral directions corresponded to the platform’s moving distances of 2.67%, 4.00%, 5.33% and 5.33% of each subject’s height, respectively. Videos were recorded in real time during all perturbation trials to enable further observation and analysis of the balance control strategies manually. 2.4 Data Processing The kinematic data including the whole body’s COM, the hip, knee, and ankle joint motions were first processed using the Plug-in Gait full body model. Then the kinematic data and raw EMG data were further processed as below in a custom MATLAB program (MATLAB, The MathWorks, Inc., Natick, Ma, USA). The kinematic data were subtracted by the mean signal value of the 1000-ms baseline interval before the start of each perturbation for normalization. To obtain the COM displacement relative to the base of support (BOS), the COM displacement was further subtracted by the displacement of the moving platform. 5 The raw EMG signals were zeroed to the mean value of the entire perturbation trial, full-wave rectified, and low-pass filtered at 4 Hz with a bi-directional 4th order Butterworth filer to obtain the envelope, then further divided by the mean signal value of the 1000-ms baseline interval before the start of perturbation trial for normalization. 5 , 20 Temporal parameters including the onset latency, time to peak, and burst duration, together with the amplitude parameters including the peak amplitude, rate of rise, and/or agonist-antagonist CCI were analyzed through a custom MATLAB algorithm (Fig. 2 ). Within 2 seconds after the start of each perturbation, the onset was detected as the first point in time when the corresponding signal value exceeded five times of the standard deviation (SD) over the mean baseline value (mean + 5 SD), and the peak was identified as the point after the onset with the maximum signal value. 5 , 20 , 32 Within 9 seconds after the start of each perturbation, the offset was identified as the first point in time after the onset when the corresponding signal value dropped below five times the standard deviation over the mean baseline value (mean + 5 SD). 33 The baseline for the onset or offset detection was the 1000-ms interval of a signal before the start of each perturbation. The onset latency indicated the time delay from the start of perturbation to the signal onset, the time to peak indicated that from the start of perturbation to the signal peak, and the burst duration indicated that from the signal onset to offset. The rate of rise was determined as the gradient of the signal rise within a 50-ms period following the onset. 5 , 20 The agonist-antagonist CCI within the duration from two muscles’ later EMG onset to two muscles’ earlier EMG offset was calculated based on the formula in Fig. 2 . 16 , 34 , 35 For each parameter, the mean value of the three perturbations with the same direction and intensity was used in further statistical analyses. 2.5 Statistical Analyses The statistical analyses were performed using SPSS (version 25.0) with the significance level set as 0.05. To examine the difference of baseline subjective assessment data between fallers and non-fallers, the independent sample t tests or Mann-Whitney U tests were used based on the data normality for continuous variables, and Chi-square tests were used for categorical variables. For each parameter (i.e., onset latency, time to peak, peak amplitude, burst duration, peak amplitude, rate of rise, and/or agonist-antagonist CCI), a four-way analysis of variance (ANOVA) and post hoc pairwise comparisons with Bonferroni corrections were conducted to examine the effects of two fall histories, four perturbation directions, four perturbation intensities, and six postural sway directions/eight dominant-leg joint motions/eight dominant-leg muscles/four dominant-leg muscle pairs. When the onset of a signal was absent, the onset latency, time to peak, burst duration were filled with 2000 ms, 2000 ms, and 0 ms, respectively; while the peak amplitude, rate of rise, and agonis-antagonist CCI were all filled with 0. With samples of equal size, the ANOVAs were considered robust even when the assumptions of normality and homogeneity were not fully met 36 . 3. Results 3.1 Subjective Assessment Results No adverse incident happened during all the experiments. There was no significant difference in the number of medications, age, body mass, height, foot length, BMI, short FES-I score or the PASE-C score between the participated older fallers and older non-fallers (Table 1 ). Nevertheless, the Mini-BEST score of fallers was significantly lower than that of non-fallers ( p < 0.05). Table 1 Subjective assessment results (categorical variable: ratio; continuous variable: mean ± SD) of twelve subjects. Faller (n = 6, 3 male & 3 female) Non-faller (n = 6, 3 male & 3 female) Significance ( p value) Number of falls 1.3 ± 0.5 0 / Number of medications 1.0 ± 1.1 0.3 ± 0.5 0.207 Age (year) 71.5 ± 4.6 69.2 ± 2.9 0.316 Body mass (kg) 55.6 ± 8.4 61.4 ± 13.0 0.381 Height (cm) 157.9 ± 8.7 162.0 ± 7.9 0.406 BMI (kg/m 2 ) 22.2 ± 2.0 23.3 ± 4.4 0.808 Leg length (cm) 77.3 ± 6.3 80.8 ± 4.6 0.587 Dominant leg (right/left) 5/1 6/0 0.296 Short FES-I (score) 12.2 ± 2.4 11.5 ± 6.0 0.808 PASE-C (score) 139.5 ± 73.2 148.1 ± 34.6 0.802 Mini-BEST (score) 23.3 ± 1.5 26.0 ± 0.9 0.004 BMI : body mass index. FES-I : fall efficacy scale-international. PASE-C : physical activity scale of elderly-Chinese. Mini-BEST : mini-balance evaluation system test. COM : center of mass. SD : standard deviation. Add : adduction. Abd : abduction. Flex : flexion. Ext : extension. Dorsi : dorsiflexion. Plantar : plantarflexion. SD : standard deviation. EMG : electromyographic. CCI : co-contraction index. SD : standard deviation. Add : adductor. Abd : abductor. Flex : flexor. Ext : extensor. Dorsi : dorsiflexor. Plantar : plantarflexor. 3.2 Balance Control Strategies Fallers were more likely to have stepping responses than non-fallers. The unexpected translational perturbations mainly induced the feet-in-place strategies (567/576, 98.4%), and three subjects (3/12, 25.0%) had stepping responses following nine perturbations (9/576, 1.6%). Specifically, following three highest-intensity medial perturbations, one non-faller had the responses of the non-dominant leg including stepping toward the perturbation direction, performing leg abduction, and elevating the leg (3/576, 0.5%). One faller took a backward step using the non-dominant leg together with several small steps following a highest-intensity anterior perturbation (1/576, 0.2%). The other faller stepped backward using the non-dominant leg following the highest-intensity (2/576, 0.3%) and high-intensity (1/576, 0.2%) anterior perturbations. Additionally, this individual stepped toward the perturbation direction using both legs in response to a low-intensity posterior perturbation (1/576, 0.2%), and with the non-dominant leg in response to a highest-intensity medial perturbation (1/576, 0.2%). 3.3 COM Displacements The mean changes in COM displacements over time (n = 12, Fig. 3 ) together with the onset latency, time to peak, peak amplitude, and burst duration of COM displacement (mean ± SD, n = 12, Fig. 4 ) are displayed for each postural sway direction, each perturbation intensity, and each perturbation direction in participated older fallers and older non-fallers. Four-way ANOVAs showed significant interaction effects of fall history and other factors on the onset latency (fall history × postural sway direction, p < 0.05), time to peak (fall history × postural sway direction, p < 0.05), and peak amplitude (fall history × direction × postural sway direction, p < 0.05) of COM displacement. Figure 4 illustrates the significant differences between older fallers and older non-fallers. Compared to non-fallers, the fallers’ onset latency of COM displacement was significantly longer in the backward direction, but significantly shorter in the forward and downward directions ( p < 0.05); the fallers’ time to peak COM displacement was significantly longer in the backward direction, but significantly shorter in the downward direction ( p < 0.05); the fallers’ peak COM displacement was significantly larger in the forward and downward directions following anterior perturbations, in the backward direction following posterior perturbation, and in the forward direction following both the medial and lateral perturbations ( p < 0.05). Significant pairwise comparisons within the factor that had interaction with “fall history” are summarized in Appendix 2 ( p < 0.05). 3.4 Dominant-leg Joint Motions The mean changes of dominant-leg joint motions over time (n = 12, Fig. 5 ) together with the angle onset latency, time to peak angle, peak angle, and angle burst duration (mean ± SD, n = 12, Fig. 6 ) are displayed for each joint motion, each perturbation intensity, and each perturbation direction in fallers and non-fallers. Four-way ANOVAs showed significant interaction effects of fall history and other factors on the angle onset latency (fall history × direction × joint motion, p < 0.05), time to peak angle (fall history × direction × joint motion, p < 0.05), peak angle (fall history × joint motion, p < 0.05), and angle burst duration (fall history × joint motion, p < 0.05). Significant differences between fallers and non-fallers are indicated in the Fig. 6 . Compared to non-fallers, the fallers’ angle onset latency was significantly longer in the hip adduction, hip extension, and knee extension following anterior perturbations, and in the ankle plantarflexion following medial perturbations ( p < 0.05); the fallers’ time to peak angle was significantly longer in the hip adduction, hip flexion, hip extension, and knee extension following anterior perturbations as well as in the ankle plantarflexion following medial perturbations, but was significantly shorter in the ankle plantarflexion following lateral perturbations ( p < 0.05); the fallers’ peak angle was significantly larger in the hip flexion and knee flexion ( p < 0.05); the fallers’ angle burst duration was significantly longer in the ankle dorsiflexion. ( p < 0.05). Significant pairwise comparisons within the factor that had interaction with “fall history” are shown in Appendix 3 . 3.5 EMG Signals of Dominant-leg Muscles The mean changes of EMG signals over time (n = 12, Fig. 7 ) together with the EMG onset latency, rate of EMG rise, time to peak EMG amplitude, EMG burst duration, and agonist-antagonist CCI (mean ± SD, n = 12, Fig. 8 ) are presented for each dominant-leg muscle (pair), each perturbation intensity, and each perturbation direction in fallers and non-fallers. Four-way ANOVAs showed significant interaction effects of fall history and other factors on the EMG onset latency (fall history × muscle, p < 0.05), rate of EMG rise (fall history × muscle, p < 0.05), and EMG burst duration (fall history × muscle, p < 0.05; fall history × direction, p < 0.05). The main effect of fall history was observed on the time to peak EMG amplitude ( p < 0.05) and the agonist-antagonist CCI ( p < 0.05). Significant differences between fallers and non-fallers are indicated in the Fig. 8 . Compared to non-fallers, the fallers’ EMG onset latency was significantly longer for the hip flexor and hip extensor ( p < 0.05); the fallers’ rate of EMG rise was significantly smaller for the ankle dorsiflexor but was significantly larger for the knee flexor ( p < 0.05); the fallers’ time to peak EMG amplitude was significantly longer ( p < 0.05); the fallers’ EMG burst duration was significantly longer for the hip abductor and ankle dorsiflexor, but was significantly shorter for the hip flexor ( p < 0.05); the fallers’ EMG burst duration was also significantly longer following the anterior and posterior perturbations, but was significantly shorter following the medial perturbations ( p < 0.05); the fallers’ agonist-antagonist CCIs were significantly smaller in the investigated muscle pairs ( p < 0.05). Significant pairwise comparisons within the factor that had interaction with “fall history” are shown in Appendix 4 . 4. Discussion and Implications This study aimed to examine the effects of fall history on reactive standing balance in community-dwelling older adults. It is innovative in depicting the underlying neuromuscular and joint kinematic mechanisms of falls, by removing the confounding factor of age and focusing on the temporal and amplitude responses of dominant-leg muscle activities and joint motions, following the unexpected translational balance perturbations with randomly different directions and intensities. Partially in line with our hypotheses, the effects of “fall history” on the investigated outcomes during reactive balance control have interacted with the “muscle/joint motion/postural sway direction” and “perturbation direction”, but not with the “perturbation intensity”. Specifically, compared to older non-fallers, older fallers have shown slowed activation of ankle/hip muscles while tending to use suspensory strategy for reactive balance control, as supported by a series of neuromuscular alterations and joint kinematics. These new insights underlying older fallers’ reactive balance control have not only indicated the possible reasons of their declined balance capability and higher risk of falls, but also indicated their utilization of prolonged and enlarged (and even overreacted) compensatory strategies for preserving postural stability and preventing falls. 19 Developing some future assessment tools based on the identified parameter may be helpful to screen and identify the fallers from non-faller in the community-dwelling adults. Furthermore, some interventions targeting these identified alterations (that are directly related to falls) may also lead to more effective and targeted solutions for improving reactive balance control and preventing recurrent falls in older fallers. Details are discussed below. 4.1 Fallers Tended to Use Suspensory Strategies Following Unexpected Perturbations: Neuromuscular and Kinematic Mechanisms The primary finding of this study was that fallers have tended to use the suspensory strategy to maintain standing balance following the unexpected translational perturbations as compared to non-fallers. This strategy has enabled fallers to promptly compensate for their insufficient initiation of ankle and hip strategies, but it has led to their prolonged and overacted balance recovery. Fallers have exhibited a decreased speed in response to an unexpected threat of losing standing balance, as indicated by their decreased activation rate of ankle dorsiflexor and the delayed EMG onset timing of hip flexor/extensor compared to non-fallers. This could be attributed to the potential degradation in any components along the sensorimotor pathway, including sensory input (feedback from external perturbation), central organization, and motor output. 6 Ankle dorsiflexor’s activation immediately following the start of perturbation has been in the first line to resist the sudden loss of balance, as this study observed its largest rate of EMG rise among the eight dominant-leg muscles following both anteroposterior and mediolateral perturbations, and our pilot studies in young adults also reported this. 5 With ageing, humans may shift from a distal-to-proximal strategy to a proximal-to-distal strategy to maintain balance, compensating for the difficulties of generating sufficient ankle torque. 37 This study further proved that such phenomenon was more pronounced in older fallers than older non-fallers. On the other hand, fallers have shown delayed EMG onset timing of hip flexor and hip extensor, along with reduced EMG burst duration of hip flexor, compared to non-fallers. These alterations could partly restrict the initiation of the hip strategy, which is the second line of defense against the sudden loss of balance. The delayed activation of hip muscles aligns with and may be explained by previous morphological observations that, fallers had reduced density of skeletal muscle fibers and increased intramuscular adipose issues in gluteus muscles compared to non-fallers. 