‘Pushing’ versus ‘holding’ isometric muscle actions; what we know and where to go: A scoping and systematic review with meta-analyses

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Skip to main content Home About Submit ALERTS / RSS Search for this keyword Advanced Search ‘Pushing’ versus ‘holding’ isometric muscle actions; what we know and where to go: A scoping and systematic review with meta-analyses View ORCID Profile Dustin J Oranchuk , View ORCID Profile André R Nelson , View ORCID Profile Danny Lum , View ORCID Profile Alex O Natera , View ORCID Profile Frank N Bittmann , View ORCID Profile Laura V Schaefer doi: https://doi.org/10.1101/2024.11.04.24316609 Dustin J Oranchuk 1 Muscle Morphology, Mechanics, and Performance Laboratory, Department of Physical Medicine and Rehabilitation, School of Medicine, University of Colorado Anschutz Medical Campus , Aurora, CO, United States Find this author on Google Scholar Find this author on PubMed Search for this author on this site ORCID record for Dustin J Oranchuk André R Nelson 2 Institute for Health and Sport, Victoria University , Melbourne, Australia Find this author on Google Scholar Find this author on PubMed Search for this author on this site ORCID record for André R Nelson Danny Lum 3 Sport Science and Sport Medicine, High Performance Sport Institute , Singapore 4 Sport and Exercise Science, School of Allied Health, Human Services and Sport, La Trobe University , Victoria, Australia Find this author on Google Scholar Find this author on PubMed Search for this author on this site ORCID record for Danny Lum Alex O Natera 5 Sport Science, New South Wales Institute of Sport , Sydney Olympic Park, NSW, Australia Find this author on Google Scholar Find this author on PubMed Search for this author on this site ORCID record for Alex O Natera Frank N Bittmann 6 Regulative Physiology and Prevention, Department of Sport and Health Sciences, University of Potsdam , Potsdam, Germany Find this author on Google Scholar Find this author on PubMed Search for this author on this site ORCID record for Frank N Bittmann Laura V Schaefer 6 Regulative Physiology and Prevention, Department of Sport and Health Sciences, University of Potsdam , Potsdam, Germany 7 Center of Orthopaedics and Traumatology, University Hospital Brandenburg, Brandenburg Medical School Theodor Fontane , Brandenburg an der Havel, Germany 8 Faculty of Health Sciences, Joint faculty of University of Potsdam, Brandenburg Medical School Theodor Fontane and Brandenburg Technical University Cottbus-Senftenberg , Brandenburg, Germany Find this author on Google Scholar Find this author on PubMed Search for this author on this site ORCID record for Laura V Schaefer For correspondence: dr.laura.schaefer{at}mhb-fontane.de Abstract Full Text Info/History Metrics Supplementary material Data/Code Preview PDF ABSTRACT Background Pushing/pulling isometric muscle actions (PIMA) are commonly used to assess strength, fatigability, and neuromechanical function, whereas holding isometric muscle actions (HIMA), applied in rehabilitation and performance settings, remain less clearly defined and comparatively understudied. Evidence suggests that PIMA and HIMA may elicit distinct neural and cardiovascular responses, yet inconsistent operational definitions complicate interpretation and application. This review synthesized research directly comparing PIMA and HIMA to clarify their physiological profiles, identify research gaps and explore practical relevance. Methods The protocol was pre-registered with PROSPERO (CRD42024530386). Databases were searched for peer-reviewed studies comparing PIMA and HIMA. Study quality and risk-of-bias were evaluated, and meta-analyses and meta-regressions were performed on time-to-task-failure (TTF), ratings of perceived exertion (RPE), heart rate (HR), and mean arterial pressure (MAP). Results Fifty-four studies (publication year 2012.9±6.9; 1995-2024) were identified (N=919 participants; ∼29.8±10.7 years). Thirty-five studies reported performance measures, 45 examined neural outputs, and 14 assessed cardiovascular or metabolic responses. Meta-analysis revealed longer TTF for PIMA vs. HIMA at the same absolute intensity (n=407; g =−0.74, p <0.001), except for two studies on axial muscles ( g =1.78–3.59, p <0.001). Individual-study patterns suggest diminishing TTF differences at higher intensities; however, since two other studies found clear differences, this may reflect methodological heterogeneity rather than a true intensity effect. No significant differences were identified for HR, MAP, or RPE at relative time points, except for higher RPE at 50%TTF during PIMA. Qualitatively, PIMA was associated with higher peak torques and discharge rates, whereas HIMA was associated with higher burst rates, glucose uptake, and force fluctuation increases. Conclusions These mechanistic distinctions may hold practical relevance as PIMA may be beneficial for prolonged activation and agonist neuromuscular adaptations. In contrast, HIMA could provide diagnostic value, injury-prevention potential, and time-efficient muscular, neural, and cardiovascular adaptations in rehabilitation. Methods varied widely across studies, making additional meta-analyses impossible. Randomized controlled trials are required to confirm the use of PIMA vs HIMA in clinical or performance contexts. Key Points Pushing/pulling and holding isometric actions produce distinct neuromuscular and physiological responses and should not be considered equivalent. Pushing/pulling actions generally support longer force maintenance, with task- and muscle-specific exceptions. Neural, mechanical, and metabolic characteristics differ, with pushing/pulling emphasizing antagonist force output while holding is more neuromuscular complex and metabolically taxing. These differences suggest divergent applications, with pushing/pulling suited to performance goals and holding suited to rehabilitation and specific diagnostic contexts. 1 INTRODUCTION Different muscle action types (i.e., concentric, isometric, eccentric) are commonly used in research to examine acute physiological responses. They are also essential in clinical and sports performance settings to elicit specific morphological, neuromuscular, and performance adaptations through tailored movements and workloads [ 1 , 2 ]. While concentric and eccentric actions are often highlighted for their distinct metabolic costs and adaptation profiles, including neuromuscular cross-education, muscle fascicle length, and region-specific hypertrophy [ 2 ], the importance of differentiating between two distinct types of isometric muscle actions has been underemphasized. Foundational studies have long demonstrated that different types of isometric tasks exhibit distinct neurophysiological [ 3 – 6 ] and metabolic [ 7 , 8 ] characteristics. More recent investigations further highlight the need to recognize and distinguish the two types of isometric muscle actions, as they exhibit unique acute responses across multiple neurological and neuromuscular parameters [ 9 – 12 ], potentially influencing outcomes and application [ 1 , 13 ]. Research examining muscle action types has existed since at least 1895, with Adolph Fick examining cardiac muscle under isometric conditions [ 14 ]. However, isometrics are commonly thought of as a single category of muscle action. There has been a recent surge in studies comparing ‘two modes’ of isometric muscle actions [ 9 , 10 ] and hybrid muscle actions such as ‘quasi-isometrics’ [ 15 – 19 ] and ‘Adaptive Force’ [ 20 – 28 ]. The two distinctive forms are known by several names, including pushing/pulling (PIMA) and holding (HIMA) isometric muscle actions [ 9 ], force and position tasks [ 3 ], and overcoming and yielding isometrics [ 29 ]. Regardless of the semantics, PIMA involves exerting force against an immovable object (e.g., a strain-gauge, a wall, or a power rack). Conversely, HIMA is defined as maintaining a set position while resisting an external force (e.g., gravity, inertially loaded weight, cable stack, training partner, examiner) ( Figure 1 ). Download figure Open in new tab Figure 1. Examples of single-joint pushing/pulling and holding isometric muscle actions. (images adapted from Rudroff et al. [ 7 ] and Pasco et al. [ 30 ]). While PIMAs are far more common in sports medicine and physiological research [ 1 ], typically as a means to assess maximal force production capacity [ 31 ], several recent studies have utilized HIMA in rehabilitative and performance contexts [ 32 – 36 ]. In parallel with this expanding use, practitioners have shown increasing interest in whether these two forms of isometric muscle action carry distinct practical implications [ 37 – 41 ]. Despite early work demonstrating that force-and position-based tasks differ in neuromechanical control and patterns of fatigue, notable gaps and inconsistencies remain in the literature. Specifically, although multiple studies have reported differences in mechanical [ 4 , 42 ], neural [ 9 , 11 ], perceptual [ 43 , 44 ] and cardiometabolic variables [ 7 , 45 , 46 ], there is still no clear or consistently applied differentiation between PIMA and HIMA, and the physiological, mechanistic and contextual bases for these task-dependent differences remain poorly characterized. Therefore, the primary purpose of this review was to systematically gather and evaluate research directly comparing PIMA and HIMA to determine whether objective measures support their distinction. A secondary aim was to use the available evidence to inform recommendations for researchers and practitioners. 2 METHODS The protocol for the present review was prospectively registered at the National Institute of Health Research PROSPERO (CRD42024530386). This systematic review conformed to the "Preferred Reporting Items for Systematic Reviews and Meta-Analyses" (PRISMA) guidelines [ 47 ]. Therefore, no Institutional Review Board approval was necessary. 2.1 Inclusion and exclusion criteria All included studies met the inclusion criteria: 1) published in English; 2) peer-reviewed journal publications or doctoral dissertations; 3) human participants; and 4) evaluated the mechanical, neurological, perceptual, or physiological effects of PIMA and HIMA. Studies were excluded if they: 1) were conference proceedings (papers, posters, presentations); and 2) did not include both PIMA and HIMA. 2.2 Literature search strategy The initial search utilized CINAHL, Embase, MEDLINE, PubMed and Web of Science databases from inception to January 2023. Terms were searched for within the article title, abstract, and keywords using conjunctions ’OR’ and ’AND’ with truncation ‘*’. Boolean phrases comprising the search terms were: (isometric* OR static*) AND (contraction* OR action* OR task*) AND (holding OR position) AND (pushing OR force). No limiters or filters were applied. Secondary searches included: a) screening the reference lists of included studies; b) forward citation tracking; c) search alerts to monitor new search results; and d) contacting the most common authors of included outputs. Database searching was re-run regularly; with the final search in August 2025. 