Neurophysiological effects of partial gravity on bimanual control: A parabolic flight study

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Kennedy, Osmar P. Neto, Madison Weinrich, Renee Abbott, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6131347/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 16 Apr, 2026 Read the published version in npj Microgravity → Version 1 posted 10 You are reading this latest preprint version Abstract Many of the activities associated with spaceflight require individuals to use both limbs simultaneously to accomplish the task. Motor control, as well as visual performance and spatial orientation are disrupted by gravitational transitions between 1G and 0G, but very little is known about the sensorimotor deficits between 0G and 1G. The objective of this analog-based research effort is to investigate the impact of partial G-levels on bimanual coordination tasks that are operationally relevant for spaceflight. Using parabolic flight to simulate different gravity levels (0g, 0.25g, 0.5g, 0.75g, 1g), participants performed isometric force coordination tasks while electromyography (EMG) data were collected to assess neuromuscular activity. Results showed that at lower gravity levels, force production was significantly reduced, became less harmonic, and more variable, particularly during complex tasks, indicating reduced coordination stability. Additionally, EMG-EMG cross-wavelet power analysis revealed significantly lower beta-band (13–30 Hz) normalized power in 0g compared to 1g, suggesting weakened neural synchronization between limbs in the absence of gravitational loading. Partial gravity conditions partially restored both force stability and neural coupling, emphasizing the role of proprioceptive feedback in motor control. These findings highlight the importance of gravitational input for maintaining motor coordination and have practical implications for astronaut training, equipment design, and countermeasures to support performance during space missions. Biological sciences/Neuroscience Biological sciences/Physiology Health sciences/Health occupations Bimanual coordination microgravity altered gravity force control EMG Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction Bimanual coordination, the ability to synchronize actions between both limbs, is critical for a wide range of complex activities that are essential to space exploration. Astronauts are frequently required to perform intricate tasks that demand precise and coordinated arm movements, such as operating spacecraft controls, assembling equipment, and conducting scientific experiments. These tasks become particularly challenging in altered gravity environments, where the absence or reduction of gravitational forces can significantly disrupt motor control strategies and coordination 1 – 3 . While extensive research has been conducted on motor control under microgravity (0g) conditions revealing its profound impact on proprioceptive feedback and motor performance 4 – 9 , less is known about the effects of partial gravity levels (e.g., 0.25g, 0.5g, 0.75g) on motor control and coordination. In microgravity, the absence of gravitational feedback is known to impair the production of stable and consistent forces, particularly in tasks requiring bimanual coordination 2 , 10 . For example, experiments have shown that force production becomes less harmonic, characterized by increased adjustments and perturbations 10 , 11 . These disruptions are attributed to the lack of a gravitational reference, which is vital for maintaining spatial orientation and stability during movement 4 , 12 , 13 . Much of the research in microgravity has focused on unimanual tasks 1 , 5 , 7 , 14 – 19 , while little is known about how bimanual coordination is affected across different gravitational level. This gap is particularly significant because bimanual tasks often require more complex neural integration, involving both hemispheric communication and precise interlimb synchronization 20 – 22 . Unlike unimanual movements, bimanual coordination depends on dynamic interactions between homologous muscles and neural pathways, which may respond differently to changes in gravitational load 2 , 23 . Understanding how partial gravity levels influence these processes is critical for advancing our knowledge of motor control adaptations in space environments and for developing effective countermeasures to support astronaut performance during long-duration missions. Recent studies on motor control in altered gravity have underscored the complexity of coordinating bimanual movements in these environments 2 , 10 . Numerous constraints, such as proprioceptive feedback, neural crosstalk, and environmental factors, have been proposed to explain variations in motor performance across different gravity conditions 2 , 20 – 22 . Among these, neural crosstalk, which involves the interaction between ipsilateral and contralateral motor commands 20 , 24 , has been particularly emphasized as a critical factor in bimanual coordination tasks 24 – 29 . Understanding these constraints is crucial for advancing our knowledge of neuromuscular control under altered gravitational conditions. In altered gravity, research has consistently demonstrated that manual control is compromised compared to performance in 1g conditions 1 , 5 , 7 , 14 . Simulated altered gravity environments on Earth, including head-down tilt (HDT), centrifugation, parabolic flight, and water immersion, have been used to study these effects, revealing slower movement times and decreased accuracy in manual control tasks 11 , 16 , 19 , 30 . Despite these observations, the specific mechanisms or constraints leading to reduced performance in altered gravity are not fully understood. Evidence indicates that individuals rely more heavily on visual and tactile cues for spatial orientation and movement control in such environments 15 , 31 , 32 . Key findings also highlight that reduced gravity can significantly disrupt motor control and task execution 33 – 35 . For example, people exposed to altered gravity encounter challenges in performing motor tasks that require precision and stability 10 , 14 . Experiments in microgravity show that force production becomes less smooth, with increased adjustments and perturbations during motor activities 7 , 10 , 16 . This suggests that the lack of gravitational feedback hampers the ability to generate stable and consistent forces, which is particularly crucial for tasks requiring coordinated hand movements 2 , 8 , 9 . Studies in partial gravity environments, created using centrifuges or parabolic flights, provide further insights into how gravity affects motor control. These studies reveal that motor coordination and task performance are influenced by gravity levels, with performance typically declining and coordination challenges increasing as gravity decreases 1 , 8 , 9 , 36 . However, the impact of partial gravity specifically on bimanual coordination is still not fully understood, particularly regarding neural mechanisms and the interaction between homologous muscles. Electromyography (EMG) analysis, particularly using EMG–EMG cross-wavelet power and coherence, is a valuable tool for examining neural interactions between homologous muscles 37 – 39 . While cross-wavelet normalized power captures both the time and frequency dynamics of muscle activity, offering insights into the magnitude of shared neural input, coherence assesses the synchronization of muscle activity, providing a measure of neural coupling strength 40 – 43 . Previous studies have used EMG analyses to investigate neural crosstalk and coordination during various motor tasks 44 – 47 . Cross-wavelet analysis offers advantages over traditional Fourier-based methods by capturing time-frequency patterns and dynamic changes in neural interactions 41 – 43 . Coherence complements this by revealing frequency-specific neural synchronization, which may vary under different gravitational loads 10 , 48 – 50 . However, the application of EMG-EMG cross-wavelet normalized power and coherence in altered gravity environments is relatively underexplored, and their combined potential to reveal the neural mechanisms behind coordination deficits in space missions remains an open question. Given the critical role of bimanual coordination in space exploration and the impact of altered gravity on motor control, this study seeks to bridge the gap in understanding the effects of partial gravity levels on bimanual coordination as well as the underlying neural mechanisms. While existing research has highlighted general disruptions in motor performance under reduced gravity 6 , 8 , 19 , there is a lack of detailed investigations into the effects of varying levels of partial gravity on neural interactions and coordination patterns. The primary objective of this study is to examine the neurophysiological effects of partial gravity on bimanual control, specifically investigating the influence of varying gravitational levels on motor coordination and neural interactions between homologous muscles. Participants performed isometric force coordination tasks during parabolic flight, which provided exposure to distinct gravitational conditions (0g, 0.25g, 0.5g, 0.75g, 1g), with data collected on force production and muscle activity using force and EMG sensors, respectively. The study evaluated bimanual force production stability and harmonicity across these gravity levels. A secondary objective is to identify and characterize the neural mechanisms underlying coordination deficits observed under different gravitational loads using two EMG-EMG wavelet analyses methods: EMG–EMG cross-wavelet normalized power and EMG-EMG coherence. This dual approach aims to enhance our understanding of the neural adaptations necessary for effective motor control in altered gravity by evaluating both the relative magnitude and synchronization of neural interactions between homologous muscles in response to partial gravity. The theoretical implications of this study lie in advancing the understanding of neuromuscular control under altered gravity conditions. By revealing how partial gravity affects neural interactions and coordination dynamics, the study contributes to the broader knowledge of motor control mechanisms in space environments. Practically, the findings may provide new insights into the design of training programs and equipment for astronauts, aiming to improve operational performance and safety during space missions. Understanding these effects will be crucial for preparing astronauts for the challenges of future missions, including the planned return to the Moon and eventual human exploration of Mars. Results We investigated the effects of varying gravitational levels on bimanual coordination and neural interactions, using parabolic flight to simulate gravity conditions of 0g, 0.25g, 0.5g, 0.75g, and 1g. Participants performed two coordination tasks (1:1 and 1:2) under each condition, while electromyography (EMG) data were collected to assess muscle activity and neural coupling between homologous muscles. Task performance was evaluated using metrics such as force harmonicity, mean force, force variability, and inter-peak intervals. Additionally, EMG-EMG analyses focused on cross-wavelet normalized power and coherence, across different gravitational loads. Unimanual Analyses Harmonicity (H) The analysis of harmonicity using mixed linear models revealed significant main effects of Gravity (F(4, 205.09) = 2.63, p = 0.036), Task (F(1, 205.00) = 72.42, p < 0.001), and Limb (F(1, 205.00) = 72.87, p < 0.001). Harmonicity values were significantly lower at 0g (0.858 ± 0.209) compared to 0.75g (0.938 ± 0.159), indicating reduced force smoothness in microgravity. Harmonicity was also higher during the 1:1 task (0.959 ± 0.116) compared to the 1:2 task (0.830 ± 0.246), and greater in the right limb (0.960 ± 0.103) than the left limb (0.830 ± 0.252), consistent with all participants being right-hand dominant. A significant Gravity × Task interaction (F(4, 205.00) = 3.05, p = 0.018) indicated that harmonicity was particularly lower during the 1:2 task in microgravity. Post-hoc comparisons showed that harmonicity during the 1:2 task was significantly reduced at 0g (0.742 ± 0.245) compared to 0.25g (0.845 ± 0.261; estimate = -0.103, SE = 0.033, df = 205, p = 0.020), 0.75g (0.883 ± 0.212; estimate = -0.131, SE = 0.034, df = 205, p = 0.002), and 1g (0.865 ± 0.239; estimate = -0.124, SE = 0.033, df = 205, p = 0.003). A significant Task × Limb interaction (F(1, 205.00) = 74.32, p < 0.001) revealed that task-related differences in harmonicity were more pronounced in the left limb. Specifically, harmonicity in the left limb was significantly lower during the 1:2 task (0.700 ± 0.282) compared to the 1:1 task (0.962 ± 0.117; estimate = 0.259, SE = 0.021, df = 205, p < 0.001). Furthermore, harmonicity for the left limb during the 1:2 task was significantly lower in 0g compared to 0.25g (estimate = -0.199, SE = 0.047, df = 205, p = 0.006), 0.75g (estimate = -0.251, SE = 0.049, df = 205, p < 0.001), and 1g (estimate = -0.218, SE = 0.047, df = 205, p = 0.001), highlighting the specific vulnerability of left-limb coordination under microgravity conditions. The contrast between 0g and 0.50g was not statistically different (estimate = -0.158, SE = 0.047, df = 205, p= 0.104) (Figure 1A). Mean Force The analysis of mean force using mixed linear models revealed significant main effects for Gravity and Task, but no significant effects for Limb or any interactions. A significant main effect of Gravity was found (F (4, 205.01) = 3.08, p = 0.017), indicating that mean force differed across the various gravity conditions (Figure 1B). Specifically, post-hoc analyses revealed that the mean force at 0g (8.89 ± 3.17 N) was significantly lower than at 1g (9.58 ± 3.14 N; estimate = -0.690, SE = 0.249, df = 205, t.ratio = -2.771, p = 0.048). Additionally, the mean force at 0.25g (8.86 ± 2.86 N) was significantly lower than at 1g (estimate = -0.714, SE = 0.249, df = 205, t.ratio = -2.866, p = 0.037). A significant main effect of Task was also detected (F (1, 204.99) = 7.64, p = 0.006), with lower mean force observed during the 1:1 task (8.88 ± 2.67 N) compared to the 1:2 task (9.32 ± 3.17 N; estimate = -0.439, SE = 0.159, df = 205, t.ratio = -2.763, p = 0.006) (Figure 1B). Force Variability (SD of force and CV of force) The analysis of force variability, measured as the standard deviation (SD) of force, revealed a significant main effect of Limb but no significant effects of Gravity or Task, nor any interactions (Figure 1C). The main effect of Limb, F (1, 204.99) = 6.74, p = 0.010, indicated that force variability was higher in the left limb (6.48 ± 2.31 N) compared to the right limb (6.17 ± 2.27 N; estimate = 0.304, SE = 0.117, df = 205, p = 0.010). We also examined the coefficient of variation (CV) of force to further explore the variability in force. Similar to the SD analysis, the CV analysis revealed a significant main effect of Limb, F(1, 204.83) = 15.28, p < 0.001, with the CV of force being higher in the left limb (0.711 ± 0.088) compared to the right limb (0.666 ± 0.106; estimate = 0.044, SE = 0.011, df = 205 , p < 0.001). This finding suggests greater variability in force production for the left limb across conditions. No significant main effects of Gravity (F (4, 205.11) = 0.58, p = 0.678) or Task (F (1, 204.83) = 3.32, p = 0.070) were found, and no significant interactions were observed (p > 0.275). Although the three-way interaction was not significant, post-hoc analysis revealed a marginally significant difference in CV during the 1:2 task between the left limb at 0g and 0.5g (estimate = 0.124, SE = 0.035, df = 205, p = 0.067) (Figure 1D). Inter-peak Interval (IPI) The inter-peak interval analysis using mixed linear models revealed significant main effects for Gravity, Task, and Limb, but no significant interactions involving Gravity. A significant main effect of Gravity was observed (F(4, 205.05) = 2.82, p = 0.026), indicating that IPI values increased at lower gravity levels (Figure 2A). Post-hoc comparisons showed that IPI at 0g (1.53 ± 0.67 s) was significantly higher than at 0.75g (1.27 ± 0.67 s; estimate = 0.225, SE = 0.069, df = 205, p = 0.012). There was also a strong main effect of Task (F (1, 205.00) = 235.99, p < 0.001), with significantly lower IPI values observed during the 1:1 task (1.08 ± 0.40 s) compared to the 1:2 task (1.74 ± 0.89 s; estimate = -0.659, SE = 0.043, df = 205, t.ratio = -15.362, p < 0.001). Additionally, a significant main effect of Limb was detected, F(1, 205.00) = 170.19, p < 0.001, with IPI being higher in the left limb (1.69 ± 0.91 s) compared to the right limb (1.13 ± 0.43 s; estimate = 0.560, SE = 0.043, df = 205, p < 0.001). In addition, a significant Task x Limb interaction was identified (F(1, 205.00) = 171.56, p < 0.001). This interaction indicates that the task effect on IPI was more pronounced for the left limb, with IPI values in the left limb for the 1:1 task (1.08 ± 0.40 s) being substantially lower compared to the 1:2 task (2.30 ± 0.87 s; estimate = -1.221, SE = 0.061, df = 205, p < 0.001). For the right limb, however, IPI did not differ significantly between tasks (estimate = -0.097, SE = 0.061, df = 205, p = 0.111) (Figure 2A). Inter-peak Interval Variability The analysis of inter-peak interval variability (SD IPI) using mixed linear models showed significant main effects for Gravity, Task, and Limb, but no significant interactions involving Gravity (Figure 2B). A main effect of Gravity was detected (F (4, 204.98) = 4.48, p = 0.002), indicating that IPI variability was highest in microgravity (0g) and decreased progressively with increasing gravitational load. Post-hoc analyses revealed that IPI variability at 0g (0.198 ± 0.145) was significantly higher compared to both 0.75g (0.115 ± 0.073; estimate = 0.059, SE = 0.019, df = 205, p = 0.021) and 1g (0.139 ± 0.111; estimate = 0.059, SE = 0.019, df = 205, p = 0.017). This suggests that the absence of gravitational cues in microgravity leads to less stable timing control increasing variability in motor coordination 12,51 . There was also a significant main effect of Task (F (1, 204.90) = 69.24, p < 0.001), with lower IPI variability observed during the 1:1 task (0.108 ± 0.122) compared to the 1:2 task (0.208 ± 0.155; estimate = -0.099, SE = 0.012, df = 205, p < 0.001). The increased variability during the 1:2 task reflects the greater complexity and demands of maintaining an asymmetric coordination pattern. The main effect of Limb was also significant, F (1, 204.90) = 14.45, p < 0.001, showing higher IPI variability in the left limb (0.181 ± 0.172) compared to the right limb (0.135 ± 0.115; estimate = 0.045, SE = 0.012, df = 205, p < 0.001). This indicates that the non-dominant (left) limb exhibited less stable timing control, particularly under conditions of increased task complexity. In addition, a significant Task x Limb interaction was identified, F (1, 204.90) = 10.57, p = 0.001. This interaction demonstrated that the task effect on IPI variability was more pronounced in the left limb, with IPI variability values in the left limb in the 1:1 task (0.111 ± 0.142) being significantly lower compared to the 1:2 task (0.250 ± 0.172; estimate = -0.138, SE = 0.017, df = 205, p < 0.001). In the right limb, the difference between tasks was smaller but still significant, with lower IPI variability in the 1:1 task (0.105 ± 0.099) compared to the 1:2 task (0.165 ± 0.123; estimate = -0.061, SE = 0.017, df = 205, p < 0.001). Although both limbs showed increased variability during the 1:2 task, the effect was more pronounced in the left limb, suggesting greater difficulty in maintain stable timing with the non-dominant limb under complex coordination demands 52 . Bimanual Analysis - Performance Inter-peak Interval (IPI) Ratio The analysis of mean inter-peak interval (IPI) ratio revealed significant main effects for Gravity and Task, as well as a significant Gravity x Task interaction (Figure 3). A main effect of Gravity was detected (F (4, 96.70) = 5.15, p = 0.001), indicating that IPI ratio differed across the various gravity conditions. Post-hoc analyses showed that IPI ratio was lower at 0g (1.42 ± 0.45) compared to all other gravity levels, including 0.25g (1.50 ± 0.52; estimate = -0.077, SE = 0.021, df = 97, p = 0.003), 0.5g (1.49 ± 0.52; estimate = -0.072, SE = 0.021, df = 97, p = 0.006), 0.75g (1.49 ± 0.51; estimate = -0.068, SE = 0.021, df = 98, p = 0.015), and 1g (1.50 ± 0.51; estimate = -0.076, SE = 0.021, df = 97, p = 0.003). The lower IPI ratio at 0g suggests participants experienced greater difficulty maintaining the intended timing ratio in the absence of gravitational cues. A strong main effect of Task was also found (F (1, 96.32) = 5482.23, p < 0.001), with a markedly lower IPI ratio during the 1:1 task (0.99 ± 0.02) compared to the 1:2 task (1.97 ± 0.12; estimate = -0.972, SE = 0.013, df = 97, p < 0.001). The large difference reflects the expected coordination demands, as the 1:2 task naturally requires a ratio near 2.0, while the 1:1 task requires equal timing between the limbs (i.e., IPI ratio = 1.0). The Gravity x Task interaction was significant (F (4, 96.32) = 5.50, p < 0.001), indicating that the effect of gravity on IPI ratio was more pronounced during the 1:2 task. Post-hoc comparisons within the 1:2 task revealed that at 0g, the IPI ratio (1.85 ± 0.17) was significantly lower compared to 0.25g (2.01 ± 0.09; estimate = -0.159, SE = 0.029, df = 97, p < 0.001), 0.5g (2.00 ± 0.09; estimate = -0.152, SE = 0.029, df = 97, p < 0.001), 0.75g (1.99 ± 0.06; estimate = -0.136, SE = 0.030, df = 98, p < 0.001), and 1g (1.99 ± 0.08; estimate = -0.148, SE = 0.029, df = 97, p < 0.001) (Figure 3B). The IPI ratio at 0g deviated further than the ideal 2.0 ratio, indicating that microgravity impaired participants’ ability to maintain the target 1:2 coordination pattern. This suggests greater difficulty in sustaining rhythmic control when gravitational cues are absent 26,51 . Bimanual Analysis - EMG EMG-EMG Cross-Wavelet Normalized Power and EMG-EMG Coherence The analysis of EMG-EMG coherence, considering both interference EMG and high-pass filtered rectified EMG signals, yielded no significant gravity-related results (p > 0.05). However, these non-significant findings should be interpreted cautiously, as trends may reflect subtle neural adaptations not fully captured due to methodological constraints. The analysis of EMG-EMG cross-wavelet normalized power revealed that the main effect of Frequency was highly significant (F (4,514.23) = 923.73, p < 0.001), demonstrating marked differences in normalized power across the five frequency bands. Figure 4 shows an example of the EMG amplitude time series for the left and right limbs during the 1:2 task under microgravity (left) and 1g (right) gravity conditions, along with the corresponding EMG-EMG normalized cross-wavelet periodogram and global spectrum. Normalized cross-wavelet power highlights periods of significant coherence across the 16-64 Hz (beta and gamma bands). The colormap represents normalized cross-wavelet power, with red regions indicating stronger coupling between the left and right limb muscle activity. Additionally, neither Gravity nor Task had a significant main effect. Importantly, the Gravity x Frequency interaction approached significance (F (16,514.23) = 1.53, p = 0.083), suggesting a trend toward differential effects of gravity across frequency bands. Despite these results, the post-hoc comparisons within the 13-30 Hz frequency band revealed a significant difference between the 0g and 1g conditions (p=0.006). Thus, in the 13-30 Hz band, normalized power for the 1g gravity condition (15.075 ± 5.077) was significantly greater than for the 0g condition (10.771 ± 3.102; estimate = -4.441, SE = 1.306, df = 514.7, p = 0.006) (Figure ). These trends in neural synchronization may reflect underlying adaptations to altered gravity, although interpretations should be made cautiously given the proximity of some effects to statistical significance thresholds. Potential methodological factors, such as limitations in signal detection at lower frequencies when using interference EMG, could influence the sensitivity of these analyses to subtle changes in neural coupling 61,62 . Discussion The study investigated the impact of microgravity and partial gravity on bimanual coordination and EMG-EMG cross-wavelet power using parabolic flight. Our primary findings revealed that participants maintained effective timing of bimanual force production across all gravity conditions. However, force production was noticeably less harmonic in microgravity (0g) than at higher gravity levels, particularly during the more complex (1:2) task, indicating increased force corrections and perturbations in the 0g condition. Additionally, mean force production was significantly lower in 0g compared to 1g, suggesting that gravitational loading plays a critical role in sustaining force output. Force variability was also greater in the left limb, reflecting asymmetries in coordination stability. In terms of neuromuscular activation, EMG-EMG cross-wavelet power revealed significantly reduced beta-band (13–30 Hz) power in 0g, implying diminished interlimb neural synchronization under microgravity. These results highlight the nuanced interplay between gravity and motor control, underscoring the value of time-frequency methods (e.g., cross-wavelet power) for probing the neural mechanisms underlying bimanual coordination challenges in altered gravity environments. Gravity Level By examining gravity levels ranging from 0 g to 1 g (including 0.25g, 0.5g, 0.75g), this study provides insights into how decreasing gravity alters motor control and coordination stability. The most pronounced impairments emerged under microgravity (0 g) during the performance of the complex task (1:2), where force production became less stable and more variable. Participants exhibited greater force distortions, adjustments, and perturbations, aligning with prior work showing that microgravity disrupts the sensory feedback and proprioceptive cues critical for precise movement coordination 12 , 63 . These findings have practical relevance for astronauts, who often report difficulty maintaining fine motor control in microgravity 64 . In contrast, higher partial gravity conditions (0.75 g) facilitated more stable and harmonic force production during the complex task, mirroring studies suggesting some gravitational load approximating Earth’s conditions helps stabilize motor control 14 , 65 , 66 . As gravity increased toward 1g, participants showed improved coordination and fewer force distortions, indicating that partial gravity can mitigate some of the deficits that appear in microgravity. Force Production The differences in force production between 0 g and higher gravity levels expand on existing literature by showing that harmonicity and coordination stability degrade significantly in microgravity 5 , 11 , 67 , especially in more complex tasks. The observed increases in force corrections and distortions in the complex task further support the view that the lack of a reliable gravitational reference undermines precise motor control 12 , 51 . Without gravity, participants likely experienced disrupted proprioceptive feedback, making it more difficult to maintain smooth, rhythmic force patterns 14 . In contrast, partial gravity conditions (e.g., 0.75 g) provided a partial restoration of Earth-like proprioceptive cues, resulting in improved control compared to 0 g 12 , 13 , 68 . This suggests that even moderate gravitational loading helps stabilize motor output by enhancing sensorimotor integration. In addition to the observed differences in harmonicity, analyses of mean force and force variability (standard deviation [SD] and coefficient of variation [CV]) provide further insights into how gravity influences bimanual coordination. Mean force production increased significantly with gravity, with participants generating higher forces under 1 g compared to microgravity (0 g). Our findings suggest that the presence of gravitational loading facilitates not only stronger but also more consistent force output 3 , 4 , 14 . The likely mechanism involves enhanced proprioceptive and tactile feedback under higher gravity, which supports more effective motor unit recruitment and sustained force generation 30 , 51 . In contrast, in 0 g, the absence of gravitational load reduces the activation of stabilizing muscles, leading to lower mean force levels as participants rely more heavily on feedforward control strategies rather than feedback-driven adjustments 69 , 70 . Taken together, these results clarify how gradient changes in gravitational force systematically influence motor performance. Even moderate reintroduction of load (e.g., 0.75 g) had appreciable benefits for both force stability and consistency 14 , highlighting the critical role of gravitational forces in supporting not only the magnitude but also the precision of bimanual coordination. EMG-EMG Cross-Wavelet Power Although a standard EMG-EMG coherence analysis did not yield clear gravity-related effects, EMG-EMG cross-wavelet normalized power captured meaningful differences across frequency bands. Crucially, we found a significant reduction in beta-band (13–30 Hz) cross-wavelet power in 0g compared to 1g, suggesting reduced shared neural activity (common neural drive) and decreased synchronization between left-right muscle activity under microgravity conditions 75 , 76 . This result suggests that typical beta-band coupling, often associated with interlimb coordination and refined motor output, may rely on the presence of gravitational loading 42 , 43 . Rather than showing increased beta-band synchronization to compensate for disrupted sensory feedback, the central nervous system (CNS) instead appears to shift or reduce its reliance on these beta-band oscillatory activity, which is typically associated with interlimb coordination, motor planning, and corticospinal communication, when gravity is absent. The absence of significant gravity-related effects in EMG-EMG coherence may reflect the inherent stability of neural synchronization mechanisms under short-term gravitational changes. Coherence measures long-range neural connectivity, which may require more prolonged exposure to altered environments to exhibit measurable adaptations 5 , 17 , 34 , 77 . Additionally, methodological factors, such as the high-pass filtering inherent in wireless EMG systems, could have attenuated low-frequency components critical for detecting subtle coherence differences in the alpha (8–13 Hz) and lower beta bands (13–20 Hz) 78 – 80 . Interestingly, while coherence did not show significant modulation, cross-wavelet power analysis revealed notable gravity-related differences, particularly in the beta-band (13–30 Hz). This suggests that time-frequency methods like cross-wavelet power may be more sensitive to transient neuromuscular adaptations, capturing dynamic fluctuations in interlimb coordination that coherence metrics might overlook 41 – 43 . These findings underscore how the CNS adapts under altered gravity: in the absence of normal proprioceptive cues, the usual beta-band neural coupling may be harder to sustain, leading to more variable and less harmonic force output. Partial gravity, on the other hand, provides intermediate levels of loading that can partially restore these typical motor patterns 14 , 66 . Future studies with longer-duration microgravity exposure or alternative environments (e.g., space missions, bed rest studies) may provide further insights into how neural synchronization mechanisms adapt over time. Additionally, refining EMG processing techniques to enhance low-frequency signal fidelity could improve the sensitivity of coherence analyses in detecting subtle neuromuscular changes related to gravity. Theoretical Implications These findings contribute to our understanding of coordination dynamics under altered gravity 81 , 82 . In line with prior evidence that microgravity disrupts proprioceptive feedback 12 , 83 , our results demonstrate that altered-gravity not only affects overall movement control but also alters the specific frequency-dependent coupling patterns of interlimb EMG signals. The decreased beta-band (13–30 Hz) cross-wavelet power in 0g implies a shift or attenuation of the usual synchronous neural processes, indicating that typical corticospinal or sensorimotor integration in this band may be less effective without gravitational input 22 , 80 , 84 . Meanwhile, partial gravity appears to facilitate partial reengagement of those neural mechanisms 65 , 66 , 85 , as suggested by more stable force outputs. By applying cross-wavelet analysis, we advance beyond traditional coherence and time-domain measures 42 , 43 , 86 , offering a more detailed understanding of how neuromuscular control strategies adjust to gravitational changes. Practical implications These results carry clear implications for space missions and astronaut training. Microgravity impairs manual control 64 , thus, training protocols that incorporate partial-gravity simulations (e.g., via centrifugation) may improve astronauts’ ability to generate stable force outputs 87 – 90 . This aligns with previous work indicating that carefully designed training can mitigate some motor deficits seen in space 5 , 91 . Furthermore, equipment and tasks could be redesigned to better accommodate the lower beta‐band synchronization or to provide augmented feedback that compensates for reduced proprioceptive cues 10 , 30 , 92 . By targeting the frequency bands known to be vulnerable in microgravity (13–30 Hz), engineers and trainers can help astronauts maintain more precise bimanual coordination in orbit and beyond. Limitations Several limitations warrant consideration. First, the participant sample consisted primarily of highly skilled individuals (e.g., fighter pilots and participants with prior parabolic flight experience). While this minimizes variability in motor performance, it limits the generalizability of the findings to broader populations, particularly individuals with less motor training or experience in altered gravity environments 93 – 95 . Consequently, caution is advised when extrapolating these results to astronauts with varying training backgrounds or to entirely untrained individuals. Second, the use of parabolic flights provides only short-duration exposures to each gravity level, which may not capture the full range of physiological adaptations that occur during extended space missions 3 , 17 . This temporal limitation restricts our understanding of long-term neuromuscular adjustments to sustained microgravity or partial gravity environments. Third, another potential limitation relates to the use of wireless EMG systems, particularly concerning their built-in hardware filters and wireless transmission characteristics 78 , 96 , 97 . These systems apply a default high-pass filter (typically around 20 Hz), which may partially attenuate signals in the alpha (5–13 Hz) and lower beta (13–20 Hz) frequency bands. This could result in an underestimation of power in these ranges, potentially affecting the sensitivity of cross-wavelet analyses 41 , 98 . Additionally, while wireless systems enhance participant mobility, they are susceptible to data packet loss and minor latency fluctuations, which may introduce subtle artifacts, especially in low-frequency bands 78 , 96 , 97 . Although these factors were mitigated through careful preprocessing, they represent inherent technical limitations that could influence the interpretation of frequency-domain EMG measures. Fourth, while EMG–EMG cross-wavelet power is a valuable tool for assessing time–frequency coupling, potential sources of signal noise, such as electrode shifts, variations in skin impedance, and minor fluctuations in signal quality, could reduce measurement precision 48 , 99 , 100 . Variations in electrode placement, signal quality, and calibration of force transducers could affect the accuracy and reliability of the coherence data 48 , 99 , 100 . These factors may introduce subtle artifacts that could influence the accuracy of the frequency-domain analyses. Future direction Further investigation is needed to pinpoint how intermediate gravity environments, such as those on the Moon (~ 0.16 g) or Mars (~ 0.38 g), affect fine motor tasks and EMG coupling. Exploring a broader set of motor tasks (e.g., tool use, maintenance activities, or emergency procedures) and combining cross-wavelet with additional neural imaging methods (EEG, fNIRS) could clarify how the CNS reorganizes under partial gravity. Additionally, research on countermeasures, including novel training regimens and feedback devices, could help astronauts to maintain more effectively bimanual coordination 72 . Such work would not only strengthen mission readiness but also offer translational benefits for rehabilitation contexts on Earth, where reduced load or novel sensory conditions (e.g., virtual reality) can similarly challenge motor coordination. Conclusions This study provides critical insights into the neurophysiological effects of altered gravity on bimanual coordination and motor control, with a particular focus on the impact of microgravity and partial gravity conditions. The findings demonstrate that coordination stability and force production are significantly influenced by gravitational loading, with notable impairments observed in microgravity (0 g) and partial restoration of motor control as gravity levels approach Earth-like conditions. Moreover, analyses of EMG–EMG cross-wavelet normalized power revealed reduced interlimb synchronization in the beta frequency band (13–30 Hz) under microgravity conditions, suggesting that gravity not only affects mechanical performance but also alters the underlying neural coupling mechanisms responsible for coordinated motor output. These findings highlight the importance of gravitational input for maintaining optimal neuromuscular control and emphasize the role of both proprioceptive feedback and central neural processing in adapting to altered gravity environments. The study’s results have practical implications for space missions, particularly regarding astronaut training, equipment design, and operational protocols aimed at mitigating the adverse effects of microgravity on motor performance 64 . While the study has several limitations, including the use of short-duration parabolic flight exposures and a highly skilled participant sample, it provides a strong foundation for future research. Ultimately, this research contributes to a growing body of knowledge on human motor control in altered gravity environments, with relevance not only for space exploration but also for applications in rehabilitation, robotics, and neuromechanics on Earth. Methods Participants Twelve healthy adults (6 males; mean age = 40.2 years, SD = 8.7 years) participated in this study. All participants were confirmed to be right-limb dominant using the Edinburgh Handedness Survey 101 and had normal or corrected-to-normal vision. Individuals with high susceptibility to motion sickness were excluded. Most participants (10 out of 12) had previous parabolic flight experience. Written informed consent was obtained from all participants. The study was approved by the NASA Johnson Space Center Institutional Review Board (IRB) (STUDY00000329), the Texas A&M University IRB (STUDY2024-0425), and the Comité de Protection des Personnes Nord Ouest II (Avis no. 22.04602.000171), adhering to the Declaration of Helsinki for research involving human subjects. Parabolic Flight The study was conducted over four parabolic flight days aboard Novespace’s Airbus A-310 Zero G. The first flight focused on microgravity (0g), with 30 parabolas during which all 12 participants completed 10 trials. The subsequent three flights (4 participants per flight) covered partial gravity conditions (0.25g, 0.5g, and 0.75g) with 10 parabolas per gravity level. The duration of altered gravity was approximately ~20 seconds (0g), ~30 seconds (0.25g), ~40 seconds (0.5g), and ~50 seconds (0.75g), with pull-up and pull-out phases at 1.8g (~20 seconds each). 1g data were collected pre- and post-flight on the ground, but only pre-flight data from the partial gravity day were analyzed to minimize potential training or fatigue effects. Apparatus Participants were sat in modified commercial airline seats equipped with custom-mounted aluminum instrument trays affixed to the armrests, ensuring their elbows were kept in contact with the instrument trays and were positioned at a 90-degree angle for stability (see Figure 6). Each arm was outfitted with an adjustable static force measurement system, comprising a force transducer and an amplifier that converted the applied force into a voltage signal. The force transducers were adjusted so that each participant’s wrist contacted the load cells in a position optimal for producing isometric force using the left and right triceps brachii muscles. These signals were recorded using an AD converter (NI USB-6210 Board, National Instruments Corp, Austin, TX, USA) connected to a computer programmed to sample at 200 Hz. Muscle activity was recorded using a wireless electromyography (EMG) system (Delsys Inc., Boston, MA, USA) at a sampling rate of 1000 Hz. Each participant was equipped with two EMG sensors positioned over the belly of the left and right triceps brachii muscles. Data from four participants were collected simultaneously, with individual force sensors and visual feedback monitors connected to a dedicated laptop per subject. EMG signals from all 4 subjects were synchronized to the same EMG recording base connected to an additional EMG-dedicated laptop. All data were collected and stored using custom LabVIEW software installed on all five laptops. This setup ensured continuous data recording during microgravity or partial gravity periods and an additional 15 seconds during hypergravity (~1.8g) phases. To maintain stability during altered gravity conditions, the trays featured 3D-printed wrist cuffs to hold participants' arms in place, preventing wrist flexion or the lifting of arms off the apparatus. In addition, the trays also featured elbow rests to maintain proper wrist alignment with the load cells. The position of each participant was continuously monitored to ensure that proper alignment was maintained throughout the experiment Participants received real-time feedback on their force production via 3D mobile theater goggles (Bigeyes H3; Vision Electronics Co.), which displayed a cursor representing their output to facilitate performance monitoring and adjustments during tasks. Bimanual Coordination Tasks Participants were required to perform two bimanual coordination tasks: 1:1 in-phase task and 1:2 multi-frequency task. The 1:1 task required participants to generate synchronized isometric force pulses with both arms simultaneously while the 1:2 task required participants to produce two force pulses with the right arm for every force pulse generated with the left arm (force coordination patterns shown in Figure 7 A and C). While performing the tasks, participants wore a head-mounted display showing a Lissajous plot to guide performance, consisting in a goal template and a cursor representing the forces produced by each arm (Figure 7 B and D). The cursor moved horizontally (left to right) as force was exerted by the right arm and vertically (bottom to top) as force was exerted by the left arm. The goal template outlined the specific force patterns needed for each coordination task. The size of the template was adjusted individually so that the maximum force required corresponded to 20% of each participant's maximum voluntary isometric contraction (MVC). Data Collection Procedure Participants were trained on the coordination tasks one day before the first flight to familiarize themselves with the experimental procedures. Training consisted of 14 practice trials for each coordination task (1:1 in-phase and 1:2 multi-frequency), with each trial lasting 30 seconds. Training began with the 1:1 task, followed by the 1:2 task. After training, baseline data were collected with two 30-second trials for each task in the same sequence as the training. During flight data collection, participants completed two trials of the 1:1 task and three trials of the 1:2 task for each gravitational condition included in the experiment (0g, 0.25g, 0.50g, 0.75g). Trials began at the onset of the microgravity or partial gravity period and continued throughout its duration, extending an additional 15 seconds into the subsequent hypergravity phase. However, for the present analysis, only data collected during the microgravity or partial gravity period were included. For each participant, performance metrics (described below) for each task and gravity level were averaged across trials (two trials for the 1:1 task and three trials for the 1:2 task). Measures and data reduction All data collection and analysis were performed using MATLAB (v2020a, The MathWorks, Inc., Natick, MA). Force time series data were collected from each arm and processed to extract key unimanual and bimanual measures. Signals from the force time series were low-pass filtered with a second-order dual-pass Butterworth filter at a 10 Hz cutoff frequency to remove noise. A 3-point difference algorithm was then used to calculate force velocity and force acceleration from the force time series. Signals were detrended and normalized to a range of ‒1 to 1 for consistency across cycles. Data were analyzed for both unimanual (force harmonicity, mean force, force variability, inter-peak interval (IPI), and inter-peak interval variability (standard deviation of the IPI)) and bimanual (IPI ratio, force-force wavelet coherence, EMG-EMG cross-wavelet power, and EMG-EMG coherence) measures to assess participants' ability to produce and time isometric force pulses across gravity conditions. Unimanual Measures. Force Harmonicity (H) : Force harmonicity (H) quantifies the harmonic or inharmonic nature of force production by analyzing distortions in the acceleration trace for each limb’s force-time series 26,27,29 . To compute H in each trial, non-overlapping windows were established between force-velocity zero-crossings 102 , allowing for half-cycle segmentation of the force data. Within each window, distortions in the force acceleration trace were identified. When a single peak appeared in the half-cycle acceleration trace, H was set to 1, reflecting smooth, undistorted force production. If distortions were present, H was calculated as the minimum-to-maximum acceleration ratio within the window. If the acceleration trace crossed from positive to negative (or vice versa) within the half-cycle, H was set to 0, indicating a disrupted force pattern. To obtain an overall harmonicity score, the H values from each window within a trial were averaged, yielding a global estimate for H. An H index of 1 indicates highly harmonic force production with minimal adjustments, whereas an index of 0 denotes significant inharmonicity and disruptions in force output. Mean force : Mean force was calculated to determine the control of force for each arm. It was determined by averaging the absolute force produced during each trial. Force Variability : Force variability was calculated to assess the consistency of force production for each limb within each trial. It was determined by calculating the standard deviation (SD) of the absolute force produced during each trial, with higher variability indicating less consistent force control. Additionally, the coefficient of variation (CV) of force was calculated as the ratio of the SD to mean force for each trial. This measure normalizes the variability relative to the average force output, providing a dimensionless metric to compare relative force consistency across conditions and tasks. Higher CV values indicate greater relative inconsistency in force production. Interpeak interval (IPI): IPI represents the average time between two consecutive force peaks during each trial. IPIs were computed for each limb cycle-by-cycle with each cycle representing every other zero crossing of the force signal. IPI variability (SD IPI) : SD IPI was computed as the standard deviation of IPI within each trial, indicating the stability of timing between peaks. Bimanual Measures. Inter-Peak Interval Ratio (IPI Ratio) : The IPI ratio was calculated as a measure of coordination between the two limbs based on the timing of force peaks. This metric provides a temporal measure of goal attainment, independent of coordination tendencies and actual force trajectories. To determine IPI ratio, the inter-peak intervals for each arm during a trial were calculated, and the ratio of the right limb’s cycle duration to the left limb’s cycle duration was computed. An IPI ratio of 1.0 indicates that the intervals for the right and left arms are equal, as required in the 1:1 task. An IPI ratio of 2.0 indicates that the interval for the left arm is twice that of the right arm, as required in the 1:2 task. EMG-EMG Cross-Wavelet Normalized Power: To quantify intermuscular neural coupling, we employed Morlet wavelet analysis to calculate the normalized EMG–EMG cross-wavelet scale-averaged power (EMG–EMG cross-wavelet normalized power). Following established methodologies 41,75,106 , we used the interference EMG signals from the left and right limbs, computed their cross-wavelet transform, and derived the scale-averaged power using the corresponding equation outlined by Neto et al. (2010) 62 (Equation 2). This power was then normalized by the total cross-wavelet power across the analyzed frequency range (5–150 Hz), yielding a percentage value (ranging from 0–100%) that indicates the relative contribution of each frequency band to the total shared energy between the two EMG signals. The normalized power was subsequently averaged over time to provide a stable measure of neural coupling. For all EMG frequency analysis, we focused on five frequency bands relevant to motor control: 5–13 Hz (alpha), 13–30 Hz (beta), 30–60 Hz (low gamma), 60–100 Hz (high gamma), and 100–150 Hz (ultra-high frequency). Prior to analysis, raw EMG signals were band-pass filtered between 20–450 Hz to eliminate low-frequency drift and high-frequency noise. Motion artifacts were minimized through visual inspection and automated artifact detection algorithms, and a notch filter (50/60 Hz) was applied to suppress power line interference. For high-pass filtered signals, full-wave rectification was conducted to preserve the envelope and oscillatory components critical for subsequent analyses. In the wavelet transform process, we applied a complex Morlet wavelet with a non-dimensional frequency parameter of ω₀ = 6, striking a balance between time and frequency resolution 107 . The decomposition covered frequencies from 5 to 150 Hz, with logarithmic spacing to ensure finer resolution at lower frequencies. EMG-EMG Coherence : For EMG–EMG coherence analysis, we utilized a wavelet-based coherence approach consistent with the methodology applied to force signals 42,43,106 . Coherence was computed for both interference EMG signals and high-pass filtered rectified EMG signals (filtered at 250 Hz and rectified). High-pass filtering at 250 Hz was employed to isolate high-frequency features