38 A prior study also reported delayed neuromuscular activation in reactive standing balance, with fallers exhibiting later EMG onset timing of hip abductor and knee flexor in the loading leg than non-fallers following the unexpected lateral perturbations exerted on the shoulder. 18 The discrepancy in the affected muscles could be attributed to the different perturbation methods. A series of kinematic and neuromuscular alterations in fallers when facing unexpected translational balance perturbations have indicated their prominent use of suspensory strategies as compared to non-fallers. In the absence of sufficient ankle and hip muscle activation, fallers have utilized the suspensory strategy, i.e., the third strategy to resist sudden loss of balance by lowering the COM to increase limit of stability and absorb the external perturbation. 39 – 41 as evident from their earlier onset and peak timing of downward COM displacement compared to non-fallers. The increased activation rate of knee flexor, generally decreased agonist-antagonist co-contraction of lower-limb muscles, and larger knee/hip flexion in fallers may have facilitated this strategy. Interestingly, our findings differ from a prior study that reported no differences in postural sway timing or amplitude between fallers and non-fallers following lateral shoulder-impact perturbations, 18 suggesting that different body segment perturbations may elicit distinct reactive balance control strategies. Additionally, while previous research linked greater co-contraction to more joint stability and poorer balance control, 42 , 43 this study has observed that older fallers even with poorer balance performance than non-fallers (lower Mini-BEST scores) were able to reduce agonist-antagonist co-contractions of lower-limb muscles and achieve larger knee/hip flexion for a suspensory strategy. On top of them, this study has observed fallers with the longer activation durations of ankle dorsiflexor and hip abductor together with the longer ankle dorsiflexion duration than non-fallers, which may be necessary for maintaining a knee bending posture during the suspensory strategy. Fallers’ balance control strategies in the current study, however, have required prolonged recovery time and caused overreactions. This is evidenced by their neuromuscular and kinematic alterations as below. Firstly, fallers’ delayed time to peak activation may have suggested their reduced motor unit recruitment and firing rate in response to external perturbations. 25 Secondly, fallers have shown longer time to peak hip flexion angle following anterior perturbations, longer burst durations of ankle dorsiflexion following all perturbations, and delayed timing of recovery joint motions following anterior/medial perturbations than non-fallers. Thirdly, both fallers and non-fallers have demonstrated the major postural sway that was opposite to the direction of an unexpected translational perturbation because of inertia, 5 while fallers have shown larger overshooting postural sways when recovering to initial positions following the unexpected anteroposterior perturbations, as indicated by their larger forward peak COM displacements following anterior perturbations and larger backward ones following posterior perturbations as compared to non-fallers. These findings have indicated that sudden perturbations could pose greater challenges to older fallers. Fallers’ more prominent overshoots of backward postural sways, as compared to non-fallers, have also been previously reported following the anterior waist-pull perturbations. 44 Additionally, a prior study found that fallers had more variable and delayed recovery steps than non-fallers in perturbed walking, 11 which could be corroborated by the observed fallers’ larger overshooting postural sways and delayed timing of overshooting lower-limb joint motions in this study. The slowed but exaggerated postural adjustments seemed to reveal the ineffective strategies used by the older adults with fall histories for reactive balance control. 4.2 Fallers Had Altered Responses to Different Perturbation Directions The secondary finding of this study was that fall history showed interaction effects with perturbation direction, but not with perturbation intensity, on the older adults’ neuromuscular and kinematic responses during reactive balance control. Fallers have shown distinct responses to anteroposterior and mediolateral perturbations compared to non-fallers. Regarding the kinematics, a previous study reported the larger COM path displacement in fallers compared to non-fallers following the mediolateral translational perturbations. 9 Our findings have further revealed that fallers’ larger postural sways were specifically in the forward direction following the mediolateral perturbations and fallers had overshooting postural sways following the anteroposterior perturbations. These responses could be partly attributed to fallers’ delayed timing of recovery joint motions compared to non-fallers following the anterior/medial perturbation. Regarding the neuromuscular responses, fallers have exhibited longer EMG burst durations in dominant-leg muscles following anteroposterior perturbations compared to non-fallers. This could also explain fallers’ overshooting postural sways. Additionally, this could explain why fallers had more non-dominant leg stepping following anteroposterior perturbations than non-fallers, as more body weight was loaded on the dominant leg. Conversely, fallers have exhibited shorter EMG burst durations of dominant-leg muscles than non-fallers following medial perturbations, resulting in fallers’ fewer non-dominant leg steps following medial perturbations compared to non-fallers. Notably, this study has found no differences in the responses of fallers compared to non-fallers to varied intensities of unexpected perturbations. Previous studies reported the inconsistent results regarding the interaction effect of fall history and perturbation intensity on the stepping characteristics following waist-pull perturbations. 10 , 21 Our finding has further built on evidence following the unexpected translational perturbations and suggested that fallers’ neuromuscular/kinematic responses to the different intensities of perturbations, which primarily induced feet-in-place strategies, have been similar to those of non-fallers. 4.3 Strengths and Limitations To the best of authors’ knowledge, this is the first study that has investigated the differences in eight major lower-limb muscles’ activation or lower-limb joint kinematics during reactive balance control between older fallers and older non-fallers. With the comprehensive analyses of the temporal and amplitude characteristics of these investigated signals, this study has built knowledge on the prior investigations that focused on a limited number of muscles and EMG parameters, and has addressed the gap of limited research on joint kinematics in fallers vs non-fallers. The mechanisms of fall-prone older adults’ decline of reactive balance control and compensatory strategy could be better understood with the findings of this study. This study has two limitations. Firstly, given the small sample sizes of recruited older fallers and older non-fallers, the findings of this study may be susceptible to sampling errors. Nevertheless, even by using a stringent method, i.e., post hoc pairwise comparisons with Bonferroni corrections, this study successfully identified some kinematic and neuromuscular factors to differentiate the balance control strategies between fallers and non-faller, which could provide preliminary insights for the further studies with larger sample sizes. Secondly, this study did not conduct maximal voluntary contraction tests for the eight analyzed muscle groups, but used the baseline EMG signal value in unperturbed standing for EMG amplitude normalization. It is therefore important to note that the rate of EMG rise and agonist-antagonist CCI in this study reflected the extent to which the perturbation task utilized the activation required for normal standing rather than the maximal activation. 4.4 Implications for Future Clinical Practice This study could have useful implications for the assessment and training of reactive balance control in future clinical practice. Firstly, the identified kinematic or neuromuscular factors that were directly related to falls can potentially facilitate the more sensitive instrumented assessment of reactive balance control and facilitate the earlier identification of fall-prone older adults. Further longitudinal studies with larger sample size are merited to examine the diagnostic accuracies/sensitivities/specificities of these identified more in-depth factors of reactive balance control in differentiating older adults’ fall risks. Secondly, the reactive balance training may need to be prescribed more for the community-dwelling older adults with fall histories in the future, considering their generally delayed peak activation of lower-limb muscles. A most recent review has reported that the perturbation-based balance training and stepping training can improve the slowed reaction time in confronting with sudden loss of balance. 45 This is promising as fallers’ degradation in neuromuscular timing can be modified. Our findings further imply that more focus/efforts may need to be put on the ankle and hip muscle power training in older fallers. Although the proximal hip and knee muscles were previously reported to be more affected with aging following unexpected perturbations, 46 the findings of this study may suggest that the training of ankle muscles should not be ignored, especially in older fallers. Additionally, this study could imply that therapists may notice whether the clients/patients have overshooting postural sways in sagittal plane during the reactive balance training. Giving feedback on this may potentially help improve the reactive balance control in older adults with fall histories to be more effective. 5. Conclusion Compared to non-fallers, older fallers’ kinematic and neuromuscular alterations in resisting unexpected translational perturbations could be indicators of both the decline and the compensation of reactive balance control. Fallers had a decreased activation rate in ankle dorsiflexor and delayed activation in hip flexor/extensor, thereby resorting to the suspensory strategy for quickly responding to external perturbations. The increased activation rate of knee flexor, decreased agonist-antagonist co-contraction of lower-limb muscles, enlarged knee and hip flexion, and earlier downward postural sways in fallers could be the basis of their prioritizing of suspensory strategies compared to non-fallers. However, fallers’ balance control strategies required prolonged recovery time and caused overreactions. Targeted modification of these identified fall-related factors is expected to prevent falls more effectively in older fallers. Abbreviations Abbreviation Full name AM Adductor Maximus ANOVA Analysis of Variance BF Long Head of Bicep Femoris CCI Co-Contraction Index COM Center of Mass EMG Electromyographic FES-I Falls Efficacy Scale-International MG Medial Gastrocnemius GMax Gluteus Maximus GMed Gluteus Medius Mini-BEST Mini-Balance Evaluation System Test PASE-C Chinese Version of The Physical Activity Scale for the Elderly RF Rectus Femoris SA Sartorius SD Standard Deviation SENIAM Surface Electromyography for the Non-Invasive Assessment of Muscles TA Tibialis Anterior Declarations Ethics approval and consent to participate This study has been approved by the Institutional Review Board, The Hong Kong Polytechnic University (Ethical reference number: HSEARS20201230002). Each participant gas signed the consent form. Consent for publication For the participant appeared in Figure 1, consent has been obtained from him for the publication of image. Availability of data and materials The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request. Competing interests The authors declare that they have no competing interests. Funding This work was partially supported by the Research Institute for Smart Ageing, The Hong Kong Polytechnic University [P0038945]; The Hong Kong Polytechnic University [P0034491]; and Associated Money, The Hong Kong Polytechnic University [G4Y56R006 to R.T.L.Z]. Authors' contributions R.T.L.Z.: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Data Curation, Writing- Original Draft, Visualization; T.T.M.H.: Formal analysis, Investigation, Writing- Original Draft; F.M.H.L.: Writing- Review & Editing; J.Z.L.: Investigation; Y.Y.L.: Investigation; J.S.: Writing- Review & Editing; S.W.: Writing- Review & Editing; C.Z.H.M.: Conceptualization, Methodology, Resources, Writing- Review & Editing, Supervision, Project administration, Funding acquisition. Acknowledgements Not applicable. References World Health Organization. Falls. World Health Organization. Accessed. Apr., 2023, 2023. https://www.who.int/news-room/fact-sheets/detail/falls# . Moreland B, Kakara R, Henry A. Trends in nonfatal falls and fall-related injuries among adults aged ≥ 65 years—United States, 2012–2018. Morb Mortal Wkly Rep. 2020;69(27):875. de Vries OJ, Peeters GMEEG, Elders PJM, et al. Multifactorial Intervention to Reduce Falls in Older People at High Risk of Recurrent Falls: A Randomized Controlled Trial. Arch Intern Med. 2010;170(13):1110–7. 10.1001/archinternmed.2010.169 . Kasahara S, Saito H. Mechanisms of postural control in older adults based on surface electromyography data. Human Movement Science . 2021/ 08/01/ 2021;78:102803. doi:https://doi.org/10.1016/j.humov.2021.102803. Tong CY, Zhu RT, Ling YT, et al. Muscular and Kinematic Responses to Unexpected Translational Balance Perturbation: A Pilot Study in Healthy Young Adults. Bioengineering. 2023;10(7). 10.3390/bioengineering10070831 . Osoba MY, Rao AK, Agrawal SK, Lalwani AK. Balance and gait in the elderly: A contemporary review. Laryngoscope Investig Otolaryngol Feb. 2019;4(1):143–53. 10.1002/lio2.252 . Allum JH, Carpenter MG, Honegger F, Adkin AL, Bloem BR. Age-dependent variations in the directional sensitivity of balance corrections and compensatory arm movements in man. J Physiol Jul. 2002;15(Pt 2):643–63. 10.1113/jphysiol.2001.015644 . Ma CZ-H, Lee WC-C. A wearable vibrotactile biofeedback system improves balance control of healthy young adults following perturbations from quiet stance. Human Movement Science . 2017 /10/01/ 2017;55:54–60. doi:https://doi.org/10.1016/j.humov.2017.07.006. Batcir S, Shani G, Shapiro A, Alexander N, Melzer I. The kinematics and strategies of recovery steps during lateral losses of balance in standing at different perturbation magnitudes in older adults with varying history of falls. BMC Geriatr Jul. 2020;20(1):249. 10.1186/s12877-020-01650-4 . Bair W-N, Prettyman MG, Beamer BA, Rogers MW. Kinematic and behavioral analyses of protective stepping strategies and risk for falls among community living older adults. Clin Biomech. 2016;36:74–82. Gerards MH, Meijer K, Karamanidis K, et al. Adaptability to balance perturbations during walking as a potential marker of falls history in older adults. Front sports Act living. 2021;3:132. Sturnieks DL, Menant J, Delbaere K, et al. Force-Controlled Balance Perturbations Associated with Falls in Older People: A Prospective Cohort Study. Article. PLoS ONE. 2013;8(8):e70981. 10.1371/journal.pone.0070981 . Maki BE, Holliday PJ, Topper AK. A Prospective Study of Postural Balance and Risk of Falling in An Ambulatory and Independent Elderly Population. J Gerontol. 