2.3 Study selection Database results were uploaded to the EndNote reference manager (vX9.0.3; Clarivate Analytics, Philadelphia, PA) and utilized to remove 3080 duplicates, while 22 duplicates were manually identified and removed. The remaining publications were uploaded to the Rayyan systematic review software, where titles and abstracts were independently screened for eligibility by two authors blinded to each other’s decisions. Three authors then independently screened the full texts to finalize eligibility for inclusion. All disagreements were resolved through discussion, with the lead author making the final decision. Because terminology surrounding PIMA and HIMA is inconsistent, all studies were evaluated against the operational definitions established for this review. Specifically, we examined whether the task involved force generation against an external resistance (PIMA) or position maintenance against an externally applied load (HIMA), regardless of the terminology used by the original authors (e.g., “pushing”, “force task”, “holding”, “position task”). 2.4 Data extraction A spreadsheet was used to record extracted data, including: 1) publication details; 2) participant information; 3) key equipment; 4) condition details; 5) means, standard deviation and, where available, raw data; and 6) reported statistical outputs. Corresponding authors were contacted via email if insufficient data were reported. Three authors independently completed data extraction, with the other three cross-checking the spreadsheet. Any differences were resolved via discussion, with the lead author finalizing each decision. WebPlotDigitizer (v4.1; https://automeris.io/WebPlotDigitizer/ ) was used to extract data from figures when raw values were unavailable. Images were exported from PDFs as high-resolution files. Means were estimated from the midpoint of error bars; when bars were asymmetric (e.g., triangles), the midpoint of the symmetrical error bars (SD or SE) was used instead. Upper error bars were digitized unless obscured by overlapping symbols, in which case lower bars were used. When error bars were embedded within the mean symbol, the largest plausible error was estimated by manually overlaying the bar width, which could overestimate the error for data points with minimal variability. When not otherwise stated, we assumed no missing samples and calculated standard deviations from reported standard errors using the study’s sample size. 2.5 Risk-of-bias and study quality Studies that met the inclusion criteria were assessed to determine their risk-of-bias and methodological and statistical reporting quality based on established scales. Although several risk-of-bias tools are available, most tools are designed to evaluate intervention studies. Therefore, these assessments include criteria (e.g., control group, participant allocation, participants lost before follow-up) that are irrelevant to observational and acute studies. Thus, we created unique risk-of-bias and quality assessments by combining the most relevant criteria from standard tools while adding other qualities deemed valuable by the present authorship. The classification of the research study design was based on that presented by Manjali and Gupta [ 48 ] and used questions adapted from the Centre for Evidence-Based Medicine [ 49 ]. Degree of internal validity was assessed by incorporating the Joanna Briggs Institute checklist for quasi-experimental studies [ 50 ], with the 2024 revision of the critical appraisal tool. Additional publication reporting-quality questions were posed based on statistical guidelines [ 51 ], participant characteristics (e.g., demographics, evaluated characteristics) and training status stratification [ 52 ]. Hence, the six risk-of-bias domains were ‘Clear cause-and-effect relationships’, ‘Thorough study population and participant details’ (e.g., age, sex, training and health status, maximal voluntary isometric contraction (MVIC)), ‘Reliable exposure’, ‘Reliable outcome measures’, ‘Selective reporting’, and ‘Overall’. The methodological and reporting quality domains were ‘Repeated condition and outcome measurements’, ‘Appropriate statistical analysis’, ‘Exact p -values’, ‘Effect sizes’, ‘Confidence intervals’, and ‘Overall’. Two authors independently completed the risk-of-bias and quality assessments, with a third author settling any discrepancies. Two authors who have published widely on PIMA vs HIMA were not involved in the risk-of-bias or quality assessments. The results were uploaded to the Robvis RoB 2.0 visualization tool [ 53 ], producing the ‘traffic-light’ and ‘summary’ plots. Paint-3D (Microsoft Corporation, Redmond, WA) was used to edit and combine plots for clarity. 2.6 Statistical analysis 2.6.1 Calculation of effect sizes Raw data, effect sizes, and p -values were extracted to a spreadsheet. Extracted raw data were used to calculate standardized mean differences (Cohen’s effect size [ d ]) between PIMA and HIMA conditions. Standardized mean differences were interpreted as: trivial0.80 [ 54 ]. Studies already reporting effect sizes were double-checked where possible. Results were distilled to statistically significant ( p <0.05), or ‘non-trivial’ ( d≥ 0.20) between-condition differences. Standardized mean differences were calculated for all possible time points and variables using the following equation: Percentage differences between PIMA and HIMA conditions were also calculated and reported where appropriate using the following equation: 2.6.2 Meta-analytic synthesis Meta-analytical procedures were performed using SPSS (Version 29.0, IBM Corp; Armonk, NY). Upon careful examination of the extracted data, the authors agreed that time-to-task-failure (TTF), ratings of perceived exertion (RPE), heart rate (HR), and mean arterial pressure (MAP) data were collected and reported with sufficient detail and consistency to warrant meta-analysis. For example, while several studies examined myoelectrical variables, some reported percent change, while others provided standardized effect sizes; thus, meta-analysis was contraindicated. Other variables, such as silent periods (N=3, interspike intervals (N=2), burst-rates (N=4), mechanomyography (MMG) (N=3), oxygen concentrations (N=1), kinematics (N=1), and glucose uptake (N=1), were not reported in enough studies to warrant meta-analysis. Each study examining TTF provided mean and standard deviation data required to calculate standardized mean differences between PIMA and HIMA. RPE, HR, and MAP data were often reported in figures and extracted (see data extraction). Data were uploaded directly into statistical software to avoid potential human error. However, standardized mean differences were confirmed by manual calculation. Including multiple effects from a single study violates the assumption of independence, as effects from the same study are likely to be more similar than those from other studies and would influence statistical power. However, when only a small proportion of studies report multiple outcomes, dependence might be ignored in a sensitivity analysis [ 55 , 56 ]. For TTF, four of 23 studies reported multiple outcomes with overlapping participants at different intensities [ 8 , 57 ], forearm positions [ 58 ], or muscle groups [ 59 ]. Sensitivity analysis revealed only minor differences in meta-analysis results with single and combined outcomes. Therefore, the forest plots illustrate single outcomes, while statistical results are provided for single and combined outcomes. For combined analysis, the effect sizes from a single study utilizing multiple loading intensities, age groups, or forearm orientations were averaged as a single comparison. Hedges’ g effect size with adjusted standard error was selected to correct for studies with small sample sizes. A random-effects model with restricted maximum likelihood was chosen because of substantial inter-study variability in age, sex, muscle groups, and loading intensities. Interpretation of the pooled effect size with 95% confidence intervals (95%CI) was based on the thresholds: trivial<0.20, small=0.20–0.49, medium=0.50–0.79, and large≥0.80 [ 54 ]. Were appropriate, bubble plots and subgroup analyses were employed to examine the effect of loading intensity, muscle group, or time relative to TTF. Muscle groups were ordered by typical muscle mass and coded: 1-finger abductors, 2-wrist extensors, 3-ankle dorsi-flexors, 4-elbow flexors, 5-elbow extensors, 6-knee extensors, and 7-trunk extensors [ 60 , 61 ]. 2.6.3 Statistical heterogeneity The I 2 statistic was used to evaluate study heterogeneity, representing the percentage of total variation in estimated effects across studies due to heterogeneity rather than chance. The I 2 statistic was interpreted as low50% heterogeneity [ 62 ]. Funnel plots were visually examined for excessive asymmetry or multiple studies outside the funnel, suggesting the potential for publication bias [ 63 ]. 3 RESULTS The systematic literature search ( Figure 2 ) yielded 54 studies fulfilling the inclusion criteria [ 3 – 12 , 20 – 25 , 30 , 42 – 46 , 57 – 59 , 64 – 92 ]. Forty-two studies were found through database searching [ 5 , 6 , 8 – 12 , 20 – 25 , 30 , 43 – 46 , 57 , 59 , 65 , 66 , 68 – 84 , 88 , 90 , 91 ], while 12 publications were included following reference searches [ 3 , 4 , 7 , 42 , 58 , 64 , 67 , 85 – 87 , 89 , 92 ]. Nine studies not found via database search were from the same laboratory group [ 7 , 42 , 58 , 64 , 67 , 85 – 87 , 89 ]. Thus, the primary investigator of the relevant laboratory was contacted, resulting in an additional study [ 4 ]. Download figure Open in new tab Figure 2. PRISMA diagram summarizing the literature search strategies. 