analogous to amplitude modulation, enhancing the clarity of oscillatory components in the 5–100 Hz range. This technique helps reduce amplitude cancellation effects, sharpens the resolution of motor unit action potentials, and minimizes movement-related artifacts 44,75 . Given that the wireless EMG sensors employed in this study incorporate a built-in high-pass filter around 20 Hz, interference EMG signals may not accurately capture low-frequency content below this threshold 78,96,97 . This limitation could result in false negatives when analyzing the alpha (5–13 Hz) and low-beta (13–30 Hz) bands, as these signals may lack sufficient low-frequency information. To address this, the high-pass filtered rectified EMG signals preserved critical envelope and oscillatory characteristics, enhancing the detection of coherence in these bands. Statistics Linear mixed-effects models (LMMs) were employed to analyze the effects of Gravity (0g, 0.25g, 0.50g, 0.75g, 1g), Task (1:1, 1:2), Limb (right, left) 108 , and Frequency Bands (0–1 Hz, 1–4 Hz, 4–8 Hz, 8–12 Hz, and 12–20 Hz for force data; and 5–13 Hz, 13–30 Hz, 30–60 Hz, 60–100 Hz and 100–150 Hz for EMG data) on various performance and neuromuscular metrics. Random intercepts for each participant were included to account for repeated measures and within-subject variability. The models were implemented using the lmer function in the lme4 package in R (v. 4.4.1). Performance measures, including Harmonicity, Mean Force, Force Variability, Inter-Peak Interval (IPI), and IPI Variability were analyzed with Gravity, Task, and Limb as fixed effects, including their two- and three-way interactions. IPI Ratio was analyzed with Gravity and Task as fixed effects, including their interaction (Gravity × Task). For the neuromuscular data, both EMG-EMG normalized cross-wavelet power and EMG-EMG coherence were analyzed using repeated-measures ANOVA to assess the effects of Gravity and Frequency Band. The integration of both interference and rectified EMG data in this dual approach enhances the sensitivity and robustness of the analysis, providing comprehensive insights into the neural mechanisms underlying bimanual coordination under varying gravitational conditions 41,75 . Model diagnostics were conducted to ensure assumptions of normality and homogeneity of variances. Residuals were tested for normality using the Shapiro-Wilk test, while Levene’s test was used to assess homogeneity of variances. Post hoc analyses were performed using estimated marginal means (from the emmeans package) with Tukey’s HSD correction for multiple comparisons. Pairwise comparisons were conducted between all gravity conditions (e.g., 0g, 0.25g, 0.5g, 0.75g, and 1g) to identify significant differences, with no single gravity condition (e.g.,1g) used as a reference for comparison. Statistical results are reported with F values, p-values, and additional post hoc pairwise comparisons including estimates, standard errors (SE), degrees of freedom 101 , and t-ratios. Statistical significance was set at α = 0.05 for all tests. Results are presented as mean ± standard deviation (SD) unless otherwise specified, and figures display mean ± standard error (SE). All statistical analyses were performed using R (v4.4.1) within the Visual Studio Code (v1.92.1) environment. Figures and visualizations were generated using GraphPad Prism (v10.2.3) for Windows (GraphPad Software, Boston, Massachusetts USA) and BioRender.com. Declarations Data availability The datasets analyzed for this study are publicly available, a repository can be found on GitHub: https://github.com/BHP-Lab/BimCoord/tree/main/Parabolic_Flight_11_12_Task/Data. Acknowledgements This work was supported by the NASA Human Research Program, Grant Number: 80NSSC20K1499. The authors would like to acknowledge NASA, Novespace, and the European Space Agency for their invaluable support in making this experiment possible. Special thanks go to Alexandra Jaquemet (Novespace) for her guidance during the design and execution of the experiment. We also extend our gratitude to Neil Melville 57 for his expertise and assistance throughout all phases of the project. Additionally, we are grateful to Pierre Denise and his colleagues for their support with the IRB procedures and approval. Finally, we thank all participants and collaborators who contributed to the success of this study. Author Contributions D.M.K. and A.D-A. conceived the study. Material preparation and operational readiness for the parabolic flight was conducted by D.M.K., M.W., R.A., N.K., T.W., B.J.D., and A.D-A. Data reduction, statistical analysis, and figures were performed by M.W., R.A., N.K., O.P.N., and A. D-A. The draft of the manuscript was written by D.M.K., M.W., R.A., R.R., and A.D-A. 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Cite Share Download PDF Status: Published Journal Publication published 16 Apr, 2026 Read the published version in npj Microgravity → Version 1 posted Editorial decision: Revision requested 09 Aug, 2025 Reviews received at journal 08 Aug, 2025 Reviewers agreed at journal 10 Jul, 2025 Reviews received at journal 10 Apr, 2025 Reviewers agreed at journal 18 Mar, 2025 Reviewers agreed at journal 18 Mar, 2025 Reviewers invited by journal 17 Mar, 2025 Editor assigned by journal 16 Mar, 2025 Submission checks completed at journal 16 Mar, 2025 First submitted to journal 28 Feb, 2025 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. 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Force measures (B and C) are depicted in Newtons (N). The data represent mean values for each gravity condition (0g, 0.25g, 0.5g, 0.75g, and 1g) across the 1:1 (solid lines) and 1:2 (dashed lines) tasks, separated by limb (left [L]: filled symbols; right [R]: open symbols). Error bars indicate the standard error of the mean (SE). Statistically significant main and interaction effects are shown in the individual panels. The * indicates post-hoc significant differences at p\u0026lt;0.05. The # indicates post-hoc differences approaching significance (p = 0.067).\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6131347/v1/92bc29bef5b6823dad86dc48.jpeg"},{"id":79326139,"identity":"d1607fb4-eaa4-48dd-b13c-358b32dd3e34","added_by":"auto","created_at":"2025-03-27 05:33:09","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":202949,"visible":true,"origin":"","legend":"\u003cp\u003eUnimanual coordination performance in each task (1:1 in phase and 1:2 multifrequency) as a function of gravitational condition for (A) inter-peak interval and (B) inter-peak interval variability (SD). The data represent mean values across gravity conditions (0g, 0.25g, 0.5g, 0.75g, and 1g) for the 1:1 (solid lines) and 1:2 (dashed lines) tasks, separated by limb (left [L]: filled symbols; right [R]: open symbols). Error bars indicate the standard error of the mean (SE). Statistically significant main and interaction effects are shown in the individual panels.\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6131347/v1/d956e1faa910617a40e6db5b.jpeg"},{"id":79326137,"identity":"70105068-d095-45de-8da3-fc66057686ab","added_by":"auto","created_at":"2025-03-27 05:33:08","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":232314,"visible":true,"origin":"","legend":"\u003cp\u003eBimanual coordination performance in (A) 1:1 in-phase task and (B) 1:2 multifrequency task, as a function of gravitational condition for mean inter-peak interval (IPI) ratio, with error bars representing standard errors of the mean (SE). Significant post-hoc differences (p \u0026lt; 0.05) are highlighted, indicating that IPI ratio values during the 1:2 task at 0g were significantly lower compared to other gravity conditions, reflecting the greater difficulty in maintaining the target coordination patterns under microgravity conditions for the most difficult task.\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6131347/v1/df33527f4f7ab8d26c1a5a32.jpeg"},{"id":79326138,"identity":"83674927-a1dc-4fa6-b06f-45544e9e70e1","added_by":"auto","created_at":"2025-03-27 05:33:09","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":624761,"visible":true,"origin":"","legend":"\u003cp\u003eSample EMG amplitude time series for the left and right limbs during the 1:2 task under microgravity (left) and 1g (right) gravity conditions, along with the corresponding EMG-EMG normalized cross-wavelet periodogram and global spectrum. In the wavelet periodograms, regions of higher power (red/yellow) indicate stronger interlimb coupling across the 16–64 Hz frequency range (beta and gamma bands). The global spectrum (right panels) represents the average power across time for each frequency. Dashed lines in the global spectrum indicate the significance threshold (p \u0026lt; 0.05), marking frequency ranges where coupling exceeds chance levels.\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6131347/v1/cb53e5fedb45371080bd8e4a.jpeg"},{"id":79326989,"identity":"29197af7-cb52-44ce-88d6-4249b18cc17f","added_by":"auto","created_at":"2025-03-27 05:41:09","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":586381,"visible":true,"origin":"","legend":"\u003cp\u003eBimanual coordination performance (both 1:1 in-phase task and 1:2 multifrequency task averaged together) across frequency bands as a function of gravitational condition for EMG-EMG cross-wavelet normalized power. Mean normalized power values, with error bars representing standard errors (SE), are displayed across the five frequency bands (5–13 Hz, 13–30 Hz, 30–60 Hz, 60–100 Hz, and 100–150 Hz) for each gravity condition (0g, 0.25g, 0.5g, 0.75g, and 1g). Significant differences in the 13–30 Hz band are highlighted, indicating that EMG-EMG cross-wavelet normalized power at 0g is significantly lower compared to the 1g condition, reflecting reduced neural synchronization under microgravity in this frequency range.\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6131347/v1/0718a5f8597ca17f49410831.jpeg"},{"id":79326149,"identity":"f8eb7612-9db9-4229-b498-3c807d87bf44","added_by":"auto","created_at":"2025-03-27 05:33:09","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":847128,"visible":true,"origin":"","legend":"\u003cp\u003eExperimental setup for bimanual force coordination tasks. (A) Side view showing the setup for a participant seated with arms resting on force transducers, performing bimanual coordination tasks while receiving real-time visual feedback via a head-mounted display. The force transducers measured isometric force production from each arm. (B) Aerial view of the complete setup during data collection, showing four participants seated in a row, each equipped with individual force sensors and visual feedback monitors connected to their assigned laptops. All participants' EMG sensors were wirelessly connected to a shared EMG recording base. This configuration ensured consistent and synchronized data collection of EMG and force recordings across participants. The layout was optimized to minimize motion artifacts and interference while maintaining participant comfort under parabolic flight conditions.\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6131347/v1/9a163f59cbe81e4ef9a90edc.jpeg"},{"id":79326993,"identity":"609bc950-fb4c-43cd-82ef-db5d98c32191","added_by":"auto","created_at":"2025-03-27 05:41:09","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":928626,"visible":true,"origin":"","legend":"\u003cp\u003eA) Graphic representation of the simulated goal force coordination patterns and corresponding Lissajous templates for the 1:1 (A, B) and 1:2 (C, D) tasks. In panels A and C, the right limb force is shown in grey whereas the left limb force is shown in black (in panel A, both the right and left limbs produce identical force patterns simultaneously).\u003c/p\u003e","description":"","filename":"floatimage7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6131347/v1/8fdf298e72884798ebb1e2cf.jpeg"},{"id":107351009,"identity":"918b1dc8-fd02-4256-aacc-a2ea2ca5525d","added_by":"auto","created_at":"2026-04-20 16:07:42","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4077785,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6131347/v1/364987e3-0e85-48c2-b3ee-c824857ad5b3.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Neurophysiological effects of partial gravity on bimanual control: A parabolic flight study","fulltext":[{"header":"Introduction","content":"\u003cp\u003eBimanual coordination, the ability to synchronize actions between both limbs, is critical for a wide range of complex activities that are essential to space exploration. Astronauts are frequently required to perform intricate tasks that demand precise and coordinated arm movements, such as operating spacecraft controls, assembling equipment, and conducting scientific experiments. These tasks become particularly challenging in altered gravity environments, where the absence or reduction of gravitational forces can significantly disrupt motor control strategies and coordination \u003csup\u003e\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. While extensive research has been conducted on motor control under microgravity (0g) conditions revealing its profound impact on proprioceptive feedback and motor performance \u003csup\u003e\u003cspan additionalcitationids=\"CR5 CR6 CR7 CR8\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e, less is known about the effects of partial gravity levels (e.g., 0.25g, 0.5g, 0.75g) on motor control and coordination.\u003c/p\u003e \u003cp\u003eIn microgravity, the absence of gravitational feedback is known to impair the production of stable and consistent forces, particularly in tasks requiring bimanual coordination \u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e. For example, experiments have shown that force production becomes less harmonic, characterized by increased adjustments and perturbations \u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e. These disruptions are attributed to the lack of a gravitational reference, which is vital for maintaining spatial orientation and stability during movement \u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e,\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. Much of the research in microgravity has focused on unimanual tasks \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan additionalcitationids=\"CR15 CR16 CR17 CR18\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e, while little is known about how bimanual coordination is affected across different gravitational level. This gap is particularly significant because bimanual tasks often require more complex neural integration, involving both hemispheric communication and precise interlimb synchronization \u003csup\u003e\u003cspan additionalcitationids=\"CR21\" citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. Unlike unimanual movements, bimanual coordination depends on dynamic interactions between homologous muscles and neural pathways, which may respond differently to changes in gravitational load \u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. Understanding how partial gravity levels influence these processes is critical for advancing our knowledge of motor control adaptations in space environments and for developing effective countermeasures to support astronaut performance during long-duration missions.\u003c/p\u003e \u003cp\u003eRecent studies on motor control in altered gravity have underscored the complexity of coordinating bimanual movements in these environments \u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e. Numerous constraints, such as proprioceptive feedback, neural crosstalk, and environmental factors, have been proposed to explain variations in motor performance across different gravity conditions \u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan additionalcitationids=\"CR21\" citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. Among these, neural crosstalk, which involves the interaction between ipsilateral and contralateral motor commands \u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e, has been particularly emphasized as a critical factor in bimanual coordination tasks \u003csup\u003e\u003cspan additionalcitationids=\"CR25 CR26 CR27 CR28\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. Understanding these constraints is crucial for advancing our knowledge of neuromuscular control under altered gravitational conditions.