1994;49(2):M72–84. 10.1093/geronj/49.2.M72 . Tantisuwat A, Vachalathiti R, Hiengkaew V, Assantachai P. Stepping Responses during Forward and Backward Fall Recovery between Thai Elderly Fallers and Non-fallers. J Phys THERAPY Sci JUN. 2011;23(3):373–9. 10.1589/jpts.23.373 . Journals of Gerontology Series A: Biomedical Sciences and Medical Sciences . 2013;68(12):1540–1548. Thompson JD, Plummer P, Franz JR. Age and falls history effects on antagonist leg muscle coactivation during walking with balance perturbations. Clin Biomech (Bristol Avon) Nov. 2018;59:94–100. 10.1016/j.clinbiomech.2018.09.011 . Studenski S, Chandler J. Postural Responses and Effector Factors in Persons with Unexplained Falls: Results and Methodologic Issues. Article. J Am Geriatr Soc. 1991;39(3):229–34. 10.1111/j.1532-5415.1991.tb01642.x . Claudino R, Dos Santos MJ, Mazo GZ. Delayed Compensatory Postural Adjustments After Lateral Perturbations Contribute to the Reduced Ability of Older Adults to Control Body Balance. Motor Control Oct. 2017;1(4):425–42. 10.1123/mc.2016-0005 . Lin S-I, Woollacott MH. Postural muscle responses following changing balance threats in young, stable older, and unstable older adults. J Mot Behav. 2002;34(1):37–44. Zhu RT, Lyu PZ, Li S, Tong CY, Ling YT, Ma CZ. How Does Lower Limb Respond to Unexpected Balance Perturbations? New Insights from Synchronized Human Kinetics, Kinematics, Muscle Electromyography (EMG) and Mechanomyography (MMG) Data. Biosens (Basel) Jun. 2022;18(6). 10.3390/bios12060430 . Rogers MW, Hedman LD, Johnson ME, Cain TD, Hanke TA. Lateral Stability During Forward-Induced Stepping for Dynamic Balance Recovery in Young and Older Adults. Journals Gerontology: Ser A. 2001;56(9):M589–94. 10.1093/gerona/56.9.M589 . Maki BE, Holliday PJ, Topper AK. A prospective study of postural balance and risk of falling in an ambulatory and independent elderly population. J Gerontol Mar. 1994;49(2):M72–84. 10.1093/geronj/49.2.m72 . Batcir S, Shani G, Shapiro A, Alexander N, Melzer I. The kinematics and strategies of recovery steps during lateral losses of balance in standing at different perturbation magnitudes in older adults with varying history of falls. BMC Geriatr JUL. 2020;20(1):249. 10.1186/s12877-020-01650-4 . Fujimoto M, Bair WN, Rogers MW. Center of pressure control for balance maintenance during lateral waist-pull perturbations in older adults. J Biomech APR. 2015;13(6):963–8. 10.1016/j.jbiomech.2015.02.012 . de Freitas PB, Knight CA, Barela JA. Postural reactions following forward platform perturbation in young, middle-age, and old adults. Journal of electromyography and kinesiology . 2010 /08/01/ 2010;20(4):693–700. doi:https://doi.org/10.1016/j.jelekin.2009.11.009. Tinetti ME, Speechley M, Ginter SF. Risk factors for falls among elderly persons living in the community. N Engl J Med Dec. 1988;29(26):1701–7. 10.1056/nejm198812293192604 . Yardley L, Beyer N, Hauer K, Kempen G, Piot-Ziegler C, Todd C. Development and initial validation of the Falls Efficacy Scale-International (FES-I). Age and Ageing . 2005;34(6):614–619. 10.1093/ageing/ afi196%J Age and Ageing. Ku P-W, Sun W, Chen L-J. Reliability and Validity of the Chinese Version of the Physical Activity Scale for the Elderly. Sport Exerc Res. 2013;10/01:15:309–19. King L, Horak F. On the Mini-BESTest: Scoring and the Reporting of Total Scores. Phys Ther. 2013;93(4):571–5. 10.2522/ptj.2013.93.4.571 . Hermens HJ, Freriks B, Disselhorst-Klug C, Rau G. Development of recommendations for SEMG sensors and sensor placement procedures. Journal of Electromyography and Kinesiology . 2000 /10/01 / 2000;10(5):361–374. doi:https://doi.org/10.1016/S1050-6411(00)00027-4. Vicon Motion Systems Limited. Plug-in Gait Reference Guide. https://docs.vicon.com/display/Nexus212/Plug-in+Gait+Reference+Guide . Ling YT, Ma CZ, Shea QT, Zheng Y-P. Sonomechanomyography (SMMG): Mapping of Skeletal Muscle Motion Onset during Contraction Using Ultrafast Ultrasound Imaging and Multiple Motion Sensors. Sensors. 2020;20(19). 10.3390/s20195513 . Hesam-Shariati N, Trinh T, Thompson-Butel AG, Shiner CT, McNulty PA. A longitudinal electromyography study of complex movements in Poststroke therapy. 1: heterogeneous changes despite consistent improvements in clinical assessments. Front Neurol. 2017;8:340. Di Nardo F, Morano M, Strazza A, Fioretti S. Muscle Co-Contraction Detection in the Time-Frequency Domain. Sens (Basel) Jun. 2022;28(13). 10.3390/s22134886 . Falconer K. Quantitative assessment of cocontraction at the ankle joint in walking. Electromyogr Clin Neurophysiol. 1985;25:135–48. Portney LG. Foundations of clinical research: applications to evidence-based practice. FA Davis; 2020. Woollacott MH. 8 Age-Related Changes in Posture and Movement. J Gerontol. 1993;48(SpecialIssue):56–60. 10.1093/geronj/48.Special_Issue.56 . Inacio M, Ryan AS, Bair W-N, Prettyman M, Beamer BA, Rogers MW. Gluteal muscle composition differentiates fallers from non-fallers in community dwelling older adults. BMC Geriatr . 2014/03/25 2014;14(1):37. 10.1186/1471-2318-14-37 . Nijhuis LBO, Bloem BR, Carpenter MG, Allum JHJ. Incorporating voluntary knee flexion into nonanticipatory balance corrections. J Neurophysiol Nov. 2007;98(5):3047–59. 10.1152/jn.01303.2006 . Cheng KB. Does knee motion contribute to feet-in-place balance recovery? J Biomech . 2016/06/14/ 2016;49(9):1873–1880. https://doi.org/10.1016/j.jbiomech.2016.04.026 . Kasahara S, Saito H, Anjiki T, Osanai H. The effect of aging on vertical postural control during the forward and backward shift of the center of pressure. Gait Posture. 2015;42(4):448–54. Schulz BW, Jongprasithporn M, Hart-Hughes SJ, Bulat T. Effects of step length, age, and fall history on hip and knee kinetics and knee co-contraction during the maximum step length test. Article. Clin Biomech. 2013;28(8):933–40. 10.1016/j.clinbiomech.2013.08.002 . Falk J, Strandkvist V, Pauelsen M, Vikman I, Nyberg L, Röijezon U. Increased co-contraction reaction during a surface perturbation is associated with unsuccessful postural control among older adults. BMC Geriatr . 2022/05/19 2022;22(1):438. 10.1186/s12877-022-03123-2 . Ho CY, Bendrups AP. Ankle Reflex Stiffness During Unperceived Perturbation of Standing in Elderly Subjects. Journals Gerontology: Ser A. 2002;57(9):B344–50. 10.1093/gerona/57.9.B344 . Bhagwat AP, Deodhe NP. The Effect of Perturbation-Based Balance Training vs Step Training on Reaction Time in Older Persons: A Review. Cureus. 2023;15(11):e48104. 10.7759/cureus.48104 . /11/01 2023. Hall CD, Jensen JL. Age-related differences in lower extremity power after support surface perturbations. J Am Geriatr Soc. 2002;50(11):1782–8. Additional Declarations No competing interests reported. Supplementary Files Appendices14.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4422750","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":307887740,"identity":"2e55881a-f60f-4380-8f93-7a8a180d94a0","order_by":0,"name":"Ringo Tang-Long Zhu","email":"","orcid":"","institution":"Department of Biomedical Engineering, The Hong Kong Polytechnic University, Hong Kong SAR","correspondingAuthor":false,"prefix":"","firstName":"Ringo","middleName":"Tang-Long","lastName":"Zhu","suffix":""},{"id":307887741,"identity":"0e3692f4-d561-4710-90ed-3d4e5c19efbe","order_by":1,"name":"Timmi Tim Mei Hung","email":"","orcid":"","institution":"Department of Biomedical Engineering, The Hong Kong Polytechnic University, Hong Kong SAR","correspondingAuthor":false,"prefix":"","firstName":"Timmi","middleName":"Tim Mei","lastName":"Hung","suffix":""},{"id":307887742,"identity":"af317d02-8b3d-411e-be56-3bff92728ad6","order_by":2,"name":"Freddy Man Hin Lam","email":"","orcid":"","institution":"Department of Rehabilitation Sciences, The Hong Kong Polytechnic University, Hong Kong SAR","correspondingAuthor":false,"prefix":"","firstName":"Freddy","middleName":"Man Hin","lastName":"Lam","suffix":""},{"id":307887743,"identity":"f22c997c-6efe-4e60-a2f2-c17a240197db","order_by":3,"name":"Jun-Zhe Li","email":"","orcid":"","institution":"Department of Biomedical Engineering, The Hong Kong Polytechnic University, Hong Kong SAR","correspondingAuthor":false,"prefix":"","firstName":"Jun-Zhe","middleName":"","lastName":"Li","suffix":""},{"id":307887744,"identity":"d6892878-cbb3-4846-be7c-fb0807916ff8","order_by":4,"name":"Yu-Yan Luo","email":"","orcid":"","institution":"Department of Biomedical Engineering, The Hong Kong Polytechnic University, Hong Kong SAR","correspondingAuthor":false,"prefix":"","firstName":"Yu-Yan","middleName":"","lastName":"Luo","suffix":""},{"id":307887745,"identity":"2bf045c8-1958-4bd6-8db0-0aada6145029","order_by":5,"name":"Jingting Sun","email":"","orcid":"","institution":"Future Architecture and Urban Research Institute, Tongji Architectural Design (Group) Co., Ltd., Shanghai","correspondingAuthor":false,"prefix":"","firstName":"Jingting","middleName":"","lastName":"Sun","suffix":""},{"id":307887746,"identity":"9e68362a-c0fe-47c5-91fc-1923beaf45a5","order_by":6,"name":"Shujun Wang","email":"","orcid":"","institution":"Department of Biomedical Engineering, The Hong Kong Polytechnic University, Hong Kong SAR","correspondingAuthor":false,"prefix":"","firstName":"Shujun","middleName":"","lastName":"Wang","suffix":""},{"id":307887747,"identity":"c25e6073-c8e0-4fce-9183-4d04c3ce63af","order_by":7,"name":"Christina Zong-Hao Ma","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA2UlEQVRIiWNgGAWjYFACHhBhw8DADuYxMzBIsBGlJQ2smCQth0nQwt9/9gDTjZrzefzMPGYSDBXWiQ3SbQl4tUgcOJfAnHPsdrFkM0jLmfTEBpljB/BqMWDsMWDOYbuduOEwUAtj2+HEBon0BvxamHmAWv6dg2r5R4wWNqCW3LYDUC0NIC1p+B0mcYbH4HBuXzLQL2zFFgnH0o3bJNIS8Grh7z9j+Djnm10eP3vzxhsfaqxl+yXSDPBqAQGQO4AGcxiASAaCEQkDQMXsD4hVPApGwSgYBSMMAADxQD+Pp67TmAAAAABJRU5ErkJggg==","orcid":"","institution":"Department of Biomedical Engineering, The Hong Kong Polytechnic University, Hong Kong SAR","correspondingAuthor":true,"prefix":"","firstName":"Christina","middleName":"Zong-Hao","lastName":"Ma","suffix":""}],"badges":[],"createdAt":"2024-05-15 05:44:20","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4422750/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4422750/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":57396863,"identity":"afec0c06-ed0e-448b-ab0a-ee6e32624abc","added_by":"auto","created_at":"2024-05-30 07:12:04","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":377338,"visible":true,"origin":"","legend":"\u003cp\u003eThe moving-platform perturbation system with the subject.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4422750/v1/0b233b69c1897cc2810ad01d.png"},{"id":57396870,"identity":"584a3de4-4723-4c24-b93c-77db7b9b5a72","added_by":"auto","created_at":"2024-05-30 07:12:05","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":150248,"visible":true,"origin":"","legend":"\u003cp\u003eIllustrations of the analyzed temporal and amplitude parameters. EMG: electromyographic.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4422750/v1/c70e03e80cb960d19d2498ab.png"},{"id":57396869,"identity":"bed88552-131e-4655-88fe-5af38156400d","added_by":"auto","created_at":"2024-05-30 07:12:05","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":238134,"visible":true,"origin":"","legend":"\u003cp\u003eThe mean forward/backward, medial/lateral, and upward/downward COM displacements in fallers (n = 6) and non-fallers (n = 6) following perturbations with different directions and intensities. The red dotted line denotes the start of balance perturbation. COM: center of mass.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4422750/v1/0e955723cd3d41eed2996455.png"},{"id":57396867,"identity":"2708c0c7-1221-431a-bcb7-20cc4ddcf85b","added_by":"auto","created_at":"2024-05-30 07:12:05","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":361729,"visible":true,"origin":"","legend":"\u003cp\u003eSee image above for figure legend\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4422750/v1/d02ee2ecb9e84b75790594aa.png"},{"id":57396868,"identity":"fda41fb5-832a-45cb-b7b6-cf2c7a0ef31c","added_by":"auto","created_at":"2024-05-30 07:12:05","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":277659,"visible":true,"origin":"","legend":"\u003cp\u003eThe mean change for each of the eight dominant-leg joint motions in fallers (n = 6) and non-fallers (n = 6) following perturbations with different directions and intensities.\u003c/p\u003e\n\u003cp\u003eThe red dotted line denotes the start of balance perturbation. \u003cstrong\u003eAdd\u003c/strong\u003e: adduction. \u003cstrong\u003eAbd\u003c/strong\u003e: abduction. \u003cstrong\u003eFlex\u003c/strong\u003e: flexion. \u003cstrong\u003eExt\u003c/strong\u003e: extension. \u003cstrong\u003eDorsi\u003c/strong\u003e: dorsiflexion. \u003cstrong\u003ePlantar\u003c/strong\u003e: plantarflexion. \u003cstrong\u003eSD\u003c/strong\u003e: standard deviation.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4422750/v1/65b1f4dd19800694d09a778f.png"},{"id":57396862,"identity":"3e9e4478-7337-4de9-8d04-ed4ecc117843","added_by":"auto","created_at":"2024-05-30 07:12:04","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":399616,"visible":true,"origin":"","legend":"\u003cp\u003eSee image above for figure legend\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-4422750/v1/0bfcaee2b180e8e630fd64b6.png"},{"id":57396866,"identity":"a250cce2-cc91-44c8-a1cd-6e92f5b569e0","added_by":"auto","created_at":"2024-05-30 07:12:04","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":443445,"visible":true,"origin":"","legend":"\u003cp\u003eThe mean EMG signal change for each of the eight dominant-leg muscles in fallers (n = 6) and non-fallers (n = 6) following perturbations with different directions and intensities. The red dotted line denotes the start of balance perturbation.\u003c/p\u003e\n\u003cp\u003eEMG: electromyographic. CCI: co-contraction index. SD: standard deviation. GMed: gluteus medius. SA: sartorius. RF: rectus femoris. TA: tibialis anterior. AM: adductor magnus. GMax: gluteus maximus. BF: biceps femoris. GM: gastrocnemius medialis. Add: adductor. Abd: abductor. Flex: flexor. Ext: extensor. Dorsi: dorsiflexor. Plantar: plantarflexor.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-4422750/v1/34717998f570dab67beddab6.png"},{"id":57396861,"identity":"cc4dd99f-8c52-44f5-8824-0db6650e945e","added_by":"auto","created_at":"2024-05-30 07:12:04","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":230251,"visible":true,"origin":"","legend":"\u003cp\u003eSee image above for figure legend\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-4422750/v1/9ee51c3fb8359ab255e2c20d.png"},{"id":66201513,"identity":"5084d418-56dd-441c-aaec-ad44ce7fd412","added_by":"auto","created_at":"2024-10-08 15:32:07","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3517311,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4422750/v1/16aa91c1-4db0-426a-8442-09ef4470e572.pdf"},{"id":57396864,"identity":"0e60da8a-c36c-4c52-b6bf-8d1c8a28c6a9","added_by":"auto","created_at":"2024-05-30 07:12:04","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":24042,"visible":true,"origin":"","legend":"","description":"","filename":"Appendices14.docx","url":"https://assets-eu.researchsquare.com/files/rs-4422750/v1/09e6c2617b1e8f555504e800.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Older Fallers and Non-fallers’ Neuromuscular and Kinematic Alterations in Reactive Balance Control: Indicators of Balance Decline or Compensation?","fulltext":[{"header":"1. Background","content":"\u003cp\u003eFalls and fall consequences in older adults burden the society heavily and are global health issues.\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e Annually, around one in three older adults falls, one in ten older adults has fall-related injuries, and 684,000 fall-related deaths happen worldwide.