3.1 Study characteristics The 54 studies included 919 participants (∼619 male, ∼263 female, [mean±standard deviation] ∼29.8±10.7 years) [ 3 – 12 , 20 – 25 , 30 , 42 – 46 , 57 – 59 , 64 – 92 ] with publication years ranging from 1995 [ 91 ] to 2024 [ 20 ], and mean publication year of 2012.9±6.9. Demographic data are approximate because sex [ 54 , 75 , 91 ] and age [ 54 , 91 ] were missing or only partially reported in several studies [ 3 , 8 , 11 , 12 , 42 , 77 , 89 ]. Ten studies were exclusively male [ 5 – 7 , 12 , 20 – 22 , 25 , 58 , 82 ], 41 were mixed sex [ 3 , 4 , 8 – 11 , 23 , 24 , 30 , 42 – 46 , 57 , 59 , 65 – 79 , 81 , 83 – 90 , 92 ], with three not reporting sex [ 64 , 80 , 91 ]. Forty-six studies examined young adults (mean: 21-33.5 years) [ 3 – 6 , 8 – 12 , 21 – 25 , 42 – 46 , 57 – 59 , 65 , 67 – 71 , 74 – 90 , 92 ], two included older participants (mean: 64-72 years) [ 20 , 73 ], four young and old sub-groups (mean: 47-52 years) [ 7 , 30 , 66 , 72 ], with two not reporting age [ 64 , 91 ]. Single studies concentrated on specific samples, such as strength and endurance athletes [ 21 ], soccer players [ 23 ], patients with knee osteoarthritis [ 20 ], or participants undergoing experimental joint pain [ 43 , 44 ]. All 54 studies utilized single-joint actions at the same absolute load [ 3 – 12 , 20 – 25 , 30 , 42 – 46 , 57 – 59 , 64 – 92 ]. The most common agonist muscle group was the elbow flexors (n=25) [ 3 , 4 , 6 , 8 , 21 – 24 , 30 , 46 , 57 – 59 , 67 , 70 , 71 , 73 , 75 , 76 , 78 , 82 , 85 , 87 , 90 , 91 ], followed by the knee extensors (n=8) [ 7 , 12 , 43 , 44 , 68 , 83 , 84 , 86 ]. Finger abductors (n=6) [ 5 , 11 , 65 , 77 , 80 , 89 ], wrist extensors (n=3) [ 42 , 64 , 66 ] and flexors (n=2) [ 45 , 64 ], ankle dorsi- (n=3) [ 72 , 74 , 81 ] and plantar-flexors (n=3) [ 69 , 79 , 92 ], elbow extensors (n=4) [ 9 , 10 , 25 , 91 ], hip/trunk extensors (n=2) [ 59 , 88 ], knee flexors (n=1) [ 20 ], and hip flexors (n=1) [ 23 ] were also agonist muscle groups investigated. Bittmann et al. [ 23 ] (elbow/hip flexion) and Thomas et al. [ 59 ] (trunk extension/elbow flexion) compared PIMA and HIMA across multiple agonist groups. One study included different forearm postures during elbow flexions [ 85 ], while another examined elbow flexion with supinated and neutral forearms [ 58 ]. Muscle action intensities differed widely, ranging from 3.8-100% (mean: ∼34.6±27.8%) of pushing or pulling MVIC. Most studies (n=40) investigated intensities ≤30% MVIC [ 3 – 8 , 11 , 30 , 42 – 44 , 57 – 59 , 64 – 74 , 77 – 80 , 82 – 92 ], while 14 studies investigated intensities of 40-90% MVIC [ 5 , 8 – 10 , 12 , 25 , 45 , 46 , 57 , 65 , 69 , 75 , 76 , 81 ], and five studies used adaptive muscle action of >100% MVIC (gradual increase in applied force from submaximal to supramaximal intensities in one trial) [ 20 – 24 ]. Twelve included multiple intensities [ 3 , 5 , 8 , 10 , 30 , 57 , 65 , 66 , 69 , 75 , 79 , 91 ]. Isometric tasks were taken to failure in 30 studies [ 4 , 5 , 7 – 10 , 20 – 25 , 42 , 46 , 57 – 59 , 67 , 68 , 70 , 72 – 74 , 76 , 78 , 84 , 86 – 88 , 90 ]. 3.2 Risk-of-bias and study quality Overall risk-of-bias was minimal ( Figure 3 ), with 44 studies assessed as ‘low’ [ 5 – 12 , 20 – 25 , 30 , 42 – 46 , 58 , 59 , 65 – 68 , 70 , 72 , 74 – 85 , 87 – 90 ], ten as having ‘some concerns’ [ 3 , 4 , 57 , 64 , 69 , 71 , 73 , 86 , 91 , 92 ], and zero studies flagged for ‘high’ overall risk-of-bias. Five studies lacked detail regarding study participant [ 3 , 64 , 69 , 91 , 92 ], and were substantially older (2005.2±8.1, 1995-2015) than the average publication date (2012.9±6.9), suggesting improved reporting and peer-review standards. Neither Buchanan and Lloyd [ 91 ] nor Baudry and Enoka [ 64 ] reported sex, age, height, mass, or any other demographic information or baseline characteristics. Inclusion and exclusion criteria, physical activity level, and baseline characteristics (e.g., strength, power) were not reported by Garner et al. [ 69 ]. Finally, neither Magalhães [ 92 ] nor Mathis et al. [ 3 ] reported height, mass, inclusion/exclusion criteria, or any baseline characteristics. While no other studies or categories received a ‘high’ risk-of-bias classification, the most common concern was the lack of in-house or intra-study reliability statistics for most outcome measures [ 3 – 8 , 11 , 30 , 42 – 45 , 57 – 59 , 64 – 75 , 79 – 92 ]. Like the participant details category, studies reporting reliabilities [ 9 , 10 , 12 , 20 – 25 , 46 , 76 – 78 ] have substantially more recent publication dates (2020±4.6) than those that did not (2010.7±6) [ 3 – 8 , 11 , 30 , 42 – 45 , 57 – 59 , 64 – 75 , 79 – 92 ]. Download figure Open in new tab Figure 3. Risk-of-bias assessment illustrations of individual studies ( a ), and all 54 studies pooled ( b ). Overall quality scores were mostly ‘medium’ [ 3 – 8 , 11 , 30 , 42 – 45 , 57 – 59 , 64 – 76 , 78 – 90 ], with 12 studies receiving ‘high’ scores [ 9 , 10 , 12 , 20 – 25 , 46 , 77 , 92 ] ( Figure 4 ). The oldest study [ 91 ] was deemed ‘low’ quality. While all studies were considered to have utilized appropriate statistical analyses (e.g., parametric vs non-parametric tests, use of post-hoc corrections), several only sometimes [ 7 , 20 , 21 , 43 , 57 , 58 , 76 , 79 , 84 , 86 , 90 ] or never [ 4 , 59 , 68 , 73 , 75 , 87 , 91 ] reported exact p -values. Fifteen studies provided effect sizes for all critical conditions and outcome measures [ 9 , 10 , 12 , 20 – 25 , 46 , 59 , 76 , 77 , 90 , 92 ], while confidence interval reporting was only fully present in four studies [ 20 , 21 , 25 , 92 ]. Interestingly, only two studies achieving ‘high’ quality were over eight years old [ 77 , 92 ], with a mean publication year of 2020.6±3.4 [ 9 , 10 , 12 , 20 – 25 , 46 , 77 , 92 ], highlighting the relative improvement in methodological and statistical reporting quality in recent years. Download figure Open in new tab Figure 4. Methodological and reporting quality of individual studies (a), and all 54 studies pooled (b). 3.3 Summary of isometric muscle action differences An overview of the primary non-meta-analytic outcomes, including the total number and types of comparisons (e.g., TTF, RPE increases, discharge rates) and the frequency of statistically significant or potentially meaningful differences, is presented in Table 1 . All comparisons were at the same absolute load, with quasi-identical joint angles/positions. 360 comparisons were identified, with all comparisons performed at the same absolute load. Fifty-two comparisons were significantly higher for PIMA ( p <0.05). In contrast, 93 comparisons were significantly higher for HIMA. 201 comparisons were statistically similar between conditions. Notably, an opposite trend emerged when examining standardized effect sizes without considering p-values, with 72 comparisons favoring PIMA, 50 favoring HIMA, and 45 trivial effects ( d or g <0.20); partially caused by missing effect sizes when significantly higher values were reported for HIMA. View this table: View inline View popup Download powerpoint Table 1. Overview of outcomes, including the number of outcomes, direction of statistical significance ( p >0.05) and standardized effect size. 3.4 Performance and mechanical parameters The 35 studies examining performance and biomechanical differences between PIMA and HIMA are detailed in Supplementary Table 1 [ 4 – 10 , 12 , 20 – 24 , 42 , 44 , 46 , 57 – 59 , 67 , 68 , 70 – 74 , 76 , 78 , 82 , 84 – 88 , 90 ]. Twenty-three studies compared TTF [ 4 , 5 , 7 – 9 , 42 , 46 , 57 – 59 , 67 , 68 , 70 , 72 – 74 , 76 , 78 , 84 , 86 – 88 , 90 ], while two studies investigated the failure rate during paired personal interactions [ 10 , 25 ]. Twenty-two studies quantified force or position fluctuations or variability throughout each condition [ 4 – 6 , 8 , 12 , 42 , 44 , 57 , 58 , 67 , 68 , 70 – 74 , 82 , 84 – 88 ], 20 studies measured MVIC reductions [ 5 – 8 , 21 , 42 , 57 , 67 , 68 , 70 – 74 , 78 , 82 , 85 , 86 , 88 , 90 ], five compared the maximal muscle action force capacity [ 20 – 24 ] and 13 compared RPE [ 4 , 5 , 57 , 58 , 68 , 70 – 74 , 82 , 86 , 90 ]. 3.4.1 Time-to-task-failure Twenty-three studies investigating TTF (n=407) showed almost exclusively longer TTFs for PIMA (718±252 s) vs. HIMA (588±202 s), with a mean percentage difference of −22.4±30.2% (range: −56.2 to 61.5%) [ 4 , 5 , 7 – 9 , 42 , 46 , 57 – 59 , 67 , 68 , 70 , 72 – 74 , 76 , 78 , 84 , 86 – 88 , 90 ]. Although single outcomes were based on partially overlapping or similar participants, the methods (high/low intensities, different forearm positions, different muscles) and results differed vastly. Thus, a combination of outcomes would result in considerable information loss, including levelling of single outcomes. According to Becker [ 56 ], and Park and Beretvas [ 55 ] dependence can be ignored if only a few studies report multiple outcomes. Sensitivity analysis showed only small differences in Hedges’ g of meta-analyses of single vs combined outcomes ( g =−0.74 vs −0.70). To avoid loss of information, the forest plot ( Figure 5a ) illustrates the 30 single outcomes across 23 studies. TTF was significantly longer during PIMA with a moderate to large effect ( g= −0.74, 95%CI: −1.18 to −0.30, p <0.001, n=407). This significant effect is highlighted by all but two appendicular-focused outputs (both elbow flexors) falling to the right of the neutral (grey) line [ 57 , 90 ]. Combining outcomes provided a similar result ( g =−0.70, 95%CI: −1.13 to −0.27, p <0.001, n=358). Download figure Open in new tab Figure 5. Meta-analysis forest plot ( a ) comparing time to task failure (TTF) between PIMA and HIMA at matched absolute target forces with funnel plot ( b ) and bubble chart regressions for loading intensity ( c ) and appendicular muscle subgroups ( d ). Twenty-six of 30 outcomes demonstrated shorter TTF for HIMA with percentage differences from ‒56.2% to ‒1.3% ( g =−3.38 to −0.02). Twenty-four outcomes showed non-trivial effect sizes ( g ≥0.20) [ 4 , 5 , 7 – 9 , 42 , 46 , 57 – 59 , 67 , 68 , 70 , 72 – 74 , 76 , 78 , 84 , 86 , 87 ], while two were negligible ( g <0.20) [ 5 , 58 ]. The latter occurred for the first dorsal interosseous with intensities of 60% MVIC [ 5 ] and elbow flexors with vertical forearm position at 20% MVIC [ 58 ]. The remaining four outcomes showed the opposite (longer TTF for HIMA) with percentage differences of 1-61% ( g =0.07-3.59) [ 57 , 59 , 88 , 90 ]. Two of these studies examined axial muscles (trunk extensors) and showed longer TTF for HIMA with large effects (n=34; g =2.63, 95%CI: 0.86-4.40, p <0.001) [ 59 , 88 ]. For appendicular muscles, 26 of 28 outcomes showed higher TTF for PIMA, with only Williams et al. [ 90 ] reporting the opposite (54%, g =0.75, 15% MVIC, elbow flexors). Rudroff et al. [ 57 ] found similar between-muscle action TTFs at 60% MVIC (%Δ=1%, g =0.07, elbow flexors). Meta-analysis of TTF with only appendicular muscles, including all outcomes, demonstrated a large effect favoring PIMA (n=373; g =−0.90, 95%CI: −1.18 to −0.62, p <0.001). However, in the three studies that tested multiple intensities, the PIMA–HIMA difference in TTF progressively narrowed as absolute load increased [ 5 , 8 , 57 ]. Across the three studies assessing multiple intensities, non-weighted Hedges’ g values were large and negative at 20% MVIC (−3.38, −1.35, −0.70) but contracted markedly at 60% MVIC (0.07, −0.18, −0.54), indicating attenuation of the PIMA–HIMA difference with increasing load ( Figure 5a ). However, these trends are contradicted by Schaefer et al. [ 9 ], who utilized high intensities (80% MVIC), and reported significantly longer TTF with PIMA vs HIMA ( g =−1.16). The above meta-analyses were highly heterogeneous (I²=66% [appendicular], 84% [axial], 85% [combined], 88% [all outcomes]), as confirmed via funnel plot ( Figure 5b ), supporting the use of the random-effects model. Subgroup analysis and meta-regression bubble plots determined negligible effects of loading intensity ( Figure 5c ) and appendicular muscle size ( Figure 5d ). It must be noted that 23 of 30 investigations used intensities ≤30% MVIC [ 4 , 5 , 7 , 8 , 42 , 57 – 59 , 67 , 68 , 70 , 72 – 74 , 78 , 84 , 86 – 88 , 90 ]. Five of the remaining seven performed at higher intensities (45-80% MVIC) showed moderate to high effect sizes ( g =−1.37 to −0.35) [ 8 , 9 , 46 , 57 , 76 ], while the other two were negligible [ 5 , 57 ]. Related to TTF, Schaefer and Bittmann examined the ‘failure rate’ during personal interaction of the elbow extensors at 90% MVIC (one partner performed PIMA and the other performed HIMA, in randomized order) [ 10 ]. In most trials (85%, p <0.001), the partner performing HIMA left the isometric position first, indicating that the pushing partner could maintain the set position longer, supporting the above results for higher intensities. 