\u003c/p\u003e \u003cp\u003eIn altered gravity, research has consistently demonstrated that manual control is compromised compared to performance in 1g conditions \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. Simulated altered gravity environments on Earth, including head-down tilt (HDT), centrifugation, parabolic flight, and water immersion, have been used to study these effects, revealing slower movement times and decreased accuracy in manual control tasks \u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. Despite these observations, the specific mechanisms or constraints leading to reduced performance in altered gravity are not fully understood. Evidence indicates that individuals rely more heavily on visual and tactile cues for spatial orientation and movement control in such environments \u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e,\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eKey findings also highlight that reduced gravity can significantly disrupt motor control and task execution \u003csup\u003e\u003cspan additionalcitationids=\"CR34\" citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. For example, people exposed to altered gravity encounter challenges in performing motor tasks that require precision and stability \u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. Experiments in microgravity show that force production becomes less smooth, with increased adjustments and perturbations during motor activities \u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. This suggests that the lack of gravitational feedback hampers the ability to generate stable and consistent forces, which is particularly crucial for tasks requiring coordinated hand movements \u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e. Studies in partial gravity environments, created using centrifuges or parabolic flights, provide further insights into how gravity affects motor control. These studies reveal that motor coordination and task performance are influenced by gravity levels, with performance typically declining and coordination challenges increasing as gravity decreases\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. However, the impact of partial gravity specifically on bimanual coordination is still not fully understood, particularly regarding neural mechanisms and the interaction between homologous muscles.\u003c/p\u003e \u003cp\u003eElectromyography (EMG) analysis, particularly using EMG\u0026ndash;EMG cross-wavelet power and coherence, is a valuable tool for examining neural interactions between homologous muscles \u003csup\u003e\u003cspan additionalcitationids=\"CR38\" citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e. While cross-wavelet normalized power captures both the time and frequency dynamics of muscle activity, offering insights into the magnitude of shared neural input, coherence assesses the synchronization of muscle activity, providing a measure of neural coupling strength \u003csup\u003e\u003cspan additionalcitationids=\"CR41 CR42\" citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e. Previous studies have used EMG analyses to investigate neural crosstalk and coordination during various motor tasks \u003csup\u003e\u003cspan additionalcitationids=\"CR45 CR46\" citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e. Cross-wavelet analysis offers advantages over traditional Fourier-based methods by capturing time-frequency patterns and dynamic changes in neural interactions \u003csup\u003e\u003cspan additionalcitationids=\"CR42\" citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e. Coherence complements this by revealing frequency-specific neural synchronization, which may vary under different gravitational loads\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan additionalcitationids=\"CR49\" citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e. However, the application of EMG-EMG cross-wavelet normalized power and coherence in altered gravity environments is relatively underexplored, and their combined potential to reveal the neural mechanisms behind coordination deficits in space missions remains an open question.\u003c/p\u003e \u003cp\u003eGiven the critical role of bimanual coordination in space exploration and the impact of altered gravity on motor control, this study seeks to bridge the gap in understanding the effects of partial gravity levels on bimanual coordination as well as the underlying neural mechanisms. While existing research has highlighted general disruptions in motor performance under reduced gravity \u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e, there is a lack of detailed investigations into the effects of varying levels of partial gravity on neural interactions and coordination patterns. The primary objective of this study is to examine the neurophysiological effects of partial gravity on bimanual control, specifically investigating the influence of varying gravitational levels on motor coordination and neural interactions between homologous muscles. Participants performed isometric force coordination tasks during parabolic flight, which provided exposure to distinct gravitational conditions (0g, 0.25g, 0.5g, 0.75g, 1g), with data collected on force production and muscle activity using force and EMG sensors, respectively. The study evaluated bimanual force production stability and harmonicity across these gravity levels. A secondary objective is to identify and characterize the neural mechanisms underlying coordination deficits observed under different gravitational loads using two EMG-EMG wavelet analyses methods: EMG\u0026ndash;EMG cross-wavelet normalized power and EMG-EMG coherence. This dual approach aims to enhance our understanding of the neural adaptations necessary for effective motor control in altered gravity by evaluating both the relative magnitude and synchronization of neural interactions between homologous muscles in response to partial gravity.\u003c/p\u003e \u003cp\u003eThe theoretical implications of this study lie in advancing the understanding of neuromuscular control under altered gravity conditions. By revealing how partial gravity affects neural interactions and coordination dynamics, the study contributes to the broader knowledge of motor control mechanisms in space environments. Practically, the findings may provide new insights into the design of training programs and equipment for astronauts, aiming to improve operational performance and safety during space missions. Understanding these effects will be crucial for preparing astronauts for the challenges of future missions, including the planned return to the Moon and eventual human exploration of Mars.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eWe investigated the effects of varying gravitational levels on bimanual coordination and neural interactions, using parabolic flight to simulate gravity conditions of 0g, 0.25g, 0.5g, 0.75g, and 1g. Participants performed two coordination tasks (1:1 and 1:2) under each condition, while electromyography (EMG) data were collected to assess muscle activity and neural coupling between homologous muscles. Task performance was evaluated using metrics such as force harmonicity, mean force, force variability, and inter-peak intervals. Additionally, EMG-EMG analyses focused on cross-wavelet normalized power and coherence, across different gravitational loads.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eUnimanual Analyses\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eHarmonicity (H)\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eThe analysis of harmonicity using mixed linear models revealed significant main effects of Gravity (F(4, 205.09) = 2.63, p = 0.036), Task (F(1, 205.00) = 72.42, p \u0026lt; 0.001), and Limb (F(1, 205.00) = 72.87, p \u0026lt; 0.001). Harmonicity values were significantly lower at 0g (0.858 \u0026plusmn; 0.209) compared to 0.75g (0.938 \u0026plusmn; 0.159), indicating reduced force smoothness in microgravity. Harmonicity was also higher during the 1:1 task (0.959 \u0026plusmn; 0.116) compared to the 1:2 task (0.830 \u0026plusmn; 0.246), and greater in the right limb (0.960 \u0026plusmn; 0.103) than the left limb (0.830 \u0026plusmn; 0.252), consistent with all participants being right-hand dominant.\u003c/p\u003e\n\u003cp\u003eA significant Gravity \u0026times; Task interaction (F(4, 205.00) = 3.05, p = 0.018) indicated that harmonicity was particularly lower during the 1:2 task in microgravity. Post-hoc comparisons showed that harmonicity during the 1:2 task was significantly reduced at 0g (0.742 \u0026plusmn; 0.245) compared to 0.25g (0.845 \u0026plusmn; 0.261; estimate = -0.103, SE = 0.033, df = 205, p = 0.020), 0.75g (0.883 \u0026plusmn; 0.212; estimate = -0.131, SE = 0.034, df = 205, p = 0.002), and 1g (0.865 \u0026plusmn; 0.239; estimate = -0.124, SE = 0.033, df = 205, p = 0.003).\u003c/p\u003e\n\u003cp\u003eA significant Task \u0026times; Limb interaction (F(1, 205.00) = 74.32, p \u0026lt; 0.001) revealed that task-related differences in harmonicity were more pronounced in the left limb. Specifically, harmonicity in the left limb was significantly lower during the 1:2 task (0.700 \u0026plusmn; 0.282) compared to the 1:1 task (0.962 \u0026plusmn; 0.117; estimate = 0.259, SE = 0.021, df = 205, p \u0026lt; 0.001).\u003c/p\u003e\n\u003cp\u003eFurthermore, harmonicity for the left limb during the 1:2 task was significantly lower in 0g compared to 0.25g (estimate = -0.199, SE = 0.047, df = 205, p = 0.006), 0.75g (estimate = -0.251, SE = 0.049, df = 205, p \u0026lt; 0.001), and 1g (estimate = -0.218, SE = 0.047, df = 205, p = 0.001), highlighting the specific vulnerability of left-limb coordination under microgravity conditions. The contrast between 0g and 0.50g was not statistically different (estimate = -0.158, SE = 0.047, df = 205, p= 0.104) (Figure 1A).\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eMean Force\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eThe analysis of mean force using mixed linear models revealed significant main effects for Gravity and Task, but no significant effects for Limb or any interactions. A significant main effect of Gravity was found (F (4, 205.01) = 3.08, p = 0.017), indicating that mean force differed across the various gravity conditions (Figure 1B). Specifically, post-hoc analyses revealed that the mean force at 0g (8.89 \u0026plusmn; 3.17 N) was significantly lower than at 1g (9.58 \u0026plusmn; 3.14 N; estimate = -0.690, SE = 0.249, df = 205, t.ratio = -2.771, p = 0.048). Additionally, the mean force at 0.25g (8.86 \u0026plusmn; 2.86 N) was significantly lower than at 1g (estimate = -0.714, SE = 0.249, df = 205, t.ratio = -2.866, p = 0.037).\u003c/p\u003e\n\u003cp\u003eA significant main effect of Task was also detected (F (1, 204.99) = 7.64, p = 0.006), with lower mean force observed during the 1:1 task (8.88 \u0026plusmn; 2.67 N) compared to the 1:2 task (9.32 \u0026plusmn; 3.17 N; estimate = -0.439, SE = 0.159, df = 205, t.ratio = -2.763, p = 0.006) (Figure 1B).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eForce Variability (SD of force and CV of force)\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eThe analysis of force variability, measured as the standard deviation (SD) of force, revealed a significant main effect of Limb but no significant effects of Gravity or Task, nor any interactions (Figure 1C). The main effect of Limb, F (1, 204.99) = 6.74, p = 0.010, indicated that force variability was higher in the left limb (6.48 \u0026plusmn; 2.31 N) compared to the right limb (6.17 \u0026plusmn; 2.27 N; estimate = 0.304, SE = 0.117, df = 205, p = 0.010).\u003c/p\u003e\n\u003cp\u003eWe also examined the coefficient of variation (CV) of force to further explore the variability in force. Similar to the SD analysis, the CV analysis revealed a significant main effect of Limb, F(1, 204.83) = 15.28, p \u0026lt; 0.001, with the CV of force being higher in the left limb (0.711 \u0026plusmn; 0.088) compared to the right limb (0.666 \u0026plusmn; 0.106; estimate = 0.044, SE = 0.011, df = 205 , p \u0026lt; 0.001). This finding suggests greater variability in force production for the left limb across conditions. No significant main effects of Gravity (F (4, 205.11) = 0.58, p = 0.678) or Task (F (1, 204.83) = 3.32, p = 0.070) were found, and no significant interactions were observed (p \u0026gt; 0.275). Although the three-way interaction was not significant, post-hoc analysis revealed a marginally significant difference in CV during the 1:2 task between the left limb at 0g and 0.5g (estimate = 0.124, SE = 0.035, df = 205, p = 0.067) (Figure 1D).\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eInter-peak Interval (IPI)\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eThe inter-peak interval analysis using mixed linear models revealed significant main effects for Gravity, Task, and Limb, but no significant interactions involving Gravity. A significant main effect of Gravity was observed (F(4, 205.05) = 2.82, p = 0.026), indicating that IPI values increased at lower gravity levels (Figure 2A). Post-hoc comparisons showed that IPI at 0g (1.53 \u0026plusmn; 0.67 s) was significantly higher than at 0.75g (1.27 \u0026plusmn; 0.67 s; estimate = 0.225, SE = 0.069, df = 205, p = 0.012).\u003c/p\u003e\n\u003cp\u003eThere was also a strong main effect of Task (F (1, 205.00) = 235.99, p \u0026lt; 0.001), with significantly lower IPI values observed during the 1:1 task (1.08 \u0026plusmn; 0.40 s) compared to the 1:2 task (1.74 \u0026plusmn; 0.89 s; estimate = -0.659, SE = 0.043, df = 205, t.ratio = -15.362, p \u0026lt; 0.001). Additionally, a significant main effect of Limb was detected, F(1, 205.00) = 170.19, p \u0026lt; 0.001, with IPI being higher in the left limb (1.69 \u0026plusmn; 0.91 s) compared to the right limb (1.13 \u0026plusmn; 0.43 s; estimate = 0.560, SE = 0.043, df = 205, p \u0026lt; 0.001).\u003c/p\u003e\n\u003cp\u003eIn addition, a significant Task x Limb interaction was identified (F(1, 205.00) = 171.56, p \u0026lt; 0.001). This interaction indicates that the task effect on IPI was more pronounced for the left limb, with IPI values in the left limb for the 1:1 task (1.08 \u0026plusmn; 0.40 s) being substantially lower compared to the 1:2 task (2.30 \u0026plusmn; 0.87 s; estimate = -1.221, SE = 0.061, df = 205, p \u0026lt; 0.001). For the right limb, however, IPI did not differ significantly between tasks (estimate = -0.097, SE = 0.061, df = 205, p = 0.111) (Figure 2A).\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eInter-peak Interval Variability\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eThe analysis of inter-peak interval variability (SD IPI) using mixed linear models showed significant main effects for Gravity, Task, and Limb, but no significant interactions involving Gravity (Figure 2B). A main effect of Gravity was detected (F (4, 204.98) = 4.48, p = 0.002), indicating that IPI variability was highest in microgravity (0g) and decreased progressively with increasing gravitational load. Post-hoc analyses revealed that IPI variability at 0g (0.198 \u0026plusmn; 0.145) was significantly higher compared to both 0.75g (0.115 \u0026plusmn; 0.073; estimate = 0.059, SE = 0.019, df = 205, p = 0.021) and 1g (0.139 \u0026plusmn; 0.111; estimate = 0.059, SE = 0.019, df = 205, p = 0.017). This suggests that the absence of gravitational cues in microgravity leads to less stable timing control increasing variability in motor coordination\u003csup\u003e12,51\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThere was also a significant main effect of Task (F (1, 204.90) = 69.24, p \u0026lt; 0.001), with lower IPI variability observed during the 1:1 task (0.108 \u0026plusmn; 0.122) compared to the 1:2 task (0.208 \u0026plusmn; 0.155; estimate = -0.099, SE = 0.012, df = 205, p \u0026lt; 0.001). The increased variability during the 1:2 task reflects the greater complexity and demands of maintaining an asymmetric coordination pattern. The main effect of Limb was also significant, F (1, 204.90) = 14.45, p \u0026lt; 0.001, showing higher IPI variability in the left limb (0.181 \u0026plusmn; 0.172) compared to the right limb (0.135 \u0026plusmn; 0.115; estimate = 0.045, SE = 0.012, df = 205, p \u0026lt; 0.001). This indicates that the non-dominant (left) limb exhibited less stable timing control, particularly under conditions of increased task complexity.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn addition, a significant Task x Limb interaction was identified, F (1, 204.90) = 10.57, p = 0.001. This interaction demonstrated that the task effect on IPI variability was more pronounced in the left limb, with IPI variability values in the left limb in the 1:1 task (0.111 \u0026plusmn; 0.142) being significantly lower compared to the 1:2 task (0.250 \u0026plusmn; 0.172; estimate = -0.138, SE = 0.017, df = 205, p \u0026lt; 0.001). In the right limb, the difference between tasks was smaller but still significant, with lower IPI variability in the 1:1 task (0.105 \u0026plusmn; 0.099) compared to the 1:2 task (0.165 \u0026plusmn; 0.123; estimate = -0.061, SE = 0.017, df = 205, p \u0026lt; 0.001). Although both limbs showed increased variability during the 1:2 task, the effect was more pronounced in the left limb, suggesting greater difficulty in maintain stable timing with the non-dominant limb under complex coordination demands\u003csup\u003e52\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eBimanual Analysis -\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003ePerformance\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eInter-peak Interval (IPI) Ratio\u003c/em\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe analysis of mean inter-peak interval (IPI) ratio revealed significant main effects for Gravity and Task, as well as a significant Gravity x Task interaction (Figure 3). A main effect of Gravity was detected (F (4, 96.70) = 5.15, p = 0.001), indicating that IPI ratio differed across the various gravity conditions. Post-hoc analyses showed that IPI ratio was lower at 0g (1.42 \u0026plusmn; 0.45) compared to all other gravity levels, including 0.25g (1.50 \u0026plusmn; 0.52; estimate = -0.077, SE = 0.021, df = 97, p = 0.003), 0.5g (1.49 \u0026plusmn; 0.52; estimate = -0.072, SE = 0.021, df = 97, p = 0.006), 0.75g (1.49 \u0026plusmn; 0.51; estimate = -0.068, SE = 0.021, df = 98, p = 0.015), and 1g (1.50 \u0026plusmn; 0.51; estimate = -0.076, SE = 0.021, df = 97, p = 0.003). \u0026nbsp;The lower IPI ratio at 0g suggests participants experienced greater difficulty maintaining the intended timing ratio in the absence of gravitational cues.