\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e However, even the multi-factorial fall-prevention management has shown relatively limited success in fall reduction, especially in older adults with fall histories, i.e., fallers.\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e Given that balance and gait disorders are the second leading causes of falls except accidents,\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e some in-depth physiological alterations of balance control in older fallers that have remained unidentified may be modifiable to prevent older adults\u0026rsquo; future falls more effectively.\u003c/p\u003e \u003cp\u003eSeveral balance control strategies with the involvement of lower limbs have been proposed based on the analyses of kinematics (i.e., postural sways, joint motions) and neuromuscular activities (i.e., electromyographic [EMG] signals). The feet-in-place strategy is commonly employed to keep the whole-body center of mass (COM) within the base of support (BOS) when external perturbations are not large, which comprises a single or a combination of the ankle strategy, hip strategy, and suspensory strategy (bending knees to lower COM for stability).\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e The stepping strategy is used to establish a new BOS when the feet-in-place strategy is not enough to overcome the increasing perturbation intensity.\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e Compared to young adults, older adults tended to rely more on the proximal lower-limb joint motions and muscles than the distal ones, and may use the stepping strategy for reactive/compensatory/automatic balance control following unexpected perturbations.\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e Apart from the age-related changes in the responses of multiple muscles/joints, prior studies have also shown the interaction effects of age with the perturbation direction and perturbation intensity on the balance control strategies.\u003csup\u003e\u003cspan additionalcitationids=\"CR7\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e Nevertheless, the identified age-related kinematic and neuromuscular alterations underlying the reactive balance control may not be directly indicative of fall risks, due to the potential existence of the confounding factor of age. Specific investigations and comparisons of the older adults with and without fall histories (i.e., fallers vs. non-fallers, and excluding the confounding factor of age) are therefore warranted to identify further balance control alterations in older individuals who are prone to falls, and to identify the fall-related factors.\u003c/p\u003e \u003cp\u003ePrevious studies have intensively analyzed the stepping strategies and whole-body postural sways to compare the reactive balance control between fallers vs non-fallers,\u003csup\u003e\u003cspan additionalcitationids=\"CR10 CR11 CR12 CR13 CR14\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e while the differences in specific joint motions or muscle activities were less focused.\u003csup\u003e\u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e Firstly, lower-limb muscle activities during reactive balance control have primarily been examined within a restricted number of lower-limb muscles, i.e., the ankle dorsiflexor/plantarflexor,\u003csup\u003e16\u0026ndash;18\u003c/sup\u003e knee flexor/extensor,\u003csup\u003e16,18\u003c/sup\u003e and hip abductor.\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e The difference on hip adductor and hip flexor/extensor activation across fallers and non-fallers remained unknown. Secondly, prior investigations of lower-limb muscle activities have examined only one single EMG parameter in each study.\u003csup\u003e\u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e Fallers were reported to exhibit longer EMG onset latency of ankle dorsiflexor following anterior translational perturbations during standing,\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e longer EMG onset latencies of hip abductor and knee flexor in the weight-bearing leg following lateral shoulder-impact perturbations during standing,\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e and no significantly different agonist-antagonist co-contraction index (CCI) of ankle dorsiflexor-plantarflexor or knee flexor-extensor following optical flow perturbations during walking as compared to non-fallers.\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e The existing analysis of timing and amplitude characteristics of EMG signals may have been insufficient, since only the delayed muscular reaction was identified to differentiate fallers from non-fallers.\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e,\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e Thirdly, regarding joint kinematics, interestingly, no prior studies seemed to have compared them in fallers vs. non-fallers during reactive balance control to the best of authors\u0026rsquo; knowledge. Although fallers exhibited decreased range of motion in lower-limb joints than non-fallers,\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e it has been unclear whether the lower-limb joint motions during reactive balance control differ between fallers and non-fallers. More comprehensive analyses of lower-limb muscle activities and joint kinematics are needed to facilitate the understanding of older fallers\u0026rsquo; balance control strategies.\u003c/p\u003e \u003cp\u003eReactive balance control strategies are influenced by both the perturbation direction and perturbation intensity,\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e while there is still insufficient evidence to determine whether fallers and non-fallers respond differently to diverse directions or intensities of balance perturbations. Regarding the perturbation intensity, a previous study reported that fallers and non-fallers\u0026rsquo; difference in stepping strategy was more pronounced following a higher intensity of mediolateral perturbation,\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e whereas another study did not observe an interaction effect of fall history and perturbation intensity on the reactive stepping strategy following unexpected anterior perturbations.\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e Regarding the perturbation direction, prior studies also reported inconsistent differences in postural sway between fallers and non-fallers when responding to unexpected anteroposterior\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e or mediolateral perturbations.\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e,\u003cspan additionalcitationids=\"CR23\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e The underlying reasons for these inconsistent findings have not been thoroughly understood/explained. Analyzing neuromuscular responses and joint kinematics during reactive balance control can potentially help better explain how the fall-prone older adults respond to varied levels of threats of suddenly losing balance, which may also provide useful insights for clinical assessments of reactive balance.\u003c/p\u003e \u003cp\u003eThe main aim of this study was therefore to explore the older fallers\u0026rsquo; neuromuscular and kinematic alterations of lower limbs during reactive balance control as compared to non-fallers. Specifically, this study had the research question of how EMG/angle signals varied among the eight different dominant-leg muscles/joint motions, different fall histories, directions, and intensities of unexpected translational perturbations. In addition, how the COM displacements varied among the six different postural sway directions (i.e., forward/backward, medial/lateral, and upward/downward), different fall histories, perturbation directions, and perturbation intensities were investigated. The timing parameters including onset latency, time to peak, and burst duration and the amplitude parameters including the rate of rise, peak amplitude, and/or agonist-antagonist CCI were analyzed for these signals. We hypothesized that the analyzed parameters during reactive balance control would be affected by the interaction of fall history, muscle/joint motion/postural sway direction, perturbation direction, and perturbation intensity. Further for the simple main effects of fall history, based on the previously available findings related to ageing\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e and fall histories,\u003csup\u003e\u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e we extrapolated that fallers would have the delayed timing and larger amplitudes of proximal muscles\u0026rsquo; activation/joint motions than non-fallers following a high intensity of unexpected anterior or lateral balance perturbation.\u003c/p\u003e"},{"header":"2. Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Study Design and Subjects\u003c/h2\u003e \u003cp\u003eThis study was a pilot observational cross-sectional study. Subjects were recruited through convenience sampling. Inclusion criteria were: \u003cb\u003e1)\u003c/b\u003e aged 65 years old or over, \u003cb\u003e2)\u003c/b\u003e living in the community independently and been able to walk for 400 m without any assistance, and \u003cb\u003e3)\u003c/b\u003e fallers (with at least one fall within the past one year) or non-fallers (with no fall within the past one year) in matched age and gender. Exclusion criteria were: \u003cb\u003e1)\u003c/b\u003e being hospitalized or living in nursing homes for more than six months in the past year; \u003cb\u003e2)\u003c/b\u003e experienced fall(s) due to traffic or occupational accidents; \u003cb\u003e3)\u003c/b\u003e diagnosed with cognitive impairment or severe systemic disease (e.g., neuromuscular, renal, hepatic, orthopedic, vestibular, or cardiopulmonary disorders) that impacts or limits physical activities; and \u003cb\u003e4)\u003c/b\u003e participated in any structured exercise training or strengthening exercises within the past 1 year. A total of twelve older participants were finally eligible for this study. Before being tested, each subject has read and signed an informed consent to participate in this study (Ethics approval agency: Institutional Review Board, The Hong Kong Polytechnic University; Ethical reference number: HSEARS20201230002). Each subject participated in the experiment once, involving subjective assessments and perturbation trials.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Subjective Assessments\u003c/h2\u003e \u003cp\u003e The collection of demographic data (e.g., age, gender, height, body mass), medical history, and fall history was first conducted, followed by the assessments using questionnaires/scale. A fall is defined as an event coming to rest inadvertently on the ground or floor or other lower level and not resulting from an intrinsic or overwhelming hazard.\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e The short Falls Efficacy Scale-International (FES-I) and the Chinese Version of the Physical Activity Scale for the Elderly (PASE-C) were introduced to the subject for the measurement of their fear of falling and physical activity level.\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e The Mini-Balance Evaluation System Test (Mini-BEST) was used to assess the subject\u0026rsquo;s functional balance performance including the anticipatory postural control, reactive postural control, sensory orientation, and dynamic gait.\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e Then the subject\u0026rsquo;s dominant leg was determined for the placement of EMG sensors.\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e All subjective assessments were conducted by the same examiner.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Perturbation Trials\u003c/h2\u003e \u003cdiv id=\"Sec6\" class=\"Section3\"\u003e \u003ch2\u003e2.3.1 Experimental Set-Up\u003c/h2\u003e \u003cp\u003eA moving-platform perturbation system was used to induce the unexpected translational perturbations (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), with technical details reported in a previous study.\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e Generally, the platform can move horizontally at a random starting time, with random moving direction and random moving distance/velocity/acceleration (related to different intensity) to constitute an unexpected balance perturbation to the subject standing on it. The whole-body kinematics were collected using an 8-camera motion capture system (Nexus 2.11, Vicon Motion Systems Ltd., Yarnton, UK) that sampled at 250 Hz. An eight-channel Trigno Wireless Biofeedback System (Delsys Inc, Natick, MA, USA) that sampled at 2000 Hz was used to record the muscular electrical activities. The data collection was synchronized for the three systems.\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section3\"\u003e \u003ch2\u003e2.3.2 Protocol of Perturbation Trials\u003c/h2\u003e \u003cp\u003eThe procedure of perturbation trials was briefed to the subject first. Subjects were informed in advance to wear their daily footwear, except impractical shoes such as sandals, high heels, ballet shoes and slippers. Each subject was given an identical type of tight shirt and shorts, to optimize the Vicon motion capture and the placement of retroreflective markers and EMG sensors. Before the perturbation trials, EMG sensors and retroreflective markers were placed on the subject. The eight wireless surface EMG sensors were placed on the eight dominant-leg muscles according to the recommendation of Surface ElectroMyoGraphy for the Non-Invasive Assessment of Muscles (SENIAM) project (\u003cb\u003eAppendix 1\u003c/b\u003e).\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e The major muscles relevant to ankle, knee and hip joint motions were selected, including tibialis anterior (TA), gastrocnemius medialis (GM), rectus femoris (RF), long head of bicep femoris (BF), sartorius (SA), gluteus maximus (GMax), gluteus medius (GMed), and adductor maximus (AM). A standard skin preparation procedure included shaving, cleaning and slightly abraded with alcohol wipes before adhering the EMG electrodes. The sensors were applied on the skin with double-sided tapes (Trigno Sensor Adhesive Interface, Delsys, Boston, MA) with medical tapes to enhance fixation. Then a set of 39 retroreflective markers were attached to the bony landmarks of the head, torso, left and right upper limbs, pelvis and left and right lower limbs.\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e All placements were conducted by the same examiner.\u003c/p\u003e \u003cp\u003eThe subject was then instructed to stand with two feet wearing shoes and shoulder-width apart on the middle of platform, and hold a light rod at waist level and close to the trunk to keep the arms from blocking the reflective markers. The subject was told to stand naturally and look forward at the beginning, try the best to maintain balance if feeling the perturbation, and then return to the original foot position marked by the dark-colored tapes as quickly as possible if they have moved the foot. A safety harness system (PG-360, Physio Gait Dynamic Unweighting System, Healthcare International Ltd., Langley, WA, USA) was equipped on each subject as a safety measure during the perturbation.