3.4.2 Fluctuations of force and position Twenty-two studies (25 outcomes) investigated fluctuations of force or position [ 4 – 6 , 8 , 12 , 42 , 44 , 57 , 58 , 67 , 68 , 70 – 74 , 82 , 84 – 88 ]. Sixteen studies (18 outcomes) compared fluctuation increases between muscle actions [ 4 , 6 , 42 , 58 , 67 , 68 , 70 – 74 , 82 , 85 – 88 ]. Of these, ten outcomes showed significantly higher increases for HIMA ( d =0.62-1.96) [ 4 , 6 , 58 , 67 , 70 , 73 , 74 , 82 , 85 , 87 ], four showed lower increases for HIMA ( d =−5.19 to −0.63) [ 42 , 68 , 71 , 86 ], while four outcomes were non-significant [ 72 , 85 , 88 ]. The other six studies (eight outcomes) examined fluctuations over the entire trial period [ 5 , 8 , 12 , 44 , 57 , 84 ]. Of these, six showed significantly lower fluctuations in HIMA (d=−3.16 to −0.53) [ 46 , 47 , 76 , 80 ], one showed significantly higher fluctuations in HIMA (d=4.05) [ 59 ], and one was non-significant (20%MVIC) [ 84 ]. While difficult to quantify without meta-analysis, no clear pattern emerged regarding absolute intensity when comparing PIMA and HIMA differences in force or position fluctuations, suggesting that relative intensity may not significantly influence these observations. 3.4.3 Maximal torques and strength declines Only one research group compared the maximal torques of HIMA (HIMA max ) and PIMA (PIMA max =MVIC) [ 20 – 24 ]. Three studies showed significantly lower HIMA max in healthy participants for elbow extensors ( d =−0.74) [ 24 ], elbow flexors ( d =−2.37) [ 21 ], and knee flexors ( d =−2.62) [ 20 ]. Nineteen studies investigated MVIC declines pre-post HIMA or PIMA tasks to failure [ 5 – 8 , 42 , 57 , 67 , 68 , 70 – 74 , 78 , 82 , 85 , 86 , 88 , 90 ]. All 25 outcomes were non-significant, though wide-ranging ( d =−0.73-1.57, −40.2-31.3%). Three outcomes (two studies) reported significance following 90%TTF tasks with larger MVIC declines following HIMA ( p ≤0.02, d =1.48-2.60, 33.2-50%) [ 7 , 67 ]. Considering the values of relative decline irrespective of the significance and TTF epoch, 15 outcomes showed higher magnitudes of decline for HIMA ( d =0.01-1.57, 13±10%) [ 5 , 7 , 8 , 42 , 57 , 67 , 68 , 71 , 78 , 85 , 88 , 90 ], where 13 revealed lower declines following HIMA ( d =0.06-0.73, −17±12%) [ 6 , 8 , 57 , 67 , 70 , 72 – 74 , 82 , 85 , 86 ], One study considered the decline in PIMA max and HIMA max after 30 repeated adaptive supramaximal HIMA trials, with HIMA leading to a 133% greater decline than PIMA [ 21 ]. 3.4.4 Ratings of perceived exertion Eleven studies investigated RPE during TTF trials [ 4 , 5 , 57 , 58 , 68 , 70 , 72 – 74 , 86 , 90 ] with two reporting RPE throughout a trial not taken to failure [ 71 , 82 ] RPE data was extracted from the figures of several studies [ 4 , 5 , 57 , 58 , 68 , 70 , 72 – 74 , 86 , 90 ]. Meta-analysis revealed that RPE did not differ significantly at the start or end of TTF (relative time point of total duration) (n=194, g =−0.02 to 0.07, p =0.43-0.84) ( Figure 6a ) [ 4 , 5 , 57 , 58 , 68 , 70 , 72 – 74 , 86 , 90 ]. However, RPE was significantly higher for PIMA at 50% TTF (n=164, g= −0.31, 95%CI: −0.55 to −0.07, p =0.01) ( Figure 6a ) [ 4 , 5 , 57 , 58 , 68 , 72 – 74 , 86 ]. Bubble plots indicate a positive effect of intensity ( Figure 6b ) and a negative effect of appendicular muscle size ( Figure 6c ), with reverse effects at task initiation (clear) and task failure (low). Considering the significantly shorter TTF for HIMA, the RPE increase rate was higher for HIMA for most studies ( Figure 7 ), further supported by two studies reporting faster RPE increases for HIMA over a set time [ 71 , 82 ]. Apart from two outcomes (both 50%TTF) [ 58 , 72 ], no apparent intramuscular differences were found with all other confidence limits crossing the overall effects. Furthermore, unlike TTF, there was no apparent effect of intensity within the studies with multiple loads [ 5 , 57 ]. Download figure Open in new tab Figure 6. Meta-analysis forest plot ( a ) comparing ratings of perceived exhaustion (RPE) between PIMA and HIMA with bubble chart regressions for loading intensity ( b ) and appendicular muscle subgroups ( c ). Download figure Open in new tab Figure 7. The relative difference in the rate of increase of ratings of perceived exertion (RPE) between HIMA and PIMA. The rate of increase was calculated from the start, end and TTF values for PIMA and HIMA, respectively. Relative differences=(HIMA-PIMA)/PIMA. 3.5 Neural and neuromuscular parameters The 45 studies examining neural or neuromuscular differences between PIMA and HIMA are detailed in Supplementary Table 2 [ 3 – 11 , 25 , 30 , 42 – 45 , 57 , 58 , 64 – 67 , 69 – 92 ]. Unlike the mechanical (above) and metabolic (below) variables, more than minimal pooling of findings via meta-analyses or otherwise was impossible due to the exceptional variety of collection methods and data processing. Nearly all studies (n=41) employed surface electromyography (sEMG) [ 3 – 5 , 7 , 8 , 11 , 30 , 42 – 45 , 57 , 58 , 64 – 67 , 69 – 92 ], while five included needle EMG [ 6 , 30 , 82 , 85 , 89 ], 12 employed intramuscular EMG [ 4 , 5 , 43 , 58 , 65 , 70 , 71 , 73 , 80 , 82 , 87 , 91 ], and three used MMG and mechanotendography (MTG), which mirror oscillatory central activity [ 9 , 10 , 25 ]. Three studies assessed neural function via electroencephalography (EEG) [ 25 , 44 , 83 ]. Twelve studies included peripheral nerve stimulation (electrical, mechanical or magnetic) [ 3 , 11 , 42 , 45 , 64 – 66 , 77 , 78 , 80 , 90 , 92 ], while four studies employed transcranial magnetic stimulation (TMS) [ 3 , 77 , 78 , 90 ]. The diverse data processing methods and outcomes included EMG or MMG amplitudes [ 4 , 7 – 10 , 42 , 44 , 57 , 58 , 64 – 67 , 69 – 71 , 73 , 78 , 82 , 85 – 87 , 91 ], root-mean-square (RMS) [ 5 , 43 , 45 , 72 , 74 – 76 , 83 , 84 , 88 , 90 ], frequency/power [ 6 , 9 , 10 , 25 , 45 , 57 , 76 , 84 , 86 , 88 ], burst rates [ 4 , 72 – 74 ], burst durations [ 4 , 73 ], discharge rates [ 6 , 30 , 43 , 67 , 71 , 82 , 85 , 89 ], interspike interval CV [ 30 , 67 , 71 , 85 ], recruitment rates and thresholds [ 30 , 67 , 75 , 82 ], H-reflex [ 78 ], heteronymous facilitation [ 42 , 64 – 66 , 77 , 80 , 92 ], D1-inhibition [ 42 , 64 , 66 , 92 ], D2-inhibition [ 92 ], T-reflex amplitude [ 65 ], silent period [ 3 , 77 , 78 , 90 ], and coactivation ratios [ 7 , 8 , 42 , 57 , 66 , 70 , 72 , 74 , 81 , 85 , 86 ]. Three studies investigated the coherence of muscle and brain activity [ 25 , 44 , 83 ]. Thirty-one studies examined antagonist muscle groups [ 3 , 5 , 7 , 8 , 30 , 42 – 44 , 57 , 58 , 65 – 67 , 70 – 76 , 78 , 80 – 87 , 91 , 92 ], while 34 assessed synergists [ 3 – 5 , 7 – 10 , 25 , 42 – 44 , 57 , 58 , 64 – 67 , 70 – 74 , 77 , 80 , 82 – 88 , 90 – 92 ]. Four studies had specific focuses, including injection-induced experimental joint pain [ 43 , 44 ], residual force enhancement [ 81 ], and skin and core temperature manipulation [ 45 ]. 3.5.1 Neuromuscular activity EMG amplitude differences (aEMG or RMS) were mostly non-significant for agonist muscles. For example, only five of 48 relevant outcomes [ 4 , 5 , 7 , 8 , 42 , 57 , 58 , 64 – 67 , 69 – 76 , 78 , 83 – 88 , 90 , 91 ] resulted in greater agonist activation during PIMA ( d =0.66-6.79, 25.5-38.1%) [ 4 , 57 , 58 , 73 , 78 ], whereas eight outcomes were greater during HIMA ( d =0.22-0.72, 11.8-25%) [ 67 , 72 , 87 , 91 ]. Eight of 17 outcomes [ 4 , 5 , 57 , 67 , 72 – 74 , 76 , 78 , 85 , 86 , 90 ] showed a significantly greater increase of EMG amplitude in agonists during HIMA ( d =0.46-0.99, 0.1-3566%) [ 5 , 57 , 67 , 72 , 74 , 86 ], with none favoring PIMA. Regarding synergist muscles, two of 15 outcomes [ 4 , 8 , 57 , 65 , 67 , 70 , 74 , 88 , 91 ] reported greater EMG amplitude during HIMA ( d =0.42-0.72, 22.8-41.4%) [ 57 ], with two favoring PIMA ( d =0.46, −21.6%) [ 74 , 91 ]. Five of 10 outcomes [ 4 , 42 , 57 , 67 , 72 , 74 ] had greater synergist EMG amplitude increases in EMG amplitude during HIMA ( d =0.43-3.16, 0.5-81%) [ 4 , 57 , 72 , 74 ], with the other five showing no significant differences. For antagonist muscles, of 17 relevant outcomes [ 7 , 8 , 42 , 64 – 67 , 70 , 71 , 75 , 76 , 78 , 83 – 87 ], only one was significant, with greater EMG amplitude for PIMA [ 84 ]. In four of nine relevant outcomes [ 5 , 67 , 72 – 74 , 76 , 78 ] the increase in antagonist EMG was significantly higher during HIMA ( d =0.40-0.82, 63-600%) [ 72 , 74 , 76 ]. Agonist EMG burst rate was significantly greater during HIMA in five of seven outcomes ( d =0.20-0.69, 35.7-210.5%) [ 4 , 72 – 74 ]. One study considered the antagonist burst rates, without significant between-task differences [ 73 ]. Discharge rates were significantly lower for HIMA in eight of 12 outcomes ( d =0.52-4.56, −28 to −10.5%) [ 30 , 67 , 71 , 82 , 85 ] while the remaining four were non-significant (e.g., in older participants or for neutral forearm position) [ 30 , 82 , 85 , 89 ]. Discharge rates declined significantly more during HIMA in all outcomes [ 71 , 82 ]. Interspike interval CV was greater or increased faster during HIMA in five of seven outcomes ( d =0.57-2.31, 23.9-625%) [ 30 , 67 , 71 , 85 ], with a single outcome favoring PIMA with large target force differences in young participants ( d =0.64, −20.3%) and a non-significant result in older participants [ 30 ]. The H-reflex amplitude significantly decreased more, and faster during HIMA ( d =1.35, 72%).