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eA strong main effect of Task was also found (F (1, 96.32) = 5482.23, p \u0026lt; 0.001), with a markedly lower IPI ratio during the 1:1 task (0.99 \u0026plusmn; 0.02) compared to the 1:2 task (1.97 \u0026plusmn; 0.12; estimate = -0.972, SE = 0.013, df = 97, p \u0026lt; 0.001). The large difference reflects the expected coordination demands, as the 1:2 task naturally requires a ratio near 2.0, while the 1:1 task requires equal timing between the limbs (i.e., IPI ratio = 1.0).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe Gravity x Task interaction was significant (F (4, 96.32) = 5.50, p \u0026lt; 0.001), indicating that the effect of gravity on IPI ratio was more pronounced during the 1:2 task. Post-hoc comparisons within the 1:2 task revealed that at 0g, the IPI ratio (1.85 \u0026plusmn; 0.17) was significantly lower compared to 0.25g (2.01 \u0026plusmn; 0.09; estimate = -0.159, SE = 0.029, df = 97, p \u0026lt; 0.001), 0.5g (2.00 \u0026plusmn; 0.09; estimate = -0.152, SE = 0.029, df = 97, p \u0026lt; 0.001), 0.75g (1.99 \u0026plusmn; 0.06; estimate = -0.136, SE = 0.030, df = 98, p \u0026lt; 0.001), and 1g (1.99 \u0026plusmn; 0.08; estimate = -0.148, SE = 0.029, df = 97, p \u0026lt; 0.001) (Figure 3B). The IPI ratio at 0g deviated further than the ideal 2.0 ratio, indicating that microgravity impaired participants\u0026rsquo; ability to maintain the target 1:2 coordination pattern. This suggests greater difficulty in sustaining rhythmic control when gravitational cues are absent \u003csup\u003e26,51\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eBimanual Analysis - EMG\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eEMG-EMG Cross-Wavelet Normalized Power and EMG-EMG Coherence\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eThe analysis of EMG-EMG coherence, considering both interference EMG and high-pass filtered rectified EMG signals, yielded no significant gravity-related results (p \u0026gt; 0.05). However, these non-significant findings should be interpreted cautiously, as trends may reflect subtle neural adaptations not fully captured due to methodological constraints.\u003c/p\u003e\n\u003cp\u003eThe analysis of EMG-EMG cross-wavelet normalized power revealed that the main effect of Frequency was highly significant (F (4,514.23) = 923.73, p \u0026lt; 0.001), demonstrating marked differences in normalized power across the five frequency bands. Figure 4 shows an example of the EMG amplitude time series for the left and right limbs during the 1:2 task under microgravity (left) and 1g (right) gravity conditions, along with the corresponding EMG-EMG normalized cross-wavelet periodogram and global spectrum. Normalized cross-wavelet power highlights periods of significant coherence across the 16-64 Hz (beta and gamma bands). The colormap represents normalized cross-wavelet power, with red regions indicating stronger coupling between the left and right limb muscle activity.\u003c/p\u003e\n\u003cp\u003eAdditionally, neither Gravity nor Task had a significant main effect. Importantly, the Gravity x Frequency interaction approached significance (F (16,514.23) = 1.53, p = 0.083), suggesting a trend toward differential effects of gravity across frequency bands. Despite these results, the post-hoc comparisons within the 13-30 Hz frequency band revealed a significant difference between the 0g and 1g conditions (p=0.006). Thus, in the 13-30 Hz band, normalized power for the 1g gravity condition (15.075 \u0026plusmn; 5.077) was significantly greater than for the 0g condition (10.771 \u0026plusmn; 3.102; estimate = -4.441, SE = 1.306, df = 514.7, p = 0.006) (Figure ).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThese trends in neural synchronization may reflect underlying adaptations to altered gravity, although interpretations should be made cautiously given the proximity of some effects to statistical significance thresholds. Potential methodological factors, such as limitations in signal detection at lower frequencies when using interference EMG, could influence the sensitivity of these analyses to subtle changes in neural coupling\u003csup\u003e61,62\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe study investigated the impact of microgravity and partial gravity on bimanual coordination and EMG-EMG cross-wavelet power using parabolic flight. Our primary findings revealed that participants maintained effective timing of bimanual force production across all gravity conditions. However, force production was noticeably less harmonic in microgravity (0g) than at higher gravity levels, particularly during the more complex (1:2) task, indicating increased force corrections and perturbations in the 0g condition. Additionally, mean force production was significantly lower in 0g compared to 1g, suggesting that gravitational loading plays a critical role in sustaining force output. Force variability was also greater in the left limb, reflecting asymmetries in coordination stability. In terms of neuromuscular activation, EMG-EMG cross-wavelet power revealed significantly reduced beta-band (13\u0026ndash;30 Hz) power in 0g, implying diminished interlimb neural synchronization under microgravity. These results highlight the nuanced interplay between gravity and motor control, underscoring the value of time-frequency methods (e.g., cross-wavelet power) for probing the neural mechanisms underlying bimanual coordination challenges in altered gravity environments.\u003c/p\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eGravity Level\u003c/h2\u003e \u003cp\u003eBy examining gravity levels ranging from 0 g to 1 g (including 0.25g, 0.5g, 0.75g), this study provides insights into how decreasing gravity alters motor control and coordination stability. The most pronounced impairments emerged under microgravity (0 g) during the performance of the complex task (1:2), where force production became less stable and more variable. Participants exhibited greater force distortions, adjustments, and perturbations, aligning with prior work showing that microgravity disrupts the sensory feedback and proprioceptive cues critical for precise movement coordination \u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e,\u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e\u003c/sup\u003e. These findings have practical relevance for astronauts, who often report difficulty maintaining fine motor control in microgravity \u003csup\u003e\u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e64\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eIn contrast, higher partial gravity conditions (0.75 g) facilitated more stable and harmonic force production during the complex task, mirroring studies suggesting some gravitational load approximating Earth\u0026rsquo;s conditions helps stabilize motor control \u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e65\u003c/span\u003e,\u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e66\u003c/span\u003e\u003c/sup\u003e. As gravity increased toward 1g, participants showed improved coordination and fewer force distortions, indicating that partial gravity can mitigate some of the deficits that appear in microgravity.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003eForce Production\u003c/h2\u003e \u003cp\u003eThe differences in force production between 0 g and higher gravity levels expand on existing literature by showing that harmonicity and coordination stability degrade significantly in microgravity \u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e,\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e67\u003c/span\u003e\u003c/sup\u003e, especially in more complex tasks. The observed increases in force corrections and distortions in the complex task further support the view that the lack of a reliable gravitational reference undermines precise motor control \u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e,\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e. Without gravity, participants likely experienced disrupted proprioceptive feedback, making it more difficult to maintain smooth, rhythmic force patterns \u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. In contrast, partial gravity conditions (e.g., 0.75 g) provided a partial restoration of Earth-like proprioceptive cues, resulting in improved control compared to 0 g \u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e,\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e68\u003c/span\u003e\u003c/sup\u003e. This suggests that even moderate gravitational loading helps stabilize motor output by enhancing sensorimotor integration.\u003c/p\u003e \u003cp\u003eIn addition to the observed differences in harmonicity, analyses of mean force and force variability (standard deviation [SD] and coefficient of variation [CV]) provide further insights into how gravity influences bimanual coordination. Mean force production increased significantly with gravity, with participants generating higher forces under 1 g compared to microgravity (0 g). Our findings suggest that the presence of gravitational loading facilitates not only stronger but also more consistent force output \u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e,\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. The likely mechanism involves enhanced proprioceptive and tactile feedback under higher gravity, which supports more effective motor unit recruitment and sustained force generation \u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e,\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e. In contrast, in 0 g, the absence of gravitational load reduces the activation of stabilizing muscles, leading to lower mean force levels as participants rely more heavily on feedforward control strategies rather than feedback-driven adjustments \u003csup\u003e\u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e69\u003c/span\u003e,\u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e70\u003c/span\u003e\u003c/sup\u003e. Taken together, these results clarify how gradient changes in gravitational force systematically influence motor performance. Even moderate reintroduction of load (e.g., 0.75 g) had appreciable benefits for both force stability and consistency \u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e, highlighting the critical role of gravitational forces in supporting not only the magnitude but also the precision of bimanual coordination.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003eEMG-EMG Cross-Wavelet Power\u003c/h2\u003e \u003cp\u003eAlthough a standard EMG-EMG coherence analysis did not yield clear gravity-related effects, EMG-EMG cross-wavelet normalized power captured meaningful differences across frequency bands. Crucially, we found a significant reduction in beta-band (13\u0026ndash;30 Hz) cross-wavelet power in 0g compared to 1g, suggesting reduced shared neural activity (common neural drive) and decreased synchronization between left-right muscle activity under microgravity conditions \u003csup\u003e\u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e75\u003c/span\u003e,\u003cspan citationid=\"CR76\" class=\"CitationRef\"\u003e76\u003c/span\u003e\u003c/sup\u003e. This result suggests that typical beta-band coupling, often associated with interlimb coordination and refined motor output, may rely on the presence of gravitational loading \u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e,\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e. Rather than showing increased beta-band synchronization to compensate for disrupted sensory feedback, the central nervous system (CNS) instead appears to shift or reduce its reliance on these beta-band oscillatory activity, which is typically associated with interlimb coordination, motor planning, and corticospinal communication, when gravity is absent.\u003c/p\u003e \u003cp\u003eThe absence of significant gravity-related effects in EMG-EMG coherence may reflect the inherent stability of neural synchronization mechanisms under short-term gravitational changes. Coherence measures long-range neural connectivity, which may require more prolonged exposure to altered environments to exhibit measurable adaptations \u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e,\u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e77\u003c/span\u003e\u003c/sup\u003e. Additionally, methodological factors, such as the high-pass filtering inherent in wireless EMG systems, could have attenuated low-frequency components critical for detecting subtle coherence differences in the alpha (8\u0026ndash;13 Hz) and lower beta bands (13\u0026ndash;20 Hz) \u003csup\u003e\u003cspan additionalcitationids=\"CR79\" citationid=\"CR78\" class=\"CitationRef\"\u003e78\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR80\" class=\"CitationRef\"\u003e80\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eInterestingly, while coherence did not show significant modulation, cross-wavelet power analysis revealed notable gravity-related differences, particularly in the beta-band (13\u0026ndash;30 Hz). This suggests that time-frequency methods like cross-wavelet power may be more sensitive to transient neuromuscular adaptations, capturing dynamic fluctuations in interlimb coordination that coherence metrics might overlook\u003csup\u003e\u003cspan additionalcitationids=\"CR42\" citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e. These findings underscore how the CNS adapts under altered gravity: in the absence of normal proprioceptive cues, the usual beta-band neural coupling may be harder to sustain, leading to more variable and less harmonic force output. Partial gravity, on the other hand, provides intermediate levels of loading that can partially restore these typical motor patterns \u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e66\u003c/span\u003e\u003c/sup\u003e. Future studies with longer-duration microgravity exposure or alternative environments (e.g., space missions, bed rest studies) may provide further insights into how neural synchronization mechanisms adapt over time. Additionally, refining EMG processing techniques to enhance low-frequency signal fidelity could improve the sensitivity of coherence analyses in detecting subtle neuromuscular changes related to gravity.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section2\"\u003e \u003ch2\u003eTheoretical Implications\u003c/h2\u003e \u003cp\u003eThese findings contribute to our understanding of coordination dynamics under altered gravity\u003csup\u003e\u003cspan citationid=\"CR81\" class=\"CitationRef\"\u003e81\u003c/span\u003e,\u003cspan citationid=\"CR82\" class=\"CitationRef\"\u003e82\u003c/span\u003e\u003c/sup\u003e. In line with prior evidence that microgravity disrupts proprioceptive feedback \u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e,\u003cspan citationid=\"CR83\" class=\"CitationRef\"\u003e83\u003c/span\u003e\u003c/sup\u003e, our results demonstrate that altered-gravity not only affects overall movement control but also alters the specific frequency-dependent coupling patterns of interlimb EMG signals. The decreased beta-band (13\u0026ndash;30 Hz) cross-wavelet power in 0g implies a shift or attenuation of the usual synchronous neural processes, indicating that typical corticospinal or sensorimotor integration in this band may be less effective without gravitational input \u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e,\u003cspan citationid=\"CR80\" class=\"CitationRef\"\u003e80\u003c/span\u003e,\u003cspan citationid=\"CR84\" class=\"CitationRef\"\u003e84\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eMeanwhile, partial gravity appears to facilitate partial reengagement of those neural mechanisms \u003csup\u003e\u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e65\u003c/span\u003e,\u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e66\u003c/span\u003e,\u003cspan citationid=\"CR85\" class=\"CitationRef\"\u003e85\u003c/span\u003e\u003c/sup\u003e, as suggested by more stable force outputs. By applying cross-wavelet analysis, we advance beyond traditional coherence and time-domain measures \u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e,\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e,\u003cspan citationid=\"CR86\" class=\"CitationRef\"\u003e86\u003c/span\u003e\u003c/sup\u003e, offering a more detailed understanding of how neuromuscular control strategies adjust to gravitational changes.