\u003c/p\u003e \u003cp\u003eEach subject then experienced four trials (each consisted of 12 random perturbations) covering a total of 48 unexpected balance perturbations (4 directions \u0026times; 4 intensities \u0026times; 3 repetitions), with 5 minutes of rest after each trial. The platform moved horizontally in a pre-determined direction and intensity first, then remained stationary for 12 seconds, and was finally pulled back to its original position. The triggering time, directions (anterior, posterior, medial, lateral), and intensities (highest, high, low, lowest) were randomized and blinded to the subject. Based on the human\u0026rsquo;s limits of stability in different directions and our pilot study results in young adults,\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e the highest intensity for the anterior, posterior, medial, and lateral directions corresponded to the platform\u0026rsquo;s moving distances of 2.67%, 4.00%, 5.33% and 5.33% of each subject\u0026rsquo;s height, respectively. Videos were recorded in real time during all perturbation trials to enable further observation and analysis of the balance control strategies manually.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Data Processing\u003c/h2\u003e \u003cp\u003eThe kinematic data including the whole body\u0026rsquo;s COM, the hip, knee, and ankle joint motions were first processed using the Plug-in Gait full body model. Then the kinematic data and raw EMG data were further processed as below in a custom MATLAB program (MATLAB, The MathWorks, Inc., Natick, Ma, USA). The kinematic data were subtracted by the mean signal value of the 1000-ms baseline interval before the start of each perturbation for normalization. To obtain the COM displacement relative to the base of support (BOS), the COM displacement was further subtracted by the displacement of the moving platform.\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e The raw EMG signals were zeroed to the mean value of the entire perturbation trial, full-wave rectified, and low-pass filtered at 4 Hz with a bi-directional 4th order Butterworth filer to obtain the envelope, then further divided by the mean signal value of the 1000-ms baseline interval before the start of perturbation trial for normalization.\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eTemporal parameters including the onset latency, time to peak, and burst duration, together with the amplitude parameters including the peak amplitude, rate of rise, and/or agonist-antagonist CCI were analyzed through a custom MATLAB algorithm (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Within 2 seconds after the start of each perturbation, the onset was detected as the first point in time when the corresponding signal value exceeded five times of the standard deviation (SD) over the mean baseline value (mean\u0026thinsp;+\u0026thinsp;5 SD), and the peak was identified as the point after the onset with the maximum signal value.\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e Within 9 seconds after the start of each perturbation, the offset was identified as the first point in time after the onset when the corresponding signal value dropped below five times the standard deviation over the mean baseline value (mean\u0026thinsp;+\u0026thinsp;5 SD).\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e The baseline for the onset or offset detection was the 1000-ms interval of a signal before the start of each perturbation. The onset latency indicated the time delay from the start of perturbation to the signal onset, the time to peak indicated that from the start of perturbation to the signal peak, and the burst duration indicated that from the signal onset to offset. The rate of rise was determined as the gradient of the signal rise within a 50-ms period following the onset.\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e The agonist-antagonist CCI within the duration from two muscles\u0026rsquo; later EMG onset to two muscles\u0026rsquo; earlier EMG offset was calculated based on the formula in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e,\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e For each parameter, the mean value of the three perturbations with the same direction and intensity was used in further statistical analyses.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e2.5 Statistical Analyses\u003c/h2\u003e \u003cp\u003eThe statistical analyses were performed using SPSS (version 25.0) with the significance level set as 0.05. To examine the difference of baseline subjective assessment data between fallers and non-fallers, the independent sample t tests or Mann-Whitney U tests were used based on the data normality for continuous variables, and Chi-square tests were used for categorical variables. For each parameter (i.e., onset latency, time to peak, peak amplitude, burst duration, peak amplitude, rate of rise, and/or agonist-antagonist CCI), a four-way analysis of variance (ANOVA) and post hoc pairwise comparisons with Bonferroni corrections were conducted to examine the effects of two fall histories, four perturbation directions, four perturbation intensities, and six postural sway directions/eight dominant-leg joint motions/eight dominant-leg muscles/four dominant-leg muscle pairs. When the onset of a signal was absent, the onset latency, time to peak, burst duration were filled with 2000 ms, 2000 ms, and 0 ms, respectively; while the peak amplitude, rate of rise, and agonis-antagonist CCI were all filled with 0. With samples of equal size, the ANOVAs were considered robust even when the assumptions of normality and homogeneity were not fully met \u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Subjective Assessment Results\u003c/h2\u003e \u003cp\u003eNo adverse incident happened during all the experiments. There was no significant difference in the number of medications, age, body mass, height, foot length, BMI, short FES-I score or the PASE-C score between the participated older fallers and older non-fallers (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Nevertheless, the Mini-BEST score of fallers was significantly lower than that of non-fallers (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSubjective assessment results (categorical variable: ratio; continuous variable: mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD) of twelve subjects.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFaller (n\u0026thinsp;=\u0026thinsp;6, 3 male \u0026amp; 3 female)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNon-faller (n\u0026thinsp;=\u0026thinsp;6, 3 male \u0026amp; 3 female)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSignificance (\u003cem\u003ep\u003c/em\u003e value)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNumber of falls\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e/\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNumber of medications\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.0\u0026thinsp;\u0026plusmn;\u0026thinsp;1.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.207\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAge (year)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e71.5\u0026thinsp;\u0026plusmn;\u0026thinsp;4.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e69.2\u0026thinsp;\u0026plusmn;\u0026thinsp;2.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.316\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBody mass (kg)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e55.6\u0026thinsp;\u0026plusmn;\u0026thinsp;8.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e61.4\u0026thinsp;\u0026plusmn;\u0026thinsp;13.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.381\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHeight (cm)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e157.9\u0026thinsp;\u0026plusmn;\u0026thinsp;8.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e162.0\u0026thinsp;\u0026plusmn;\u0026thinsp;7.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.406\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBMI (kg/m\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e22.2\u0026thinsp;\u0026plusmn;\u0026thinsp;2.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e23.3\u0026thinsp;\u0026plusmn;\u0026thinsp;4.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.808\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLeg length (cm)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e77.3\u0026thinsp;\u0026plusmn;\u0026thinsp;6.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e80.8\u0026thinsp;\u0026plusmn;\u0026thinsp;4.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.587\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDominant leg (right/left)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5/1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e6/0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.296\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eShort FES-I (score)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e12.2\u0026thinsp;\u0026plusmn;\u0026thinsp;2.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e11.5\u0026thinsp;\u0026plusmn;\u0026thinsp;6.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.808\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePASE-C (score)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e139.5\u0026thinsp;\u0026plusmn;\u0026thinsp;73.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e148.1\u0026thinsp;\u0026plusmn;\u0026thinsp;34.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.802\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMini-BEST (score)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e23.3\u0026thinsp;\u0026plusmn;\u0026thinsp;1.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e26.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e0.004\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"4\"\u003e\u003cb\u003eBMI\u003c/b\u003e: body mass index. \u003cb\u003eFES-I\u003c/b\u003e: fall efficacy scale-international. \u003cb\u003ePASE-C\u003c/b\u003e: physical activity scale of elderly-Chinese. \u003cb\u003eMini-BEST\u003c/b\u003e: mini-balance evaluation system test.\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"4\"\u003e\u003cb\u003eCOM\u003c/b\u003e: center of mass. \u003cb\u003eSD\u003c/b\u003e: standard deviation.\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"4\"\u003e\u003cb\u003eAdd\u003c/b\u003e: adduction. \u003cb\u003eAbd\u003c/b\u003e: abduction. \u003cb\u003eFlex\u003c/b\u003e: flexion. \u003cb\u003eExt\u003c/b\u003e: extension. \u003cb\u003eDorsi\u003c/b\u003e: dorsiflexion. \u003cb\u003ePlantar\u003c/b\u003e: plantarflexion. \u003cb\u003eSD\u003c/b\u003e: standard deviation.\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"4\"\u003e\u003cb\u003eEMG\u003c/b\u003e: electromyographic. \u003cb\u003eCCI\u003c/b\u003e: co-contraction index. \u003cb\u003eSD\u003c/b\u003e: standard deviation. \u003cb\u003eAdd\u003c/b\u003e: adductor. \u003cb\u003eAbd\u003c/b\u003e: abductor. \u003cb\u003eFlex\u003c/b\u003e: flexor. \u003cb\u003eExt\u003c/b\u003e: extensor. \u003cb\u003eDorsi\u003c/b\u003e: dorsiflexor. \u003cb\u003ePlantar\u003c/b\u003e: plantarflexor.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Balance Control Strategies\u003c/h2\u003e \u003cp\u003eFallers were more likely to have stepping responses than non-fallers. The unexpected translational perturbations mainly induced the feet-in-place strategies (567/576, 98.4%), and three subjects (3/12, 25.0%) had stepping responses following nine perturbations (9/576, 1.6%). Specifically, following three highest-intensity medial perturbations, one non-faller had the responses of the non-dominant leg including stepping toward the perturbation direction, performing leg abduction, and elevating the leg (3/576, 0.5%). One faller took a backward step using the non-dominant leg together with several small steps following a highest-intensity anterior perturbation (1/576, 0.2%). The other faller stepped backward using the non-dominant leg following the highest-intensity (2/576, 0.3%) and high-intensity (1/576, 0.2%) anterior perturbations. Additionally, this individual stepped toward the perturbation direction using both legs in response to a low-intensity posterior perturbation (1/576, 0.2%), and with the non-dominant leg in response to a highest-intensity medial perturbation (1/576, 0.2%).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e3.3 COM Displacements\u003c/h2\u003e \u003cp\u003eThe mean changes in COM displacements over time (n\u0026thinsp;=\u0026thinsp;12, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) together with the onset latency, time to peak, peak amplitude, and burst duration of COM displacement (mean \u0026plusmn; SD, n\u0026thinsp;=\u0026thinsp;12, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) are displayed for each postural sway direction, each perturbation intensity, and each perturbation direction in participated older fallers and older non-fallers.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFour-way ANOVAs showed significant interaction effects of fall history and other factors on the onset latency (fall history \u0026times; postural sway direction, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05), time to peak (fall history \u0026times; postural sway direction, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05), and peak amplitude (fall history \u0026times; direction \u0026times; postural sway direction, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05) of COM displacement.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e illustrates the significant differences between older fallers and older non-fallers. Compared to non-fallers, the fallers\u0026rsquo; onset latency of COM displacement was significantly longer in the backward direction, but significantly shorter in the forward and downward directions (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05); the fallers\u0026rsquo; time to peak COM displacement was significantly longer in the backward direction, but significantly shorter in the downward direction (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05); the fallers\u0026rsquo; peak COM displacement was significantly larger in the forward and downward directions following anterior perturbations, in the backward direction following posterior perturbation, and in the forward direction following both the medial and lateral perturbations (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05). Significant pairwise comparisons within the factor that had interaction with \u0026ldquo;fall history\u0026rdquo; are summarized in \u003cb\u003eAppendix 2\u003c/b\u003e (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Dominant-leg Joint Motions\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe mean changes of dominant-leg joint motions over time (n\u0026thinsp;=\u0026thinsp;12, Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) together with the angle onset latency, time to peak angle, peak angle, and angle burst duration (mean \u0026plusmn; SD, n\u0026thinsp;=\u0026thinsp;12, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e) are displayed for each joint motion, each perturbation intensity, and each perturbation direction in fallers and non-fallers.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFour-way ANOVAs showed significant interaction effects of fall history and other factors on the angle onset latency (fall history \u0026times; direction \u0026times; joint motion, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05), time to peak angle (fall history \u0026times; direction \u0026times; joint motion, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05), peak angle (fall history \u0026times; joint motion, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05), and angle burst duration (fall history \u0026times; joint motion, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05).