[ 78 ] Heteronymous facilitation was greater for HIMA in nine of 11 outcomes ( d =0.21-1.32, 5.22-34.6%) [ 64 – 66 , 77 , 80 , 92 ]. One outcome was significantly lower ( p =0.049) for HIMA at task failure [ 42 ]; differences in older adults were non-significant [ 66 ]. D1-inhibition was lower for HIMA in five of seven outcomes ( p ≤0.042, d =−0.71 to −3.6, −14.3 to −34%) [ 42 , 64 , 66 , 92 ]. One study revealed a significantly lower D1-inhibition during HIMA after 120s, with a reversed direction of effect at task termination ( d =1.57, 33%) [ 42 ]. D1-inhibition did not differ significantly between tasks for old participants [ 66 ]. The coactivation ratio was non-significant between muscle actions in ten of 13 outcomes [ 7 , 8 , 42 , 57 , 66 , 70 , 74 , 85 , 86 ]. Significantly greater coactivation was found during HIMA wrist extensions for older participants [ 66 ] and ankle dorsiflexion [ 72 ]. Schaefer and Bittmann used MMG and MTG to examine the elbow extensors during 80% and 90% MVIC muscle actions in a single-person setting [ 9 ] or during inter-participant interaction [ 10 ]. PIMA led to significantly greater triceps MMG amplitude [ 9 ], while HIMA showed significantly greater power in the MTG of the triceps tendon across 8-29 Hz frequencies [ 9 ]. For the single-person setting, the amplitude of the oblique muscle varied significantly less between HIMA vs PIMA trials [ 9 ]. In the paired setting, the obliques amplitude variation within-trials was significantly higher for HIMA [ 10 ]. While HIMA led to a significantly lower power of obliques’ MMG ( d =0.28-0.39, −33 to −50%), PIMA showed a significantly higher power-frequency ratio ( d =0.36-0.66, 24.7-57.4%) and frequency variation between trials [ 10 ]. 3.5.2 Brain activity Three studies included measures of brain activity via EEG [ 25 , 44 , 83 ]. Poortvliet et al. [ 44 , 83 ] investigated the knee extensors (10% MVIC, 30s) and reported non-significant differences between tasks for corticomuscular coherence (only central areas of EEG were used that approximated the motor cortex of the left hemisphere), but significantly lower cortico-cortical coherence for HIMA (four of five outcomes; p <0.001, d =0.09-0.27, −5 to −14.5%) and significantly higher beta-band EEG power of the left hemisphere ( p <0.05, d =0.11, 0.68%) [ 83 ]. Schaefer and Bittmann [ 25 ] examined the coherence between EEGs (central, left and right areas), MMGs (triceps brachii and ipsilateral abdominal external oblique muscles; statistics did not consider muscles separately) and force-signal during interpersonal actions of elbow extensors (70% MVIC of the weaker partner, fatiguing, n=2). Significantly higher values were revealed for HIMA regarding EEG coherence (central and left areas) to the partner’s MMG (inter-brain-muscle coherence), force-EEG-coherence (central and right area) and the frequency of inter-brain-synchrony (central vs left and central vs right) and inter-brain-muscle-coherence (EEG right vs MMG), respectively ( p ≤0.047, d =0.36-2.67, 5.1-67.6%) [ 25 ]. 3.5.3 Special factors Coletta et al. [ 45 ] reported greater RMS of the EMG signal for HIMA at modest heating and cooling conditions (±0.5°C) during 3s muscle actions. However, non-significant muscle action differences were found during 60s muscle actions at the higher heating (1°C) condition [ 45 ]. It must be noted that the RMS was significantly higher for HIMA at baseline. Marion and Power [ 81 ] compared PIMA and HIMA following active lengthening of the dorsi-flexors to examine residual force enhancement, with no significant differences between isometric tasks for any outcome (activation reduction, neuromuscular economy, coactivation). Experiments led by Poortvliet investigated the effects of hypertonic saline injections into the infrapatellar fat pad to temporarily generate knee pain during low-intensity (∼10%) knee extensor muscle actions [ 43 , 44 ]. The first study considered 35 single-motor-units (n=5), which were assessed across all conditions (no pain, pain, HIMA, PIMA) in a single session [ 43 ]. Additionally, 189 single-motor-units (n=11) were assessed over two sessions. For the 35 single-motor-units, the no-pain state showed similar RMS and discharge rates between HIMA and PIMA, where the discharge rate variability tended to be higher for PIMA ( p =0.05). Under pain, the decline in discharge rate was greater for PIMA. Furthermore, discharge rate variability between no-pain and pain states only changed significantly for PIMA. Considering all 189 single-motor-units, discharge rate was significantly higher for HIMA. However, discharge rate decreased under pain for both tasks, with a trend towards lower discharge rates for PIMA. Moreover, the number of single-motor-unit discharge rates which changed (>10% increase or decrease) from no-pain to pain-state states was twice as high for PIMA (44.9%) than HIMA (22.2%) [ 43 ]. The second study by Poortvliet et al. found no significant muscle action differences for corticomuscular coherence [ 44 ]. However, only PIMA resulted in significant changes between no-pain and pain states, with lower corticomuscular coherence under pain. Additionally, beta band power decreased while gamma band increased between no-pain and pain during PIMA. Thus, Poortvliet et al. suggested that brain activity is less affected by pain during HIMA [ 44 ]. 3.6 Cardiovascular and metabolic parameters Fourteen studies examining cardiovascular or metabolic differences between PIMA and HIMA are detailed in Supplementary Table 3 [ 4 , 5 , 7 , 8 , 46 , 57 , 58 , 70 , 72 – 74 , 82 , 86 , 87 ]. Eleven studies assessed HR and MAP [ 4 , 5 , 57 , 58 , 70 , 72 – 74 , 82 , 86 , 87 ], allowing for meta-analysis. Seven utilized the elbow flexors [ 4 , 57 , 58 , 70 , 73 , 82 , 87 ], while others employed the dorsi-flexors [ 72 , 74 ], knee extensors [ 86 ], or first dorsal interossei [ 5 ]. Two studies compared parameters of muscle oxygenation using near-infrared spectroscopy on biceps and triceps brachii during elbow flexions at 20% or 60% MVIC [ 8 ], or an O2C ® spectrophotometer on biceps brachii during elbow flexions at 60% MVIC [ 46 ], with both studies reporting non-significant between-task differences. Another study employed computed tomography with venous glucose injection to assess standardized glucose uptake of the agonist (knee extensors), antagonist, and synergist muscles at 25% MVIC [ 7 ]. While all differences were non-significant in the older population ( d =0.13 to 0.26, −7.7 to −4.3%), HIMA resulted in greater glucose uptake into agonist ( d =0.68, 32.6%), antagonist ( d =0.47, 15.4%), and hip ( d =0.19, 9.7%) muscles of the young group [ 7 ]. HR and MAP data were extracted from the figures of nine studies [ 4 , 5 , 57 , 58 , 70 , 72 – 74 , 86 ]. Meta-analyses of 11-15 outcomes (7-9 studies; 29.2±15.7% MVIC, range: 15-60%) found trivial differences in HR (n=136-170, g =−0.11 to 0.15, p =0.12-0.96) and MAP (n=136-170, g =0.04-0.18, p =0.07-0.45) at initiation, 50% TTF and at task failure ( Figure 8a , 9a ) [ 4 , 5 , 57 , 58 , 70 , 72 – 74 , 86 ]. While not reaching statistical significance, MAP at task failure was slightly greater during HIMA (n=170, g= 0.18, 95%CI: −0.01 to 0.37, p =0.07). Examining bubble plots is complicated by relatively few studies. However, HR tended to be higher for PIMA for larger appendicular muscles at task failure, with the reverse for smaller muscle groups ( Figure 8c ). The opposite appears true when examining MAP during the initial phases (0% and 50% of TTF): the larger the muscle group, the more HIMA seems to elevate MAP when compared to PIMA ( Figure 9c ). Minimal differences between muscle groups were observed for either HR or MAP, with only a single output’s confidence interval falling outside the overall effect ( Figure 9a ) [ 72 ]. Similarly, potential within-study intensity effects cannot be reliably inferred from the degree of confidence-interval overlap [ 5 , 57 ]. Finally, HR and MAP meta-analyses were almost completely homogeneous (all I 2 <0.001). Download figure Open in new tab Figure 8. Meta-analysis forest plot ( a ) comparing heart rate (HR) between PIMA and HIMA with bubble chart regressions for loading intensity ( b ) and appendicular muscle subgroups ( c ). Download figure Open in new tab Figure 9. Meta-analysis forest plot ( a ) comparing mean arterial pressure (MAP) between PIMA and HIMA with bubble chart regressions for loading intensity ( b ) and appendicular muscle subgroups ( c ). The rate of HR and MAP increase was higher during HIMA for most studies ( Figure 10 ) [ 4 , 5 , 57 , 58 , 70 , 72 – 74 , 86 ]. These data suggest metabolic conditions or consequences contributed to the mostly briefer TTF for HIMA. Download figure Open in new tab Figure 10. The relative difference between HIMA and PIMA regarding the rate of increase in heart rate (HR; dark blue) and mean arterial pressure (MAP; light blue). Note: The rate of increase was calculated from the start, end and TTF values for PIMA and HIMA, respectively. Relative differences=(HIMA-PIMA)/PIMA. 4 DISCUSSION In sports science and medicine, it is uncommon to differentiate the two isometric muscle action types. PIMA (the most common isometric muscle action) is usually assessed to quantify muscle strength. However, focus is increasingly shifting towards HIMA, which is being utilized more frequently by sports medicine and performance practitioners aiming to improve musculotendinous morphology and neuromuscular function [ 37 – 41 ]. Despite a wealth of studies comparing PIMA and HIMA, it is difficult for researchers and practitioners to understand their potential applications due to the wide range of methods and data reporting. Therefore, we identified and presented all studies directly comparing the isometric muscle action types. The first question was whether two types of isometric muscle action exist. Based on this review, the answer is yes : HIMA and PIMA can be differentiated by objective measures. When examining the 54 studies comparing both isometric types, our findings can be separated into those supported by meta-analysis and those derived from qualitative synthesis. First, meta-analytic results indicated that (1) PIMA leads to longer TTF at the same relative intensity (except for postural axial muscles), although this difference may narrow at higher intensities; and that (2) no consistent differences were evident for RPE, HR, or MAP at matched relative time points. Second, qualitative evidence from the broader literature suggested several additional patterns. PIMA was generally associated with larger force fluctuations, higher discharge rates, greater D1-inhibition, and higher peak torques. HIMA tended to show higher heteronymous facilitation, greater EMG burst rates, higher interspike interval CV, greater glucose uptake in young participants, and faster increases in force or position fluctuations, EMG amplitude, RPE, HR, and MAP. HIMA may also lead to greater declines in MVIC and higher coactivation. Non-significant or unclear differences were observed for absolute EMG amplitude and frequency, MEPs, and oxygenation. Finally, several studies reported differences between HIMA and PIMA in cortico-cortical and corticomuscular coherence, as well as differential responses to experimentally induced joint pain. These qualitative findings should be interpreted cautiously, as most variables beyond TTF, RPE, HR, and MAP were not amenable to meta-analysis and often relied on single-study comparisons or heterogeneous methodologies. 