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec18\" class=\"Section2\"\u003e \u003ch2\u003ePractical implications\u003c/h2\u003e \u003cp\u003eThese results carry clear implications for space missions and astronaut training. Microgravity impairs manual control \u003csup\u003e\u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e64\u003c/span\u003e\u003c/sup\u003e, thus, training protocols that incorporate partial-gravity simulations (e.g., via centrifugation) may improve astronauts\u0026rsquo; ability to generate stable force outputs \u003csup\u003e\u003cspan additionalcitationids=\"CR88 CR89\" citationid=\"CR87\" class=\"CitationRef\"\u003e87\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR90\" class=\"CitationRef\"\u003e90\u003c/span\u003e\u003c/sup\u003e. This aligns with previous work indicating that carefully designed training can mitigate some motor deficits seen in space \u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR91\" class=\"CitationRef\"\u003e91\u003c/span\u003e\u003c/sup\u003e. Furthermore, equipment and tasks could be redesigned to better accommodate the lower beta‐band synchronization or to provide augmented feedback that compensates for reduced proprioceptive cues \u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e,\u003cspan citationid=\"CR92\" class=\"CitationRef\"\u003e92\u003c/span\u003e\u003c/sup\u003e. By targeting the frequency bands known to be vulnerable in microgravity (13\u0026ndash;30 Hz), engineers and trainers can help astronauts maintain more precise bimanual coordination in orbit and beyond.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec19\" class=\"Section2\"\u003e \u003ch2\u003eLimitations\u003c/h2\u003e \u003cp\u003eSeveral limitations warrant consideration. First, the participant sample consisted primarily of highly skilled individuals (e.g., fighter pilots and participants with prior parabolic flight experience). While this minimizes variability in motor performance, it limits the generalizability of the findings to broader populations, particularly individuals with less motor training or experience in altered gravity environments \u003csup\u003e\u003cspan additionalcitationids=\"CR94\" citationid=\"CR93\" class=\"CitationRef\"\u003e93\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR95\" class=\"CitationRef\"\u003e95\u003c/span\u003e\u003c/sup\u003e. Consequently, caution is advised when extrapolating these results to astronauts with varying training backgrounds or to entirely untrained individuals.\u003c/p\u003e \u003cp\u003eSecond, the use of parabolic flights provides only short-duration exposures to each gravity level, which may not capture the full range of physiological adaptations that occur during extended space missions \u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. This temporal limitation restricts our understanding of long-term neuromuscular adjustments to sustained microgravity or partial gravity environments.\u003c/p\u003e \u003cp\u003eThird, another potential limitation relates to the use of wireless EMG systems, particularly concerning their built-in hardware filters and wireless transmission characteristics \u003csup\u003e\u003cspan citationid=\"CR78\" class=\"CitationRef\"\u003e78\u003c/span\u003e,\u003cspan citationid=\"CR96\" class=\"CitationRef\"\u003e96\u003c/span\u003e,\u003cspan citationid=\"CR97\" class=\"CitationRef\"\u003e97\u003c/span\u003e\u003c/sup\u003e. These systems apply a default high-pass filter (typically around 20 Hz), which may partially attenuate signals in the alpha (5\u0026ndash;13 Hz) and lower beta (13\u0026ndash;20 Hz) frequency bands. This could result in an underestimation of power in these ranges, potentially affecting the sensitivity of cross-wavelet analyses \u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e,\u003cspan citationid=\"CR98\" class=\"CitationRef\"\u003e98\u003c/span\u003e\u003c/sup\u003e. Additionally, while wireless systems enhance participant mobility, they are susceptible to data packet loss and minor latency fluctuations, which may introduce subtle artifacts, especially in low-frequency bands \u003csup\u003e\u003cspan citationid=\"CR78\" class=\"CitationRef\"\u003e78\u003c/span\u003e,\u003cspan citationid=\"CR96\" class=\"CitationRef\"\u003e96\u003c/span\u003e,\u003cspan citationid=\"CR97\" class=\"CitationRef\"\u003e97\u003c/span\u003e\u003c/sup\u003e. Although these factors were mitigated through careful preprocessing, they represent inherent technical limitations that could influence the interpretation of frequency-domain EMG measures.\u003c/p\u003e \u003cp\u003eFourth, while EMG\u0026ndash;EMG cross-wavelet power is a valuable tool for assessing time\u0026ndash;frequency coupling, potential sources of signal noise, such as electrode shifts, variations in skin impedance, and minor fluctuations in signal quality, could reduce measurement precision \u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e,\u003cspan citationid=\"CR99\" class=\"CitationRef\"\u003e99\u003c/span\u003e,\u003cspan citationid=\"CR100\" class=\"CitationRef\"\u003e100\u003c/span\u003e\u003c/sup\u003e. Variations in electrode placement, signal quality, and calibration of force transducers could affect the accuracy and reliability of the coherence data \u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e,\u003cspan citationid=\"CR99\" class=\"CitationRef\"\u003e99\u003c/span\u003e,\u003cspan citationid=\"CR100\" class=\"CitationRef\"\u003e100\u003c/span\u003e\u003c/sup\u003e. These factors may introduce subtle artifacts that could influence the accuracy of the frequency-domain analyses.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec20\" class=\"Section2\"\u003e \u003ch2\u003eFuture direction\u003c/h2\u003e \u003cp\u003eFurther investigation is needed to pinpoint how intermediate gravity environments, such as those on the Moon (~\u0026thinsp;0.16 g) or Mars (~\u0026thinsp;0.38 g), affect fine motor tasks and EMG coupling. Exploring a broader set of motor tasks (e.g., tool use, maintenance activities, or emergency procedures) and combining cross-wavelet with additional neural imaging methods (EEG, fNIRS) could clarify how the CNS reorganizes under partial gravity. Additionally, research on countermeasures, including novel training regimens and feedback devices, could help astronauts to maintain more effectively bimanual coordination \u003csup\u003e\u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e72\u003c/span\u003e\u003c/sup\u003e. Such work would not only strengthen mission readiness but also offer translational benefits for rehabilitation contexts on Earth, where reduced load or novel sensory conditions (e.g., virtual reality) can similarly challenge motor coordination.\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003eThis study provides critical insights into the neurophysiological effects of altered gravity on bimanual coordination and motor control, with a particular focus on the impact of microgravity and partial gravity conditions. The findings demonstrate that coordination stability and force production are significantly influenced by gravitational loading, with notable impairments observed in microgravity (0 g) and partial restoration of motor control as gravity levels approach Earth-like conditions.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eMoreover, analyses of EMG\u0026ndash;EMG cross-wavelet normalized power revealed reduced interlimb synchronization in the beta frequency band (13\u0026ndash;30 Hz) under microgravity conditions, suggesting that gravity not only affects mechanical performance but also alters the underlying neural coupling mechanisms responsible for coordinated motor output. These findings highlight the importance of gravitational input for maintaining optimal neuromuscular control and emphasize the role of both proprioceptive feedback and central neural processing in adapting to altered gravity environments.\u003c/p\u003e\n\u003cp\u003eThe study\u0026rsquo;s results have practical implications for space missions, particularly regarding astronaut training, equipment design, and operational protocols aimed at mitigating the adverse effects of microgravity on motor performance \u003csup\u003e64\u003c/sup\u003e. While the study has several limitations, including the use of short-duration parabolic flight exposures and a highly skilled participant sample, it provides a strong foundation for future research. Ultimately, this research contributes to a growing body of knowledge on human motor control in altered gravity environments, with relevance not only for space exploration but also for applications in rehabilitation, robotics, and neuromechanics on Earth.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003eParticipants\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTwelve healthy adults (6 males; mean age = 40.2 years, SD = 8.7 years) participated in this study. All participants were confirmed to be right-limb dominant using the Edinburgh Handedness Survey\u003csup\u003e101\u003c/sup\u003e and had normal or corrected-to-normal vision. Individuals with high susceptibility to motion sickness were excluded. Most participants (10 out of 12) had previous parabolic flight experience. Written informed consent was obtained from all participants. The study was approved by the NASA Johnson Space Center Institutional Review Board (IRB) (STUDY00000329), the Texas A\u0026amp;M University IRB (STUDY2024-0425), and the Comit\u0026eacute; de Protection des Personnes Nord Ouest II (Avis no. 22.04602.000171), adhering to the Declaration of Helsinki for research involving human subjects.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eParabolic Flight\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe study was conducted over four parabolic flight days aboard Novespace\u0026rsquo;s Airbus A-310 Zero G. The first flight focused on microgravity (0g), with 30 parabolas during which all 12 participants completed 10 trials. The subsequent three flights (4 participants per flight) covered partial gravity conditions (0.25g, 0.5g, and 0.75g) with 10 parabolas per gravity level. The duration of altered gravity was approximately ~20 seconds (0g), ~30 seconds (0.25g), ~40 seconds (0.5g), and ~50 seconds (0.75g), with pull-up and pull-out phases at 1.8g (~20 seconds each). 1g data were collected pre- and post-flight on the ground, but only pre-flight data from the partial gravity day were analyzed to minimize potential training or fatigue effects.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eApparatus\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eParticipants were sat in modified commercial airline seats equipped with custom-mounted aluminum instrument trays affixed to the armrests, ensuring their elbows were kept in contact with the instrument trays and were positioned at a 90-degree angle for stability (see Figure 6). Each arm was outfitted with an adjustable static force measurement system, comprising a force transducer and an amplifier that converted the applied force into a voltage signal. The force transducers were adjusted so that each participant\u0026rsquo;s wrist contacted the load cells in a position optimal for producing isometric force using the left and right triceps brachii muscles. These signals were recorded using an AD converter (NI USB-6210 Board, National Instruments Corp, Austin, TX, USA) connected to a computer programmed to sample at 200 Hz. Muscle activity was recorded using a wireless electromyography (EMG) system (Delsys Inc., Boston, MA, USA) at a sampling rate of 1000 Hz. Each participant was equipped with two EMG sensors positioned over the belly of the left and right triceps brachii muscles.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eData from four participants were collected simultaneously, with individual force sensors and visual feedback monitors connected to a dedicated laptop per subject. EMG signals from all 4 subjects were synchronized to the same EMG recording base connected to an additional EMG-dedicated laptop. All data were collected and stored using custom LabVIEW software installed on all five laptops. This setup ensured continuous data recording during microgravity or partial gravity periods and an additional 15 seconds during hypergravity (~1.8g) phases. To maintain stability during altered gravity conditions, the trays featured 3D-printed wrist cuffs to hold participants\u0026apos; arms in place, preventing wrist flexion or the lifting of arms off the apparatus. In addition, the trays also featured elbow rests to maintain proper wrist alignment with the load cells. The position of each participant was continuously monitored to ensure that proper alignment was maintained throughout the experiment Participants received real-time feedback on their force production via 3D mobile theater goggles (Bigeyes H3; Vision Electronics Co.), which displayed a cursor representing their output to facilitate performance monitoring and adjustments during tasks.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eBimanual Coordination Tasks\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eParticipants were required to perform two bimanual coordination tasks: 1:1 in-phase task and 1:2 multi-frequency task. The 1:1 task required participants to generate synchronized isometric force pulses with both arms simultaneously while the 1:2 task required participants to produce two force pulses with the right arm for every force pulse generated with the left arm (force coordination patterns shown in Figure 7 A and C). While performing the tasks, participants wore a head-mounted display showing a Lissajous plot to guide performance, consisting in a goal template and a cursor representing the forces produced by each arm (Figure 7 B and D). The cursor moved horizontally (left to right) as force was exerted by the right arm and vertically (bottom to top) as force was exerted by the left arm. The goal template outlined the specific force patterns needed for each coordination task. The size of the template was adjusted individually so that the maximum force required corresponded to 20% of each participant\u0026apos;s maximum voluntary isometric contraction (MVC).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Collection Procedure\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eParticipants were trained on the coordination tasks one day before the first flight to familiarize themselves with the experimental procedures. Training consisted of 14 practice trials for each coordination task (1:1 in-phase and 1:2 multi-frequency), with each trial lasting 30 seconds. Training began with the 1:1 task, followed by the 1:2 task. After training, baseline data were collected with two 30-second trials for each task in the same sequence as the training.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eDuring flight data collection, participants completed two trials of the 1:1 task and three trials of the 1:2 task for each gravitational condition included in the experiment (0g, 0.25g, 0.50g, 0.75g). Trials began at the onset of the microgravity or partial gravity period and continued throughout its duration, extending an additional 15 seconds into the subsequent hypergravity phase. However, for the present analysis, only data collected during the microgravity or partial gravity period were included. For each participant, performance metrics (described below) for each task and gravity level were averaged across trials (two trials for the 1:1 task and three trials for the 1:2 task).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMeasures and data reduction\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data collection and analysis were performed using MATLAB (v2020a, The MathWorks, Inc., Natick, MA). Force time series data were collected from each arm and processed to extract key unimanual and bimanual measures. Signals from the force time series were low-pass filtered with a second-order dual-pass Butterworth filter at a 10 Hz cutoff frequency to remove noise. A 3-point difference algorithm was then used to calculate force velocity and force acceleration from the force time series. Signals were detrended and normalized to a range of ‒1 to 1 for consistency across cycles. Data were analyzed for both unimanual (force harmonicity, mean force, force variability, inter-peak interval (IPI), and inter-peak interval variability (standard deviation of the IPI)) and bimanual (IPI ratio, force-force wavelet coherence, EMG-EMG cross-wavelet power, and EMG-EMG coherence) measures to assess participants\u0026apos; ability to produce and time isometric force pulses across gravity conditions.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003eUnimanual Measures.