\u003c/p\u003e \u003cp\u003eSignificant differences between fallers and non-fallers are indicated in the Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. Compared to non-fallers, the fallers\u0026rsquo; angle onset latency was significantly longer in the hip adduction, hip extension, and knee extension following anterior perturbations, and in the ankle plantarflexion following medial perturbations (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05); the fallers\u0026rsquo; time to peak angle was significantly longer in the hip adduction, hip flexion, hip extension, and knee extension following anterior perturbations as well as in the ankle plantarflexion following medial perturbations, but was significantly shorter in the ankle plantarflexion following lateral perturbations (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05); the fallers\u0026rsquo; peak angle was significantly larger in the hip flexion and knee flexion (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05); the fallers\u0026rsquo; angle burst duration was significantly longer in the ankle dorsiflexion. (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05). Significant pairwise comparisons within the factor that had interaction with \u0026ldquo;fall history\u0026rdquo; are shown in \u003cb\u003eAppendix 3\u003c/b\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e3.5 EMG Signals of Dominant-leg Muscles\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe mean changes of EMG signals over time (n\u0026thinsp;=\u0026thinsp;12, Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e) together with the EMG onset latency, rate of EMG rise, time to peak EMG amplitude, EMG burst duration, and agonist-antagonist CCI (mean \u0026plusmn; SD, n\u0026thinsp;=\u0026thinsp;12, Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e) are presented for each dominant-leg muscle (pair), each perturbation intensity, and each perturbation direction in fallers and non-fallers.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFour-way ANOVAs showed significant interaction effects of fall history and other factors on the EMG onset latency (fall history \u0026times; muscle, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05), rate of EMG rise (fall history \u0026times; muscle, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05), and EMG burst duration (fall history \u0026times; muscle, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05; fall history \u0026times; direction, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05). The main effect of fall history was observed on the time to peak EMG amplitude (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05) and the agonist-antagonist CCI (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05).\u003c/p\u003e \u003cp\u003eSignificant differences between fallers and non-fallers are indicated in the Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e. Compared to non-fallers, the fallers\u0026rsquo; EMG onset latency was significantly longer for the hip flexor and hip extensor (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05); the fallers\u0026rsquo; rate of EMG rise was significantly smaller for the ankle dorsiflexor but was significantly larger for the knee flexor (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05); the fallers\u0026rsquo; time to peak EMG amplitude was significantly longer (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05); the fallers\u0026rsquo; EMG burst duration was significantly longer for the hip abductor and ankle dorsiflexor, but was significantly shorter for the hip flexor (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05); the fallers\u0026rsquo; EMG burst duration was also significantly longer following the anterior and posterior perturbations, but was significantly shorter following the medial perturbations (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05); the fallers\u0026rsquo; agonist-antagonist CCIs were significantly smaller in the investigated muscle pairs (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05). Significant pairwise comparisons within the factor that had interaction with \u0026ldquo;fall history\u0026rdquo; are shown in \u003cb\u003eAppendix 4\u003c/b\u003e.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Discussion and Implications","content":"\u003cp\u003eThis study aimed to examine the effects of fall history on reactive standing balance in community-dwelling older adults. It is innovative in depicting the underlying neuromuscular and joint kinematic mechanisms of falls, by removing the confounding factor of age and focusing on the temporal and amplitude responses of dominant-leg muscle activities and joint motions, following the unexpected translational balance perturbations with randomly different directions and intensities. Partially in line with our hypotheses, the effects of \u0026ldquo;fall history\u0026rdquo; on the investigated outcomes during reactive balance control have interacted with the \u0026ldquo;muscle/joint motion/postural sway direction\u0026rdquo; and \u0026ldquo;perturbation direction\u0026rdquo;, but not with the \u0026ldquo;perturbation intensity\u0026rdquo;. Specifically, compared to older non-fallers, older fallers have shown slowed activation of ankle/hip muscles while tending to use suspensory strategy for reactive balance control, as supported by a series of neuromuscular alterations and joint kinematics. These new insights underlying older fallers\u0026rsquo; reactive balance control have not only indicated the possible reasons of their declined balance capability and higher risk of falls, but also indicated their utilization of prolonged and enlarged (and even overreacted) compensatory strategies for preserving postural stability and preventing falls.\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e Developing some future assessment tools based on the identified parameter may be helpful to screen and identify the fallers from non-faller in the community-dwelling adults. Furthermore, some interventions targeting these identified alterations (that are directly related to falls) may also lead to more effective and targeted solutions for improving reactive balance control and preventing recurrent falls in older fallers. Details are discussed below.\u003c/p\u003e \u003cdiv id=\"Sec17\" class=\"Section2\"\u003e \u003ch2\u003e4.1 Fallers Tended to Use Suspensory Strategies Following Unexpected Perturbations: Neuromuscular and Kinematic Mechanisms\u003c/h2\u003e \u003cp\u003eThe primary finding of this study was that fallers have tended to use the suspensory strategy to maintain standing balance following the unexpected translational perturbations as compared to non-fallers. This strategy has enabled fallers to promptly compensate for their insufficient initiation of ankle and hip strategies, but it has led to their prolonged and overacted balance recovery.\u003c/p\u003e \u003cp\u003eFallers have exhibited a decreased speed in response to an unexpected threat of losing standing balance, as indicated by their decreased activation rate of ankle dorsiflexor and the delayed EMG onset timing of hip flexor/extensor compared to non-fallers. This could be attributed to the potential degradation in any components along the sensorimotor pathway, including sensory input (feedback from external perturbation), central organization, and motor output.\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e Ankle dorsiflexor\u0026rsquo;s activation immediately following the start of perturbation has been in the first line to resist the sudden loss of balance, as this study observed its largest rate of EMG rise among the eight dominant-leg muscles following both anteroposterior and mediolateral perturbations, and our pilot studies in young adults also reported this.\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e With ageing, humans may shift from a distal-to-proximal strategy to a proximal-to-distal strategy to maintain balance, compensating for the difficulties of generating sufficient ankle torque.\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e This study further proved that such phenomenon was more pronounced in older fallers than older non-fallers. On the other hand, fallers have shown delayed EMG onset timing of hip flexor and hip extensor, along with reduced EMG burst duration of hip flexor, compared to non-fallers. These alterations could partly restrict the initiation of the hip strategy, which is the second line of defense against the sudden loss of balance. The delayed activation of hip muscles aligns with and may be explained by previous morphological observations that, fallers had reduced density of skeletal muscle fibers and increased intramuscular adipose issues in gluteus muscles compared to non-fallers.\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e A prior study also reported delayed neuromuscular activation in reactive standing balance, with fallers exhibiting later EMG onset timing of hip abductor and knee flexor in the loading leg than non-fallers following the unexpected lateral perturbations exerted on the shoulder.\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e The discrepancy in the affected muscles could be attributed to the different perturbation methods.\u003c/p\u003e \u003cp\u003eA series of kinematic and neuromuscular alterations in fallers when facing unexpected translational balance perturbations have indicated their prominent use of suspensory strategies as compared to non-fallers. In the absence of sufficient ankle and hip muscle activation, fallers have utilized the suspensory strategy, i.e., the third strategy to resist sudden loss of balance by lowering the COM to increase limit of stability and absorb the external perturbation.\u003csup\u003e\u003cspan additionalcitationids=\"CR40\" citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e as evident from their earlier onset and peak timing of downward COM displacement compared to non-fallers. The increased activation rate of knee flexor, generally decreased agonist-antagonist co-contraction of lower-limb muscles, and larger knee/hip flexion in fallers may have facilitated this strategy. Interestingly, our findings differ from a prior study that reported no differences in postural sway timing or amplitude between fallers and non-fallers following lateral shoulder-impact perturbations,\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e suggesting that different body segment perturbations may elicit distinct reactive balance control strategies. Additionally, while previous research linked greater co-contraction to more joint stability and poorer balance control,\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e,\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e this study has observed that older fallers even with poorer balance performance than non-fallers (lower Mini-BEST scores) were able to reduce agonist-antagonist co-contractions of lower-limb muscles and achieve larger knee/hip flexion for a suspensory strategy. On top of them, this study has observed fallers with the longer activation durations of ankle dorsiflexor and hip abductor together with the longer ankle dorsiflexion duration than non-fallers, which may be necessary for maintaining a knee bending posture during the suspensory strategy.\u003c/p\u003e \u003cp\u003eFallers\u0026rsquo; balance control strategies in the current study, however, have required prolonged recovery time and caused overreactions. This is evidenced by their neuromuscular and kinematic alterations as below. Firstly, fallers\u0026rsquo; delayed time to peak activation may have suggested their reduced motor unit recruitment and firing rate in response to external perturbations.\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e Secondly, fallers have shown longer time to peak hip flexion angle following anterior perturbations, longer burst durations of ankle dorsiflexion following all perturbations, and delayed timing of recovery joint motions following anterior/medial perturbations than non-fallers. Thirdly, both fallers and non-fallers have demonstrated the major postural sway that was opposite to the direction of an unexpected translational perturbation because of inertia,\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e while fallers have shown larger overshooting postural sways when recovering to initial positions following the unexpected anteroposterior perturbations, as indicated by their larger forward peak COM displacements following anterior perturbations and larger backward ones following posterior perturbations as compared to non-fallers. These findings have indicated that sudden perturbations could pose greater challenges to older fallers. Fallers\u0026rsquo; more prominent overshoots of backward postural sways, as compared to non-fallers, have also been previously reported following the anterior waist-pull perturbations.\u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e Additionally, a prior study found that fallers had more variable and delayed recovery steps than non-fallers in perturbed walking,\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e which could be corroborated by the observed fallers\u0026rsquo; larger overshooting postural sways and delayed timing of overshooting lower-limb joint motions in this study. The slowed but exaggerated postural adjustments seemed to reveal the ineffective strategies used by the older adults with fall histories for reactive balance control.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec18\" class=\"Section2\"\u003e \u003ch2\u003e4.2 Fallers Had Altered Responses to Different Perturbation Directions\u003c/h2\u003e \u003cp\u003eThe secondary finding of this study was that fall history showed interaction effects with perturbation direction, but not with perturbation intensity, on the older adults\u0026rsquo; neuromuscular and kinematic responses during reactive balance control.\u003c/p\u003e \u003cp\u003eFallers have shown distinct responses to anteroposterior and mediolateral perturbations compared to non-fallers. Regarding the kinematics, a previous study reported the larger COM path displacement in fallers compared to non-fallers following the mediolateral translational perturbations.\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e Our findings have further revealed that fallers\u0026rsquo; larger postural sways were specifically in the forward direction following the mediolateral perturbations and fallers had overshooting postural sways following the anteroposterior perturbations. These responses could be partly attributed to fallers\u0026rsquo; delayed timing of recovery joint motions compared to non-fallers following the anterior/medial perturbation. Regarding the neuromuscular responses, fallers have exhibited longer EMG burst durations in dominant-leg muscles following anteroposterior perturbations compared to non-fallers. This could also explain fallers\u0026rsquo; overshooting postural sways. Additionally, this could explain why fallers had more non-dominant leg stepping following anteroposterior perturbations than non-fallers, as more body weight was loaded on the dominant leg. Conversely, fallers have exhibited shorter EMG burst durations of dominant-leg muscles than non-fallers following medial perturbations, resulting in fallers\u0026rsquo; fewer non-dominant leg steps following medial perturbations compared to non-fallers.\u003c/p\u003e \u003cp\u003eNotably, this study has found no differences in the responses of fallers compared to non-fallers to varied intensities of unexpected perturbations. Previous studies reported the inconsistent results regarding the interaction effect of fall history and perturbation intensity on the stepping characteristics following waist-pull perturbations.\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e Our finding has further built on evidence following the unexpected translational perturbations and suggested that fallers\u0026rsquo; neuromuscular/kinematic responses to the different intensities of perturbations, which primarily induced feet-in-place strategies, have been similar to those of non-fallers.