4.1 Distinguishing holding and pushing isometric muscle actions 4.1.1 Muscle fatigability TTF at the same intensity was the most frequently investigated parameter, resulting in the clearest findings. TTF was significantly longer in appendicular muscles for PIMA, with a large meta-analytic effect ( g =0.90). These findings suggest several factors must contribute to reduced endurance with HIMA. However, 12 of 30 outcomes were non-significant, with a trend suggesting that the phenomenon was more evident at low intensities (≤30% MVIC), thus, the difference may be attenuated with increasing intensity [ 5 , 8 , 57 ]. Only six studies investigated TTF with intensities ≥50% MVIC [ 5 , 8 , 9 , 46 , 57 , 76 ] with non-significant differences between the isometric types for five outcomes. Schaefer et al. [ 9 ], who found significantly shorter TTF for HIMA at 80% MVIC, used a pneumatic device to exert an external force during the holding task against the participant without permitted position corrections. In contrast, other studies employed inertial loads, with four explicitly allowing position correction [ 5 , 8 , 57 , 76 ]. It is suggested that minor muscle shortenings during these position corrections might alter neuromuscular control processes in favor of PIMA. Dech et al. [ 46 ] supports this, as dumbbell-loaded HIMA were examined without and with brief, subtle muscle twitches at 60% MVIC. TTF during HIMA with twitches was larger than without and did not differ from PIMA [ 46 ]. However, some studies investigating low intensities also allowed position corrections during HIMA, finding significant differences. Consequently, intensity could play a role in whether minor muscle shortenings can alter neuromuscular control. Differences between the isometric types, even at high intensities, are further supported by the investigation during paired measurements (90% MVIC) [ 10 ]. In most cases (85%), the partner performing HIMA failed first. As the settings for HIMA and PIMA were identical (load compliance, i.e., position stability and correction possibilities), the differences must have resulted from the assigned tasks. Axial muscle TTF was significantly longer during HIMA [ 59 , 88 ], with minimal between-muscle differences (all subgroup confidence intervals crossing the overall effect), likely reflecting different habitual neuromuscular functions compared with appendicular muscle groups. Axial muscles are routinely engaged in stabilizing and “holding” the torso during standing, walking, and upright sitting, and therefore habitually perform tasks analogous to HIMA. In contrast, appendicular muscles more commonly execute voluntary movements resembling PIMA. In addition, during axial muscle testing, participants must support not only the external load, but also a substantial portion of their body mass, which may have caused the actual intensity experienced to exceed the nominal target. Although the available results are limited to two studies, these patterns suggest that axial and appendicular muscles may need to be considered separately in both assessment and treatment contexts. The significant TTF differences between HIMA and PIMA suggest that, at least for appendicular muscles, HIMA is more challenging to maintain than PIMA. Hence, factors contributing to this phenomenon must be present. One factor might be the relative proportion of peak force achieved via each muscle action. The intensity in nearly all studies was based on PIMA max (MVIC). HIMA max has been estimated to be ∼77% of PIMA max [ 20 , 21 , 24 ]. Thus, the absolute loads were identical, but the relative loading presumably was not. Fluctuations in force and/or position tended to increase more rapidly during HIMA compared to PIMA, whereas absolute fluctuations were generally greater during PIMA. Similar findings for TTF and force decrements revealed no substantial intermuscular differences, suggesting minimal influence of total muscle mass or body region. However, the relatively small number of studies and outcome measures limits the ability to draw definitive conclusions about between-muscle differences. Overall, the question remains about why lower peak forces and shorter TTF occur during HIMA. 4.1.2 Neurophysiological, metabolic, and cardiovascular considerations In line with shorter TTF, several parameters increased faster during HIMA (e.g., fluctuations, RPE, MAP, HR, EMG amplitude, burst rate), indicating that HIMA is more strenuous than PIMA. Since the perceived effort and MAP are mediated by central processes [ 72 , 93 ], HIMA is assumed to require greater central processing. This is further underpinned by the absence of differences in muscle oxygenation as a possible reason for task differences [ 8 , 46 ]. Studies have shown that HIMA involves greater supraspinal and spinal activation compared to PIMA, as evidenced by, for example, spinal processing reflected in earlier and greater changes in motor-unit recruitment and rate coding during HIMA [ 4 , 5 , 57 , 67 , 74 , 75 , 82 , 86 ]. Moreover, the lower D1-inhibition and higher heteronymous facilitation during HIMA indicate reduced presynaptic inhibition and enhanced reliance on proprioceptive input [ 42 , 64 – 66 , 77 , 80 , 92 ]. Presynaptic inhibition is primarily controlled by supraspinal centers and sensory feedback [ 94 ], reinforcing the relevance of supraspinal pathways when distinguishing between isometric tasks. Accordingly, a strategy for HIMA was suggested “in which supraspinal centers choose to enhance the contribution of muscle afferents to the synaptic input that converges onto spinal motor neurons” [ 66 , 95 ], aligning with findings that HIMA requires greater proprioceptive information [ 11 , 77 , 82 ]. These patterns indicate that HIMA engages more complex central modulation and sensorimotor integration than PIMA as suggested by several researchers [ 3 , 5 , 11 , 12 , 25 , 30 , 57 , 58 , 64 , 70 , 73 , 75 , 82 , 87 ]. The given justification is the higher instability compared to the PIMA setting, requiring higher reflex responsiveness [ 64 ]. Indeed, 45 of 53 studies used inertial loading, or similar methods, to realize HIMA, allowing movement across muscle shortening and lengthening. In contrast, four studies employed a pneumatic device that impedes muscle shortening [ 4 , 18 , 20 , 23 ], while another four utilized personal interaction [ 10 , 22 , 23 , 25 ]. In those studies, the ‘stability’ or ‘freedom-to-move’ was similar between tasks. Nevertheless, HIMA and PIMA still showed different outcomes, suggesting that, at least for those studies, the differences between the isometric types could not be explained by stability or degrees of freedom. Thus, other factors must contribute to the muscle action differences. While load compliance (i.e., force vs position control) plays a major role in shaping neuromuscular demands, it is important to clarify that HIMA and PIMA are not synonymous with closed- and open-chain kinetic exercises. Both HIMA and PIMA can be performed under either chain condition, depending on joint fixation, limb support, or load configuration. The key distinction lies in whether the individual is actively generating force against an immovable object (PIMA) or reactively stabilizing against an imposed force (HIMA), regardless of limb constraint. Equating these with closed- or open-chain exercises risks oversimplifying their neural and sensorimotor distinctions, which are central to this review. Overall, our findings support the argument that intrinsic differences in neural control, rather than task mechanics alone, underpin the distinct responses observed between PIMA and HIMA. 4.1.3 Neural mechanisms during HIMA and PIMA The meta-analytic finding of shorter TTF during HIMA raises the question of underlying neural mechanisms. Available evidence suggests differences at both supraspinal and spinal levels. At the supraspinal level, Poortvliet et al. [ 44 , 83 ] reported lower cortico-cortical coherence but higher beta band power during HIMA, which they interpreted as reflecting greater subcortical involvement. Schaefer and Bittmann [ 25 ] found higher inter-brain-muscle coherence for HIMA. Despite large methodological differences between these studies, both suggest that HIMA and PIMA engage qualitatively different neural processing. These supraspinal differences may be linked to distinct roles of afferent input. A potential paradox arises from Poortvliet et al.’s [ 44 , 83 ] finding that experimental pain produced greater motor modifications during PIMA than HIMA—seemingly inconsistent with HIMA involving heightened sensory processing [ 4 , 5 , 43 , 58 , 74 , 84 ]. Yunoki et al. [ 11 ] proposed that sensory input unrelated to movement is suppressed during PIMA (especially cutaneous information), while HIMA maintains greater openness to proprioceptive feedback. Under this framework, HIMA would be selectively more responsive to proprioceptive input than PIMA. Supporting this interpretation, Kirimoto et al. [ 77 ] and Mottram et al. [ 6 , 82 ] demonstrated increased sensorimotor modulation and interspike interval variability during HIMA, indicating enhanced feedback-dependent regulation. Collectively, these findings suggest that HIMA relies on continuous proprioceptive updating integrated across supraspinal and spinal levels, consistent with models of optimal feedback control [ 95 , 96 ]. The persistence of TTF differences even in paired settings with identical mechanical conditions and high target force [ 10 ] reinforces that these reflect genuine differences in control strategy rather than task artifacts. Accordingly, classification, diagnostics, and training interventions should not treat isometric actions as a monolith, but rather tailor strategies based on whether the muscle action is more ’holding’ or ’pushing’ in nature. In particular, HIMA may serve as a more sensitive probe of sensorimotor integration capacity. Confirmation through further research with standardized protocols is needed. 