\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eForce Harmonicity (H)\u003c/em\u003e: Force harmonicity (H) quantifies the harmonic or inharmonic nature of force production by analyzing distortions in the acceleration trace for each limb\u0026rsquo;s force-time series \u003csup\u003e26,27,29\u003c/sup\u003e. To compute H in each trial, non-overlapping windows were established between force-velocity zero-crossings \u003csup\u003e102\u003c/sup\u003e, allowing for half-cycle segmentation of the force data. Within each window, distortions in the force acceleration trace were identified. When a single peak appeared in the half-cycle acceleration trace, H was set to 1, reflecting smooth, undistorted force production. If distortions were present, H was calculated as the minimum-to-maximum acceleration ratio within the window. If the acceleration trace crossed from positive to negative (or vice versa) within the half-cycle, H was set to 0, indicating a disrupted force pattern. To obtain an overall harmonicity score, the H values from each window within a trial were averaged, yielding a global estimate for H. An H index of 1 indicates highly harmonic force production with minimal adjustments, whereas an index of 0 denotes significant inharmonicity and disruptions in force output.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eMean force\u003c/em\u003e: Mean force was calculated to determine the control of force for each arm. It was determined by averaging the absolute force produced during each trial.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eForce Variability\u003c/em\u003e: Force variability was calculated to assess the consistency of force production for each limb within each trial. It was determined by calculating the standard deviation (SD) of the absolute force produced during each trial, with higher variability indicating less consistent force control. Additionally, the coefficient of variation (CV) of force was calculated as the ratio of the SD to mean force for each trial. This measure normalizes the variability relative to the average force output, providing a dimensionless metric to compare relative force consistency across conditions and tasks. Higher CV values indicate greater relative inconsistency in force production.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eInterpeak interval (IPI):\u0026nbsp;\u003c/em\u003eIPI represents the average time between two consecutive force peaks during each trial. IPIs were computed for each limb cycle-by-cycle with each cycle representing every other zero crossing of the force signal.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eIPI variability (SD IPI)\u003c/em\u003e: SD IPI was computed as the standard deviation of IPI within each trial, indicating the stability of timing between peaks.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003eBimanual Measures.\u0026nbsp;\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eInter-Peak Interval Ratio (IPI Ratio)\u003c/em\u003e: The IPI ratio was calculated as a measure of coordination between the two limbs based on the timing of force peaks. This metric provides a temporal measure of goal attainment, independent of coordination tendencies and actual force trajectories. To determine IPI ratio, the inter-peak intervals for each arm during a trial were calculated, and the ratio of the right limb\u0026rsquo;s cycle duration to the left limb\u0026rsquo;s cycle duration was computed. An IPI ratio of 1.0 indicates that the intervals for the right and left arms are equal, as required in the 1:1 task. An IPI ratio of 2.0 indicates that the interval for the left arm is twice that of the right arm, as required in the 1:2 task.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eEMG-EMG Cross-Wavelet Normalized Power:\u0026nbsp;\u003c/em\u003eTo quantify intermuscular neural coupling, we employed Morlet wavelet analysis to calculate the normalized EMG\u0026ndash;EMG cross-wavelet scale-averaged power (EMG\u0026ndash;EMG cross-wavelet normalized power). Following established methodologies \u003csup\u003e41,75,106\u003c/sup\u003e, we used the interference EMG signals from the left and right limbs, computed their cross-wavelet transform, and derived the scale-averaged power using the corresponding equation outlined by Neto et al. (2010)\u003csup\u003e62\u003c/sup\u003e (Equation 2).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\" width=\"470\" height=\"127\"\u003e\u003c/p\u003e\n\u003cp\u003eThis power was then normalized by the total cross-wavelet power across the analyzed frequency range (5\u0026ndash;150 Hz), yielding a percentage value (ranging from 0\u0026ndash;100%) that indicates the relative contribution of each frequency band to the total shared energy between the two EMG signals. The normalized power was subsequently averaged over time to provide a stable measure of neural coupling. For all EMG frequency analysis, we focused on five frequency bands relevant to motor control: 5\u0026ndash;13 Hz (alpha), 13\u0026ndash;30 Hz (beta), 30\u0026ndash;60 Hz (low gamma), 60\u0026ndash;100 Hz (high gamma), and 100\u0026ndash;150 Hz (ultra-high frequency).\u003c/p\u003e\n\u003cp\u003ePrior to analysis, raw EMG signals were band-pass filtered between 20\u0026ndash;450 Hz to eliminate low-frequency drift and high-frequency noise. Motion artifacts were minimized through visual inspection and automated artifact detection algorithms, and a notch filter (50/60 Hz) was applied to suppress power line interference. For high-pass filtered signals, full-wave rectification was conducted to preserve the envelope and oscillatory components critical for subsequent analyses. In the wavelet transform process, we applied a complex Morlet wavelet with a non-dimensional frequency parameter of \u0026omega;₀ = 6, striking a balance between time and frequency resolution\u003csup\u003e107\u003c/sup\u003e. The decomposition covered frequencies from 5 to 150 Hz, with logarithmic spacing to ensure finer resolution at lower frequencies.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eEMG-EMG Coherence\u003c/em\u003e: For EMG\u0026ndash;EMG coherence analysis, we utilized a wavelet-based coherence approach consistent with the methodology applied to force signals\u003csup\u003e42,43,106\u003c/sup\u003e. Coherence was computed for both interference EMG signals and high-pass filtered rectified EMG signals (filtered at 250 Hz and rectified). High-pass filtering at 250 Hz was employed to isolate high-frequency features analogous to amplitude modulation, enhancing the clarity of oscillatory components in the 5\u0026ndash;100 Hz range. This technique helps reduce amplitude cancellation effects, sharpens the resolution of motor unit action potentials, and minimizes movement-related artifacts \u003csup\u003e44,75\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eGiven that the wireless EMG sensors employed in this study incorporate a built-in high-pass filter around 20 Hz, interference EMG signals may not accurately capture low-frequency content below this threshold\u003csup\u003e78,96,97\u003c/sup\u003e. This limitation could result in false negatives when analyzing the alpha (5\u0026ndash;13 Hz) and low-beta (13\u0026ndash;30 Hz) bands, as these signals may lack sufficient low-frequency information. To address this, the high-pass filtered rectified EMG signals preserved critical envelope and oscillatory characteristics, enhancing the detection of coherence in these bands.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eStatistics\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eLinear mixed-effects models (LMMs) were employed to analyze the effects of Gravity (0g, 0.25g, 0.50g, 0.75g, 1g), Task (1:1, 1:2), Limb (right, left)\u003csup\u003e108\u003c/sup\u003e, and Frequency Bands (0\u0026ndash;1 Hz, 1\u0026ndash;4 Hz, 4\u0026ndash;8 Hz, 8\u0026ndash;12 Hz, and 12\u0026ndash;20 Hz for force data; and 5\u0026ndash;13 Hz, 13\u0026ndash;30 Hz, 30\u0026ndash;60 Hz, 60\u0026ndash;100 Hz and 100\u0026ndash;150 Hz for EMG data) on various performance and neuromuscular metrics. Random intercepts for each participant were included to account for repeated measures and within-subject variability. The models were implemented using the lmer function in the lme4 package in R (v. 4.4.1).\u003c/p\u003e\n\u003cp\u003ePerformance measures, including Harmonicity, Mean Force, Force Variability, Inter-Peak Interval (IPI), and IPI Variability were analyzed with Gravity, Task, and Limb as fixed effects, including their two- and three-way interactions. IPI Ratio was analyzed with Gravity and Task as fixed effects, including their interaction (Gravity \u0026times; Task). For the neuromuscular data, both EMG-EMG normalized cross-wavelet power and EMG-EMG coherence were analyzed using repeated-measures ANOVA to assess the effects of Gravity and Frequency Band. The integration of both interference and rectified EMG data in this dual approach enhances the sensitivity and robustness of the analysis, providing comprehensive insights into the neural mechanisms underlying bimanual coordination under varying gravitational conditions \u003csup\u003e41,75\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eModel diagnostics were conducted to ensure assumptions of normality and homogeneity of variances. Residuals were tested for normality using the Shapiro-Wilk test, while Levene\u0026rsquo;s test was used to assess homogeneity of variances. Post hoc analyses were performed using estimated marginal means (from the emmeans package) with Tukey\u0026rsquo;s HSD correction for multiple comparisons. Pairwise comparisons were conducted between all gravity conditions (e.g., 0g, 0.25g, 0.5g, 0.75g, and 1g) to identify significant differences, with no single gravity condition (e.g.,1g) used as a reference for comparison. Statistical results are reported with F values, p-values, and additional post hoc pairwise comparisons including estimates, standard errors (SE), degrees of freedom \u003csup\u003e101\u003c/sup\u003e, and t-ratios. Statistical significance was set at \u0026alpha; = 0.05 for all tests. Results are presented as mean \u0026plusmn; standard deviation (SD) unless otherwise specified, and figures display mean \u0026plusmn; standard error (SE).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAll statistical analyses were performed using R (v4.4.1) within the Visual Studio Code (v1.92.1) environment. Figures and visualizations were generated using GraphPad Prism (v10.2.3) for Windows (GraphPad Software, Boston, Massachusetts USA) and BioRender.com.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe datasets analyzed for this study are publicly available, a repository can be found on GitHub: https://github.com/BHP-Lab/BimCoord/tree/main/Parabolic_Flight_11_12_Task/Data.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the NASA Human Research Program, Grant Number: 80NSSC20K1499. The authors would like to acknowledge NASA, Novespace, and the European Space Agency for their invaluable support in making this experiment possible. Special thanks go to Alexandra Jaquemet (Novespace) for her guidance during the design and execution of the experiment. We also extend our gratitude to Neil Melville \u003csup\u003e57\u003c/sup\u003e for his expertise and assistance throughout all phases of the project. Additionally, we are grateful to Pierre Denise and his colleagues for their support with the IRB procedures and approval. Finally, we thank all participants and collaborators who contributed to the success of this study.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eD.M.K. and A.D-A. conceived the study. Material preparation and operational readiness for the parabolic flight was conducted by D.M.K., M.W., R.A., N.K., T.W., B.J.D., and A.D-A. Data reduction, statistical analysis, and figures were performed by M.W., R.A., N.K., O.P.N., and A. D-A. The draft of the manuscript was written by D.M.K., M.W., R.A., R.R., and A.D-A. All authors reviewed, edited, and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interest.\u003cstrong\u003e\u003cbr\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eClark, T. K., Newman, M. C., Merfeld, D. M., Oman, C. M. \u0026amp; Young, L. R. 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P.\u003cem\u003e et al.\u003c/em\u003e Morlet wavelet transforms of heart rate variability for autonomic nervous system activity. \u003cem\u003eApplied and Computational Harmonic Analysis\u003c/em\u003e \u003cstrong\u003e40\u003c/strong\u003e, 200-206 (2016). https://doi.org/https://doi.org/10.1016/j.acha.2015.07.002\u003c/li\u003e\n\u003cli\u003eClement, G.\u003cem\u003e et al.\u003c/em\u003e Assessing the effects of artificial gravity in an analog of long-duration spaceflight: The protocol and implementation of the AGBRESA bed rest study. \u003cem\u003eFront Physiol\u003c/em\u003e \u003cstrong\u003e13\u003c/strong\u003e, 976926 (2022). https://doi.org/10.3389/fphys.2022.976926\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"npj-microgravity","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npjmgrav","sideBox":"Learn more about [npj Microgravity](http://www.nature.com/npjmgrav/)","snPcode":"41526","submissionUrl":"https://submission.springernature.com/new-submission/41526/3","title":"npj Microgravity","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Bimanual coordination, microgravity, altered gravity, force control, EMG","lastPublishedDoi":"10.21203/rs.3.rs-6131347/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6131347/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eMany of the activities associated with spaceflight require individuals to use both limbs simultaneously to accomplish the task. Motor control, as well as visual performance and spatial orientation are disrupted by gravitational transitions between 1G and 0G, but very little is known about the sensorimotor deficits between 0G and 1G. The objective of this analog-based research effort is to investigate the impact of partial G-levels on bimanual coordination tasks that are operationally relevant for spaceflight. Using parabolic flight to simulate different gravity levels (0g, 0.25g, 0.5g, 0.75g, 1g), participants performed isometric force coordination tasks while electromyography (EMG) data were collected to assess neuromuscular activity. Results showed that at lower gravity levels, force production was significantly reduced, became less harmonic, and more variable, particularly during complex tasks, indicating reduced coordination stability. Additionally, EMG-EMG cross-wavelet power analysis revealed significantly lower beta-band (13\u0026ndash;30 Hz) normalized power in 0g compared to 1g, suggesting weakened neural synchronization between limbs in the absence of gravitational loading. Partial gravity conditions partially restored both force stability and neural coupling, emphasizing the role of proprioceptive feedback in motor control. These findings highlight the importance of gravitational input for maintaining motor coordination and have practical implications for astronaut training, equipment design, and countermeasures to support performance during space missions.\u003c/p\u003e","manuscriptTitle":"Neurophysiological effects of partial gravity on bimanual control: A parabolic flight study","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-03-27 05:33:04","doi":"10.21203/rs.3.rs-6131347/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-08-10T02:46:00+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-08-08T11:21:48+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"127692785372387365673243526757342631306","date":"2025-07-10T07:29:45+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-04-10T18:26:40+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"45354620708726703187747501303377599194","date":"2025-03-18T16:22:51+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"256196327713059183468116381326200377062","date":"2025-03-18T07:07:15+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-03-17T18:11:57+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-03-17T03:25:27+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-03-16T13:37:05+00:00","index":"","fulltext":""},{"type":"submitted","content":"npj Microgravity","date":"2025-02-28T20:50:44+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"npj-microgravity","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npjmgrav","sideBox":"Learn more about [npj Microgravity](http://www.nature.com/npjmgrav/)","snPcode":"41526","submissionUrl":"https://submission.springernature.com/new-submission/41526/3","title":"npj Microgravity","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"746fdafb-1dd8-40b5-b03c-49da1b2b3dc3","owner":[],"postedDate":"March 27th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":45863952,"name":"Biological sciences/Neuroscience"},{"id":45863953,"name":"Biological sciences/Physiology"},{"id":45863954,"name":"Health sciences/Health occupations"}],"tags":[],"updatedAt":"2026-04-20T16:05:28+00:00","versionOfRecord":{"articleIdentity":"rs-6131347","link":"https://doi.org/10.1038/s41526-026-00595-z","journal":{"identity":"npj-microgravity","isVorOnly":false,"title":"npj Microgravity"},"publishedOn":"2026-04-16 15:58:38","publishedOnDateReadable":"April 16th, 2026"},"versionCreatedAt":"2025-03-27 05:33:04","video":"","vorDoi":"10.1038/s41526-026-00595-z","vorDoiUrl":"https://doi.org/10.1038/s41526-026-00595-z","workflowStages":[]},"version":"v1","identity":"rs-6131347","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6131347","identity":"rs-6131347","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}