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec19\" class=\"Section2\"\u003e \u003ch2\u003e4.3 Strengths and Limitations\u003c/h2\u003e \u003cp\u003eTo the best of authors\u0026rsquo; knowledge, this is the first study that has investigated the differences in eight major lower-limb muscles\u0026rsquo; activation or lower-limb joint kinematics during reactive balance control between older fallers and older non-fallers. With the comprehensive analyses of the temporal and amplitude characteristics of these investigated signals, this study has built knowledge on the prior investigations that focused on a limited number of muscles and EMG parameters, and has addressed the gap of limited research on joint kinematics in fallers vs non-fallers. The mechanisms of fall-prone older adults\u0026rsquo; decline of reactive balance control and compensatory strategy could be better understood with the findings of this study.\u003c/p\u003e \u003cp\u003eThis study has two limitations. Firstly, given the small sample sizes of recruited older fallers and older non-fallers, the findings of this study may be susceptible to sampling errors. Nevertheless, even by using a stringent method, i.e., post hoc pairwise comparisons with Bonferroni corrections, this study successfully identified some kinematic and neuromuscular factors to differentiate the balance control strategies between fallers and non-faller, which could provide preliminary insights for the further studies with larger sample sizes. Secondly, this study did not conduct maximal voluntary contraction tests for the eight analyzed muscle groups, but used the baseline EMG signal value in unperturbed standing for EMG amplitude normalization. It is therefore important to note that the rate of EMG rise and agonist-antagonist CCI in this study reflected the extent to which the perturbation task utilized the activation required for normal standing rather than the maximal activation.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec20\" class=\"Section2\"\u003e \u003ch2\u003e4.4 Implications for Future Clinical Practice\u003c/h2\u003e \u003cp\u003eThis study could have useful implications for the assessment and training of reactive balance control in future clinical practice. Firstly, the identified kinematic or neuromuscular factors that were directly related to falls can potentially facilitate the more sensitive instrumented assessment of reactive balance control and facilitate the earlier identification of fall-prone older adults. Further longitudinal studies with larger sample size are merited to examine the diagnostic accuracies/sensitivities/specificities of these identified more in-depth factors of reactive balance control in differentiating older adults\u0026rsquo; fall risks. Secondly, the reactive balance training may need to be prescribed more for the community-dwelling older adults with fall histories in the future, considering their generally delayed peak activation of lower-limb muscles. A most recent review has reported that the perturbation-based balance training and stepping training can improve the slowed reaction time in confronting with sudden loss of balance.\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e This is promising as fallers\u0026rsquo; degradation in neuromuscular timing can be modified. Our findings further imply that more focus/efforts may need to be put on the ankle and hip muscle power training in older fallers. Although the proximal hip and knee muscles were previously reported to be more affected with aging following unexpected perturbations,\u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e the findings of this study may suggest that the training of ankle muscles should not be ignored, especially in older fallers. Additionally, this study could imply that therapists may notice whether the clients/patients have overshooting postural sways in sagittal plane during the reactive balance training. Giving feedback on this may potentially help improve the reactive balance control in older adults with fall histories to be more effective.\u003c/p\u003e \u003c/div\u003e"},{"header":"5. Conclusion","content":"\u003cp\u003eCompared to non-fallers, older fallers\u0026rsquo; kinematic and neuromuscular alterations in resisting unexpected translational perturbations could be indicators of both the decline and the compensation of reactive balance control. Fallers had a decreased activation rate in ankle dorsiflexor and delayed activation in hip flexor/extensor, thereby resorting to the suspensory strategy for quickly responding to external perturbations. The increased activation rate of knee flexor, decreased agonist-antagonist co-contraction of lower-limb muscles, enlarged knee and hip flexion, and earlier downward postural sways in fallers could be the basis of their prioritizing of suspensory strategies compared to non-fallers. However, fallers\u0026rsquo; balance control strategies required prolonged recovery time and caused overreactions. Targeted modification of these identified fall-related factors is expected to prevent falls more effectively in older fallers.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.61783439490446%\" valign=\"top\"\u003e\n \u003cp\u003e\u003cstrong\u003eAbbreviation\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"70.38216560509554%\" valign=\"top\"\u003e\n \u003cp\u003e\u003cstrong\u003eFull name\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.61783439490446%\"\u003e\n \u003cp\u003eAM\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"70.38216560509554%\"\u003e\n \u003cp\u003eAdductor Maximus\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.61783439490446%\"\u003e\n \u003cp\u003eANOVA\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"70.38216560509554%\"\u003e\n \u003cp\u003eAnalysis of Variance\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.61783439490446%\"\u003e\n \u003cp\u003eBF\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"70.38216560509554%\"\u003e\n \u003cp\u003eLong Head of Bicep Femoris\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.61783439490446%\"\u003e\n \u003cp\u003eCCI\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"70.38216560509554%\"\u003e\n \u003cp\u003eCo-Contraction Index\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.61783439490446%\"\u003e\n \u003cp\u003eCOM\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"70.38216560509554%\"\u003e\n \u003cp\u003eCenter of Mass\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.61783439490446%\"\u003e\n \u003cp\u003eEMG\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"70.38216560509554%\"\u003e\n \u003cp\u003eElectromyographic\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.61783439490446%\"\u003e\n \u003cp\u003eFES-I\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"70.38216560509554%\"\u003e\n \u003cp\u003eFalls Efficacy Scale-International\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.61783439490446%\"\u003e\n \u003cp\u003eMG\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"70.38216560509554%\"\u003e\n \u003cp\u003eMedial Gastrocnemius\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.61783439490446%\"\u003e\n \u003cp\u003eGMax\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"70.38216560509554%\"\u003e\n \u003cp\u003eGluteus Maximus\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.61783439490446%\"\u003e\n \u003cp\u003eGMed\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"70.38216560509554%\"\u003e\n \u003cp\u003eGluteus Medius\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.61783439490446%\"\u003e\n \u003cp\u003eMini-BEST\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"70.38216560509554%\"\u003e\n \u003cp\u003eMini-Balance Evaluation System Test\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.61783439490446%\"\u003e\n \u003cp\u003ePASE-C\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"70.38216560509554%\"\u003e\n \u003cp\u003eChinese Version of The Physical Activity Scale for the Elderly\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.61783439490446%\"\u003e\n \u003cp\u003eRF\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"70.38216560509554%\"\u003e\n \u003cp\u003eRectus Femoris\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.61783439490446%\"\u003e\n \u003cp\u003eSA\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"70.38216560509554%\"\u003e\n \u003cp\u003eSartorius\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.61783439490446%\"\u003e\n \u003cp\u003eSD\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"70.38216560509554%\"\u003e\n \u003cp\u003eStandard Deviation\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.61783439490446%\"\u003e\n \u003cp\u003eSENIAM\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"70.38216560509554%\"\u003e\n \u003cp\u003eSurface Electromyography for the Non-Invasive Assessment of Muscles\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.61783439490446%\"\u003e\n \u003cp\u003eTA\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"70.38216560509554%\"\u003e\n \u003cp\u003eTibialis Anterior\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e"},{"header":"Declarations","content":"\u003ch2\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e\u003c/h2\u003e\n\u003cp\u003eThis study has been approved by the Institutional Review Board, The Hong Kong Polytechnic University (Ethical reference number: HSEARS20201230002). Each participant gas signed the consent form.\u003c/p\u003e\n\u003ch2\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/h2\u003e\n\u003cp\u003eFor the participant appeared in Figure 1, consent has been obtained from him for the publication of image.\u0026nbsp;\u003c/p\u003e\n\u003ch2\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e\u003c/h2\u003e\n\u003cp\u003eThe datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e\n\u003ch2\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/h2\u003e\n\u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e\n\u003ch2\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/h2\u003e\n\u003cp\u003eThis work was partially supported by the Research Institute for Smart Ageing, The Hong Kong Polytechnic University [P0038945]; The Hong Kong Polytechnic University [P0034491]; and Associated Money, The Hong Kong Polytechnic University [G4Y56R006 to R.T.L.Z].\u003c/p\u003e\n\u003ch2\u003e\u003cstrong\u003eAuthors\u0026apos; contributions\u003c/strong\u003e\u003c/h2\u003e\n\u003cp\u003e\u003cstrong\u003eR.T.L.Z.:\u003c/strong\u003e Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Data Curation, Writing- Original Draft, Visualization; \u003cstrong\u003eT.T.M.H.:\u0026nbsp;\u003c/strong\u003eFormal analysis, Investigation, Writing- Original Draft; \u003cstrong\u003eF.M.H.L.:\u0026nbsp;\u003c/strong\u003eWriting- Review \u0026amp; Editing;\u003cstrong\u003e\u0026nbsp;J.Z.L.:\u0026nbsp;\u003c/strong\u003eInvestigation;\u003cstrong\u003e\u0026nbsp;Y.Y.L.:\u003c/strong\u003e Investigation;\u003cstrong\u003e\u0026nbsp;J.S.:\u003c/strong\u003e Writing- Review \u0026amp; Editing;\u003cstrong\u003e\u0026nbsp;S.W.:\u0026nbsp;\u003c/strong\u003eWriting- Review \u0026amp; Editing;\u003cstrong\u003e\u0026nbsp;C.Z.H.M.:\u0026nbsp;\u003c/strong\u003eConceptualization, Methodology, Resources, Writing- Review \u0026amp; Editing, Supervision, Project administration, Funding acquisition.\u003c/p\u003e\n\u003ch2\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/h2\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eWorld Health Organization. Falls. World Health Organization. Accessed. Apr., 2023, 2023. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.who.int/news-room/fact-sheets/detail/falls#\u003c/span\u003e\u003cspan address=\"https://www.who.int/news-room/fact-sheets/detail/falls#\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMoreland B, Kakara R, Henry A. Trends in nonfatal falls and fall-related injuries among adults aged\u0026thinsp;\u0026ge;\u0026thinsp;65 years\u0026mdash;United States, 2012\u0026ndash;2018. Morb Mortal Wkly Rep. 2020;69(27):875.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ede Vries OJ, Peeters GMEEG, Elders PJM, et al. Multifactorial Intervention to Reduce Falls in Older People at High Risk of Recurrent Falls: A Randomized Controlled Trial. Arch Intern Med. 2010;170(13):1110\u0026ndash;7. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1001/archinternmed.2010.169\u003c/span\u003e\u003cspan address=\"10.1001/archinternmed.2010.169\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKasahara S, Saito H. Mechanisms of postural control in older adults based on surface electromyography data. \u003cem\u003eHuman Movement Science\u003c/em\u003e. 2021/\u003cdiv class=\"ExternalRefDOI\"\u003e08/01/\u003c/div\u003e 2021;78:102803. doi:https://doi.org/10.1016/j.humov.2021.102803.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTong CY, Zhu RT, Ling YT, et al. Muscular and Kinematic Responses to Unexpected Translational Balance Perturbation: A Pilot Study in Healthy Young Adults. Bioengineering. 2023;10(7). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.3390/bioengineering10070831\u003c/span\u003e\u003cspan address=\"10.3390/bioengineering10070831\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eOsoba MY, Rao AK, Agrawal SK, Lalwani AK. Balance and gait in the elderly: A contemporary review. Laryngoscope Investig Otolaryngol Feb. 2019;4(1):143\u0026ndash;53. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1002/lio2.252\u003c/span\u003e\u003cspan address=\"10.1002/lio2.252\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAllum JH, Carpenter MG, Honegger F, Adkin AL, Bloem BR. Age-dependent variations in the directional sensitivity of balance corrections and compensatory arm movements in man. J Physiol Jul. 2002;15(Pt 2):643\u0026ndash;63. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1113/jphysiol.2001.015644\u003c/span\u003e\u003cspan address=\"10.1113/jphysiol.2001.015644\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMa CZ-H, Lee WC-C. A wearable vibrotactile biofeedback system improves balance control of healthy young adults following perturbations from quiet stance. \u003cem\u003eHuman Movement Science\u003c/em\u003e. 2017\u003cdiv class=\"ExternalRefDOI\"\u003e/10/01/\u003c/div\u003e 2017;55:54\u0026ndash;60. doi:https://doi.org/10.1016/j.humov.2017.07.006.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBatcir S, Shani G, Shapiro A, Alexander N, Melzer I. The kinematics and strategies of recovery steps during lateral losses of balance in standing at different perturbation magnitudes in older adults with varying history of falls. BMC Geriatr Jul. 2020;20(1):249. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1186/s12877-020-01650-4\u003c/span\u003e\u003cspan address=\"10.1186/s12877-020-01650-4\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBair W-N, Prettyman MG, Beamer BA, Rogers MW. Kinematic and behavioral analyses of protective stepping strategies and risk for falls among community living older adults. Clin Biomech. 2016;36:74\u0026ndash;82.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGerards MH, Meijer K, Karamanidis K, et al. Adaptability to balance perturbations during walking as a potential marker of falls history in older adults. Front sports Act living. 2021;3:132.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSturnieks DL, Menant J, Delbaere K, et al. Force-Controlled Balance Perturbations Associated with Falls in Older People: A Prospective Cohort Study. Article. PLoS ONE. 2013;8(8):e70981. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1371/journal.pone.0070981\u003c/span\u003e\u003cspan address=\"10.1371/journal.pone.0070981\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMaki BE, Holliday PJ, Topper AK. A Prospective Study of Postural Balance and Risk of Falling in An Ambulatory and Independent Elderly Population. J Gerontol. 