4.1.4 HIMA and PIMA as ‘stopped’ or ‘restricted’ lengthening or shortening actions Classically, isometric muscle action is considered a single category, defined by constant muscle length. However, our findings demonstrate that HIMA and PIMA differ systematically across multiple parameters, challenging the notion of a uniform isometric muscle action. Moreover, even during isometric tasks, fascicles shorten slightly while tendons elongate [ 97 , 98 ], and mechanical oscillations reflect that muscles continuously alternate between shortening and lengthening [ 9 , 10 , 99 ], which are similar to those during concentric and eccentric actions [ 99 ]. These observations have led to the proposal of the ’stopped action hypothesis’: HIMA and PIMA may represent restricted eccentric and concentric actions, respectively [ 9 , 10 ]. During PIMA, the participant pushes against immovable resistance—a concentric action that is mechanically prevented from producing movement. During HIMA, the participant resists an external load that would cause lengthening if force were insufficient—also known as eccentric quasi-isometric muscle action [ 15 – 19 ]—resembling a restricted eccentric action. This framework has explanatory value because lengthening muscle actions are known to require more complex neural control strategies than shortening ones [ 100 ]. If HIMA indeed resembles a restricted eccentric action, this could account for the proposed differences in motor control strategies between HIMA and PIMA [ 9 – 12 , 21 – 23 , 25 – 28 , 58 ]. However, direct comparison between isometric and anisometric actions is complicated by the fact that the latter involves macro changes in muscle length and joint movement. Despite this limitation, some similarities exist between the isometric types and their suggested anisometric counterparts. Discharge rates are systematically lower during lengthening than shortening actions [ 100 ]; similarly, discharge rates were lower during HIMA than PIMA in eight of 12 outcomes [ 30 , 67 , 71 , 82 , 85 ]. The faster RPE increase during HIMA [ 4 , 5 , 57 , 58 , 68 , 70 – 74 , 82 , 86 ] parallels the force overestimation reported for eccentric vs. concentric actions [ 93 ]. Modulations of neural activation during lengthening actions are assumed to involve supraspinal and spinal mechanisms, predominantly mediated by spinal pre- and postsynaptic inhibitory mechanisms [ 100 ]. Similar mechanisms have been suggested for HIMA [ 4 , 5 , 42 , 57 , 58 , 64 – 67 , 71 , 74 , 75 , 77 , 78 , 80 , 82 , 86 , 92 ]. Higher central activity (EEG) accompanies reduced EMG activity during lengthening versus shortening actions [ 101 ]; similarly, higher beta band power was found for HIMA [ 83 ], though data remain limited. Finally, voluntary activation is lower during maximal lengthening than shortening actions, limiting the realization of the muscle’s intrinsic force capacity [ 100 ]. A similar mechanism may explain the lower peak torque for HIMA compared to PIMA [ 20 , 21 , 24 ]. H-reflex data are insufficient for comparison, with only one study reporting decreased values during HIMA vs. PIMA [ 78 ], which, despite limited evidence, aligns with the typically depressed H-reflex during active and passive lengthening actions [ 93 , 100 ]. However, some findings are inconsistent with the hypothesis above. MEP and post-TMS silent period are reduced and briefer during lengthening versus shortening actions [ 100 ], but HIMA versus PIMA results are mixed: MEP differences were non-significant [ 3 , 77 , 78 ], and silent periods were briefer [ 77 ], non-significant [ 78 , 90 ], or longer [ 3 ]. In summary, several parallels support the ’stopped action hypothesis’, though the limited evidence base calls for further research. 4.1.5 HIMA with variable external forces – A greater challenge for motor control Research on Adaptive Force (AF) further supports the assumption of more complex control strategies during HIMA. AF refers to the neuromuscular capacity to adapt to increasing external loads and is assessed via HIMA with a non-stationary load that ramps to and beyond HIMA max , requiring constant sensorimotor updating [ 102 ]. As previously mentioned, adaptive HIMA max (i.e., maximum isometric AF) was significantly lower than PIMA max [ 20 , 21 , 24 ]. Moreover, adaptive HIMA max is reduced significantly in healthy participants by stimuli such as negative imagery or odors [ 26 – 28 ], by muscle spindle slack induced through specific pre-conditioning [ 22 , 23 ], and in patients with knee osteoarthritis [ 20 ]. Whether PIMA max would react similarly has not been directly tested, though findings from Bittmann et al. [ 23 ] suggest otherwise. In that study, adaptive HIMA max of the elbow flexors was assessed after two pre-conditioning protocols: (1) muscle action in a lengthened position with passive return to the testing (middle) position (provoking spindle slack), and (2) the same, but followed by a brief voluntary contraction (20% MVIC) in the testing position. Only protocol (1) reduced the adaptive HIMA max significantly (−47%); the brief voluntary contraction in protocol (2) restored full holding capacity. Critically, PIMA inherently begins with voluntary force production, preventing spindle slack from affecting performance. This suggests that HIMA’s reliance on reactive, feedback-dependent control renders it selectively vulnerable to proprioceptive perturbations—a vulnerability likely not shared by PIMA. Taken together, these findings indicate that HIMA—particularly adaptive HIMA—involves heightened sensitivity to afferent input, which supports the suggested differences in sensorimotor modulation between the two isometric types. The higher sensitivity likely arises from the need to regulate length-tension control while adapting to external loads continuously. This process particularly requires the integration of proprioceptive afferents via feedback control mechanisms engaging subcortical structures (thalamus, cerebellum, basal ganglia). Since these structures also process nociceptive and emotional inputs, such stimuli may influence adaptive HIMA performance. While this sensitivity enables precise adaptation, it also increases vulnerability to interfering inputs (e.g., nociception, mental distress). Under such perturbations, the neuromuscular system may fail to maintain the required position under increasing load, resulting in yielding. This has potential diagnostic implications. Assessing adaptive HIMA max , particularly under standardized stimulus conditions, could help identify impairments in sensorimotor integration that remain undetected by conventional strength testing (PIMA). Such impairments may be relevant to injury risk assessment, rehabilitation monitoring, or the detection of neuromuscular dysfunction. Since adaptive HIMA max also responds to supportive stimuli, its assessment could help identify effective and individualized therapy approaches. However, further research is needed to establish clinical utility and to clarify how different stimuli affect HIMA versus PIMA. 4.2 Practical applications 4.2.1 Injury prevention The findings of this review may have implications for injury prevention, though these remain speculative. Most injuries occur without contact [ 103 ] during muscle lengthening, when a muscle must hold or decelerate an external force [ 104 ]. Injuries induced by muscle action typically do not result from pushing/pulling isometric or shortening actions [ 106 ]. Despite this, muscle strength is usually assessed via PIMA, neglecting the adaptive holding component which is closer to the nature of injury mechanisms. In a review, Ullmann et al. [ 105 ] claimed that the positive effects of isometric exercise on injury prevention are assumed because of post-activation potentiation. However, potential distinctions between HIMA and PIMA were not considered. This raises the question whether assessing HIMA—particularly adaptive HIMA—could help identify individuals at risk. Various stimuli, including negative emotions, proprioceptive disruption and nociception reduce adaptive HIMA [ 22 , 23 , 26 – 28 ]. If such reductions also occur in real-world settings, they might explain why non-contact injuries can occur suddenly during routine movements under normal load (e.g., running, direction changes, landing) that have been performed thousands of times without incident. However, no study has directly investigated whether reduced holding capacity (muscle instability) increases injury risk or whether full holding capacity (muscle stability) prevents injuries. The evidence for isometric exercise in injury prevention is limited. Two low-quality studies suggest reduced injury rates with added isometric training [ 106 , 107 ], but the type of isometric exercise was not specified. Complex exercises involving reactive stabilization, such as the Nordic hamstring exercise, may be more relevant. Two reviews found significant reductions in hamstring injuries [ 108 , 109 ], though another reported inconclusive evidence of a protective effect [ 110 ]. Thus, it is hypothesized that (1) adaptive HIMA could uncover functional impairments undetected by conventional strength testing and (2) such impairments may be relevant for injury risk. Notably, reductions in adaptive HIMA are likely due to interfering inputs (e.g., nociception, emotional stress) rather than insufficient muscle strength, so conventional exercise may not address the underlying causes. These hypotheses require empirical validation. 