1994;49(2):M72\u0026ndash;84. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1093/geronj/49.2.M72\u003c/span\u003e\u003cspan address=\"10.1093/geronj/49.2.M72\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTantisuwat A, Vachalathiti R, Hiengkaew V, Assantachai P. Stepping Responses during Forward and Backward Fall Recovery between Thai Elderly Fallers and Non-fallers. J Phys THERAPY Sci JUN. 2011;23(3):373\u0026ndash;9. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1589/jpts.23.373\u003c/span\u003e\u003cspan address=\"10.1589/jpts.23.373\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e\u003cem\u003eJournals of Gerontology Series A: Biomedical Sciences and Medical Sciences\u003c/em\u003e. 2013;68(12):1540\u0026ndash;1548.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eThompson JD, Plummer P, Franz JR. Age and falls history effects on antagonist leg muscle coactivation during walking with balance perturbations. Clin Biomech (Bristol Avon) Nov. 2018;59:94\u0026ndash;100. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/j.clinbiomech.2018.09.011\u003c/span\u003e\u003cspan address=\"10.1016/j.clinbiomech.2018.09.011\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eStudenski S, Chandler J. Postural Responses and Effector Factors in Persons with Unexplained Falls: Results and Methodologic Issues. Article. J Am Geriatr Soc. 1991;39(3):229\u0026ndash;34. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1111/j.1532-5415.1991.tb01642.x\u003c/span\u003e\u003cspan address=\"10.1111/j.1532-5415.1991.tb01642.x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eClaudino R, Dos Santos MJ, Mazo GZ. Delayed Compensatory Postural Adjustments After Lateral Perturbations Contribute to the Reduced Ability of Older Adults to Control Body Balance. Motor Control Oct. 2017;1(4):425\u0026ndash;42. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1123/mc.2016-0005\u003c/span\u003e\u003cspan address=\"10.1123/mc.2016-0005\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLin S-I, Woollacott MH. Postural muscle responses following changing balance threats in young, stable older, and unstable older adults. J Mot Behav. 2002;34(1):37\u0026ndash;44.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhu RT, Lyu PZ, Li S, Tong CY, Ling YT, Ma CZ. How Does Lower Limb Respond to Unexpected Balance Perturbations? New Insights from Synchronized Human Kinetics, Kinematics, Muscle Electromyography (EMG) and Mechanomyography (MMG) Data. Biosens (Basel) Jun. 2022;18(6). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.3390/bios12060430\u003c/span\u003e\u003cspan address=\"10.3390/bios12060430\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRogers MW, Hedman LD, Johnson ME, Cain TD, Hanke TA. Lateral Stability During Forward-Induced Stepping for Dynamic Balance Recovery in Young and Older Adults. Journals Gerontology: Ser A. 2001;56(9):M589\u0026ndash;94. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1093/gerona/56.9.M589\u003c/span\u003e\u003cspan address=\"10.1093/gerona/56.9.M589\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMaki BE, Holliday PJ, Topper AK. A prospective study of postural balance and risk of falling in an ambulatory and independent elderly population. J Gerontol Mar. 1994;49(2):M72\u0026ndash;84. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1093/geronj/49.2.m72\u003c/span\u003e\u003cspan address=\"10.1093/geronj/49.2.m72\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBatcir S, Shani G, Shapiro A, Alexander N, Melzer I. The kinematics and strategies of recovery steps during lateral losses of balance in standing at different perturbation magnitudes in older adults with varying history of falls. BMC Geriatr JUL. 2020;20(1):249. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1186/s12877-020-01650-4\u003c/span\u003e\u003cspan address=\"10.1186/s12877-020-01650-4\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eFujimoto M, Bair WN, Rogers MW. Center of pressure control for balance maintenance during lateral waist-pull perturbations in older adults. J Biomech APR. 2015;13(6):963\u0026ndash;8. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/j.jbiomech.2015.02.012\u003c/span\u003e\u003cspan address=\"10.1016/j.jbiomech.2015.02.012\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ede Freitas PB, Knight CA, Barela JA. Postural reactions following forward platform perturbation in young, middle-age, and old adults. \u003cem\u003eJournal of electromyography and kinesiology\u003c/em\u003e. 2010\u003cdiv class=\"ExternalRefDOI\"\u003e/08/01/\u003c/div\u003e 2010;20(4):693\u0026ndash;700. doi:https://doi.org/10.1016/j.jelekin.2009.11.009.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTinetti ME, Speechley M, Ginter SF. Risk factors for falls among elderly persons living in the community. N Engl J Med Dec. 1988;29(26):1701\u0026ndash;7. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1056/nejm198812293192604\u003c/span\u003e\u003cspan address=\"10.1056/nejm198812293192604\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYardley L, Beyer N, Hauer K, Kempen G, Piot-Ziegler C, Todd C. Development and initial validation of the Falls Efficacy Scale-International (FES-I). \u003cem\u003eAge and Ageing\u003c/em\u003e. 2005;34(6):614\u0026ndash;619. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1093/ageing/\u003c/span\u003e\u003cspan address=\"10.1093/ageing/\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003eafi196%J Age and Ageing.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKu P-W, Sun W, Chen L-J. Reliability and Validity of the Chinese Version of the Physical Activity Scale for the Elderly. Sport Exerc Res. 2013;10/01:15:309\u0026ndash;19.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKing L, Horak F. On the Mini-BESTest: Scoring and the Reporting of Total Scores. Phys Ther. 2013;93(4):571\u0026ndash;5. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.2522/ptj.2013.93.4.571\u003c/span\u003e\u003cspan address=\"10.2522/ptj.2013.93.4.571\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHermens HJ, Freriks B, Disselhorst-Klug C, Rau G. Development of recommendations for SEMG sensors and sensor placement procedures. \u003cem\u003eJournal of Electromyography and Kinesiology\u003c/em\u003e. 2000\u003cdiv class=\"ExternalRefDOI\"\u003e/10/01\u003c/div\u003e/ 2000;10(5):361\u0026ndash;374. doi:https://doi.org/10.1016/S1050-6411(00)00027-4.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eVicon Motion Systems Limited. Plug-in Gait Reference Guide. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://docs.vicon.com/display/Nexus212/Plug-in+Gait+Reference+Guide\u003c/span\u003e\u003cspan address=\"https://docs.vicon.com/display/Nexus212/Plug-in+Gait+Reference+Guide\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLing YT, Ma CZ, Shea QT, Zheng Y-P. Sonomechanomyography (SMMG): Mapping of Skeletal Muscle Motion Onset during Contraction Using Ultrafast Ultrasound Imaging and Multiple Motion Sensors. Sensors. 2020;20(19). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.3390/s20195513\u003c/span\u003e\u003cspan address=\"10.3390/s20195513\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHesam-Shariati N, Trinh T, Thompson-Butel AG, Shiner CT, McNulty PA. A longitudinal electromyography study of complex movements in Poststroke therapy. 1: heterogeneous changes despite consistent improvements in clinical assessments. Front Neurol. 2017;8:340.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDi Nardo F, Morano M, Strazza A, Fioretti S. Muscle Co-Contraction Detection in the Time-Frequency Domain. Sens (Basel) Jun. 2022;28(13). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.3390/s22134886\u003c/span\u003e\u003cspan address=\"10.3390/s22134886\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eFalconer K. Quantitative assessment of cocontraction at the ankle joint in walking. Electromyogr Clin Neurophysiol. 1985;25:135\u0026ndash;48.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePortney LG. Foundations of clinical research: applications to evidence-based practice. FA Davis; 2020.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWoollacott MH. 8 Age-Related Changes in Posture and Movement. J Gerontol. 1993;48(SpecialIssue):56\u0026ndash;60. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1093/geronj/48.Special_Issue.56\u003c/span\u003e\u003cspan address=\"10.1093/geronj/48.Special_Issue.56\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eInacio M, Ryan AS, Bair W-N, Prettyman M, Beamer BA, Rogers MW. Gluteal muscle composition differentiates fallers from non-fallers in community dwelling older adults. \u003cem\u003eBMC Geriatr\u003c/em\u003e. 2014/03/25 2014;14(1):37. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1186/1471-2318-14-37\u003c/span\u003e\u003cspan address=\"10.1186/1471-2318-14-37\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eNijhuis LBO, Bloem BR, Carpenter MG, Allum JHJ. Incorporating voluntary knee flexion into nonanticipatory balance corrections. J Neurophysiol Nov. 2007;98(5):3047\u0026ndash;59. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1152/jn.01303.2006\u003c/span\u003e\u003cspan address=\"10.1152/jn.01303.2006\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCheng KB. Does knee motion contribute to feet-in-place balance recovery? \u003cem\u003eJ Biomech\u003c/em\u003e. 2016/06/14/ 2016;49(9):1873\u0026ndash;1880. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jbiomech.2016.04.026\u003c/span\u003e\u003cspan address=\"10.1016/j.jbiomech.2016.04.026\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKasahara S, Saito H, Anjiki T, Osanai H. The effect of aging on vertical postural control during the forward and backward shift of the center of pressure. Gait Posture. 2015;42(4):448\u0026ndash;54.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSchulz BW, Jongprasithporn M, Hart-Hughes SJ, Bulat T. Effects of step length, age, and fall history on hip and knee kinetics and knee co-contraction during the maximum step length test. Article. Clin Biomech. 2013;28(8):933\u0026ndash;40. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/j.clinbiomech.2013.08.002\u003c/span\u003e\u003cspan address=\"10.1016/j.clinbiomech.2013.08.002\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eFalk J, Strandkvist V, Pauelsen M, Vikman I, Nyberg L, R\u0026ouml;ijezon U. Increased co-contraction reaction during a surface perturbation is associated with unsuccessful postural control among older adults. \u003cem\u003eBMC Geriatr\u003c/em\u003e. 2022/05/19 2022;22(1):438. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1186/s12877-022-03123-2\u003c/span\u003e\u003cspan address=\"10.1186/s12877-022-03123-2\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHo CY, Bendrups AP. Ankle Reflex Stiffness During Unperceived Perturbation of Standing in Elderly Subjects. Journals Gerontology: Ser A. 2002;57(9):B344\u0026ndash;50. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1093/gerona/57.9.B344\u003c/span\u003e\u003cspan address=\"10.1093/gerona/57.9.B344\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBhagwat AP, Deodhe NP. The Effect of Perturbation-Based Balance Training vs Step Training on Reaction Time in Older Persons: A Review. Cureus. 2023;15(11):e48104. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.7759/cureus.48104\u003c/span\u003e\u003cspan address=\"10.7759/cureus.48104\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e. /11/01 2023.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHall CD, Jensen JL. Age-related differences in lower extremity power after support surface perturbations. J Am Geriatr Soc. 2002;50(11):1782\u0026ndash;8.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Community-dwelling, electromyographic (EMG), neuromuscular, kinematics, postural sways, reactive balance, falls, older adults, muscle activation.","lastPublishedDoi":"10.21203/rs.3.rs-4422750/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4422750/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e\u003cstrong\u003eBackground\u003c/strong\u003e: Falls and fall consequences in older adults are global health issues. Previous studies have compared postural sways or stepping strategies between older adults with and without fall histories, to identify the associated factors of falls. However, more in-depth neuromuscular/kinematic mechanisms have remained unclear. This study therefore aimed to comprehensively investigate and compare the muscle activities and joint kinematics during reactive balance control in older adults with different fall histories.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMethods\u003c/strong\u003e: This pilot observational study recruited six community-dwelling older fallers (≥1 fall in past one year) and six non-fallers, who received unexpected translational balance perturbations in randomized directions and intensities during natural standing. The whole-body center-of-mass (COM) displacements, eight dominant-leg joint motions and muscle electrical activities were collected, and analyzed using the temporal and amplitude parameters. Four-way ANOVA and post hoc analyses were conducted to examine the effects of fall history, perturbation direction, perturbation intensity, and postural sway/joint/muscle on each parameter.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eResults\u003c/strong\u003e: Post hoc analyses revealed that compared to older non-fallers, fallers had significantly: (a) smaller activation rate in ankle dorsiflexor, delayed activation in hip flexor/extensor, larger activation rate in knee flexor, and smaller agonist-antagonist co-contraction in lower-limb muscles; (b) larger knee/hip flexion angles, longer ankle dorsiflexion duration, and delayed timing of recovery in joint motions; and (c) earlier downward COM displacements and larger anteroposterior overshooting COM displacements following unexpected perturbations (\u003cem\u003ep\u003c/em\u003e\u0026lt; 0.05).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConclusion and Implication\u003c/strong\u003e: Compared to older non-fallers, fallers tended to use more suspensory strategies to maintain reactive standing balance. Such strategies could enable older fallers to compensate for their inadequate initiation of ankle/hip strategies, but led to prolonged and overacted balance recovery among them. This study’s comprehensive neuromuscular/kinematic analyses and controlled balance perturbation preliminarily uncovered some specific declines and ineffective strategies in fall-prone older adults during reactive balance control, which can potentially enhance the instrumented assessments for early identification of fall-prone older adults and facilitate the targeted training to prevent their falls. Further longitudinal studies are still needed to examine diagnostic accuracies of these identified neuromuscular/kinematic factors in differentiating fall risks of older people.\u003c/p\u003e","manuscriptTitle":"Older Fallers and Non-fallers’ Neuromuscular and Kinematic Alterations in Reactive Balance Control: Indicators of Balance Decline or Compensation?","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-05-30 07:11:55","doi":"10.21203/rs.3.rs-4422750/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"72e40b30-7b72-4fbb-a024-b4b3589bfc30","owner":[],"postedDate":"May 30th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-10-08T15:23:58+00:00","versionOfRecord":[],"versionCreatedAt":"2024-05-30 07:11:55","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4422750","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4422750","identity":"rs-4422750","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}