4.2.2 Rehabilitation Clinical interest in isometric exercise for conditions like tendinopathy has increased [ 111 ], though studies do not distinguish between HIMA and PIMA [ 32 , 35 , 112 – 120 ]. One study compared isometric dorsiflexion—assumed to resemble HIMA—with other interventions for Achilles tendinopathy and found significantly greater improvement compared to isotonic exercise and rest [ 112 ]. However, no study has directly compared HIMA and PIMA exercise programs. Based on the present findings, preliminary suggestions can be made. PIMA may be preferable when prolonged activation is required, given its longer TTF. HIMA may be more suitable when higher sensorimotor demands and stability control are desired, potentially relevant for neurological rehabilitation. As proposed by Bauer et al. [ 12 ], differentiating between the two isometric types according to patient needs appears warranted. Based on the findings regarding maximal holding capacity and injury prevention considerations, rehabilitation should primarily focus on muscle stability and adaptation to external loads rather than pure muscle strength. However, direct comparative studies are needed before specific recommendations can be made. 4.2.3 Sports performance Isometric training for sports performance has gained recent popularity as more evidence on its ability to improve various sports performance indicators has become available [ 1 , 33 , 34 , 121 , 122 ]. However, most of these studies used PIMA [126, 127], with only a limited number employing HIMA [ 33 , 34 ]. Additionally, no study has compared the efficacy of PIMA and HIMA for improving sports performance. Hence, it may be challenging for practitioners to determine which method to use at various training cycle phases. Nevertheless, the current review may provide some guidance on when to include HIMA or PIMA in athletic physical preparation. Firstly, HIMA may be preferred during general preparation, where the training objective is often to increase the overall strength required to endure the demands of subsequent phases, and to reduce the risk of injury as training intensity increases. This recommendation is based on the greater synergistic muscle activation during HIMA, which is vital in maintaining joint integrity during intensive movements [ 21 ]. Furthermore, appendicular muscles can sustain PIMA longer than HIMA at a given intensity, while the opposite finding was true for the trunk muscles. During the general preparation phase, coaches plan for higher training volumes, meaning that muscle action duration would be longer if isometric training is included. If PIMA were used during an exercise such as the isometric squat, the adaptation of the lower limb may be limited by the duration for which the trunk muscles could sustain a given force. However, if HIMA were to be used, the trunk muscles would likely be able to maintain the given force for as long as, or longer than, the lower limb. Hence, the adaptation to the lower limb would be less limited by the trunk. Secondly, during the training phase, PIMA would likely be preferred, as it involves exerting force against an immovable object, allowing an athlete to exert maximal force rapidly. Similarly, several studies demonstrated that PIMA allows for greater maximal torques than HIMA [ 20 , 21 , 24 ]. Hence, high-intensity PIMA would likely result in greater strength adaptations. While there is rarely only one mode of muscle action in sports, brief or prolonged isometric action is typical in many sports. Hence, the decision to use PIMA or HIMA in training may also depend on the specific sporting action(s) to be prepared for. For example, sports actions under high external loads with relatively static conditions, such as a rugby scrum or a grappling/wrestling maneuver, may be best trained using PIMA. Likewise, actions that require holdings, such as the static position held during a springboard dive or various gymnastic maneuvers, may be best prepared for using HIMA. Finally, if improving rate of force development is paramount, both PIMA and HIMA with rapid muscle actions can be utilized [ 123 ]. While it is relatively simple to perform PIMA with rapid intent, practitioners may lack insight into how to conduct rapid HIMA, including rapid landings or unconventional exercises (e.g., rapid decelerations, hamstring ‘switches’, oscillatory training), which are currently lacking in scientific literature. Furthermore, while several examples of multi-joint HIMA exist [ 33 , 34 ], no studies have compared PIMA and HIMA via closed-chain movements, presenting an exciting research opportunity. 4.3 Limitations and future research directions While the primary aims were accomplished, there are several limitations. First, we included only studies that directly compared PIMA and HIMA. While it would have been exceptionally time-consuming, all studies employing PIMA or HIMA could have been identified and synthesized. While enough studies utilized TTF as a dependent variable for confident meta-analyses, far fewer studies reported HR, MAP, or RPE. Therefore, readers should be aware of potential limitations. Similarly, the between-study variability in neurological (e.g., EEG, CT) and neuromuscular (e.g., sEMG) assessment methodology prevented us from performing further meta-analyses. Moreover, the small number of studies focusing on the same objective (e.g., brain activity, tendon oscillations, max torque) makes authoritative statements difficult. Other methods may uncover further insights, including fMRI, fascicle dynamics, echo intensity, shear wave elastography, tendon creep, and tensiomyography. The ‘stability-of-position’ factor was impossible to include in the interpretation of data (i.e., HIMA is typically more unstable than PIMA). Thus, aligning the position stability during both functions would be advisable. Readers should also be aware that the experimental pain trials included in this review were conducted in highly controlled laboratory settings, which may not be directly applicable to clinical settings. While the risk-of-bias was modest for all studies, our quality scoring criteria are somewhat subjective. As such, other researchers may have different means of assessing quality. Many included studies did not provide effect size statistics, leaving us to calculate them based on text, tables, or figure extractions, potentially (minimally) affecting some of the meta-analyses. The mean intensity of the included studies was far lower than nearly all but the most conservative rehabilitation or sports performance guidelines. While these low loads allowed for greater durations and, thus, more time intervals for comparison, future studies should examine more practically relevant intensities. An even more explicit limitation is the complete absence of studies comparing PIMA and HIMA during multi-joint movements (e.g., squat, leg press, pull-up, bench press). Researchers could also compare the two types of isometric muscle action over multiple days to examine the time course of fatigue and recovery. Moreover, investigations on how external and internal stimuli might influence HIMA and PIMA could be relevant for gathering further information on underlying control mechanisms and for diagnostics, injury mitigation and rehabilitation. Most importantly, longitudinal, randomized control trials are required to ascertain if PIMA or HIMA have any measurable advantage over the other for use in diagnostics or for inducing various rehabilitation or sports performance adaptations. 5 CONCLUSIONS This systematic review and meta-analysis advances the understanding of isometric muscle actions by clearly delineating two distinct types—PIMA and HIMA—and synthesizing evidence that they differ in meaningful neural, biomechanical, and functional ways. Objective measures largely suggest that HIMA involves more complex neural and neuromuscular control strategies than PIMA, likely reflecting neuromuscular processing similar to muscle lengthening and shortening actions, respectively. This supports the hypothesis that HIMA and PIMA may represent “stopped” anisometric actions rather than a single, uniform isometric modality. HIMA demonstrates faster rises in neuromuscular and cardiovascular responses, and exhibits heightened sensorimotor modulation, suggesting particular utility in diagnostics, injury prevention, and rehabilitation—especially when increasing external loads are incorporated—though clinical evidence remains limited. When integrated with broader physiological, biomechanical, and psychological research, HIMA may offer a time-efficient means of inducing musculoskeletal, neural, and cardiovascular adaptations. In contrast, PIMA appears more effective for sustained activation and promoting agonist-specific neuromuscular adaptations due to its greater force capacity. Collectively, the findings challenge the traditional view of isometric muscle actions as a unitary construct and highlight important implications for sports medicine and rehabilitation, where isometrics are widely used to enhance recovery, prevent injury, and improve performance. Incorporating both HIMA and PIMA into assessment, training, and rehabilitation programs may enable practitioners to target complementary adaptations, individualize care, and develop goal-specific interventions. However, substantial methodological heterogeneity across studies precluded additional meta-analyses, underscoring the need for consistent protocols and improved reporting. Future randomized trials are warranted to translate these mechanistic insights into clinical and athletic outcomes, and it is recommended that knowledge of both isometric types be integrated into research, education, and clinical practice in sports and health sciences. DECLARATIONS Funding The publication was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project no. 491466077. Conflict of interest Two authors run for-profit sports performance workshops, often focusing on isometric resistance training. All other authors have no relevant financial or non-financial interests to disclose. Data availability Data used in the meta-analyses are available upon request. Ethics approval not applicable. Consent to participate/for publication not applicable. Author contributions DJO conceptualized the review topic and formed the research team. DJO and ARN fine-tuned the research question and review methods. DJO and ARN performed the literature search and initial screenings. DJO, LVS, DL, AON and ARN assessed inclusion and quality/bias studies. DJO and LVS extracted data, while ARN and FNB checked the extracted data for accuracy. DJO, LVS and ARN performed the statistical analyses and created the figures. DJO wrote the first draft of the title, abstract, introduction and methods. DJO and LVS wrote the first draft of the results. All authors contributed to the discussion’s first draft and approved the manuscript’s final version List of abbreviations AF Adaptive force EEG Electroencephalography EMG Electromyography aEMG Electromyographic amplitude sEMG Surface electromyography HIMA Holding isometric muscle actions HR Heart rate MAP Mean arterial pressure MMG Mechanomyography MTG Mechanotendography MVIC Maximal voluntary isometric contraction PIMA Pushing/pulling isometric muscle actions PRISMA Preferred Reporting Items for Systematic Reviews and Meta-Analyses RMS Root-mean-squared RPE Ratings of perceived exertion TMS Transcranial magnetic stimulation TTF Time to task failure Data Availability All data produced in the present study are available upon reasonable request to the authors Footnotes Updated abstract and manuscript text that improves clarity regarding which conclusions are the result of meta-analysis vs more subjective interpretations. 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