Recent studies have demonstrated that isometric wrist actions performed with an open hand have a direction-dependent influence on upper arm muscle activity. It is presently unclear if these findings are translatable to motor tasks requiring a closed hand, as it common in most hand-object interactions. Thus, the purpose of the present study was to determine the influence of hand posture on upper arm muscle activity during isometric wrist exertions in different directions. Participants were seated with their forearm unsupported within a custom-built apparatus that could position an adjustable force transducer either above, below, or to the right/left of the participant's hand. Muscle activity was recorded from the biceps and triceps brachii while participants exerted 50% of their maximal force against the transducer in one of four randomized wrist actions: flexion, extension, or radial or ulnar deviation. Results demonstrated that a closed-hand posture produced greater biceps (open: 4.92 ± 4.17% maximal voluntary excitation [MVE], closed: 8.18 ± 6.44% MVE, p < .001) and triceps brachii (open: 16.91 ± 8.0% MVE, closed: 19.09 ± 8.54% MVE, p = .002) muscle activity when exerting wrist forces downwards but less activity when forces were exerted to the right. The influence of wrist exertion direction on upper arm muscle activity also depended on hand posture. These findings add valuable new evidence that a neural coupling likely exists between the distal and proximal regions of the upper extremity. They also indicate that research into open-hand postures may not fully translate to motor tasks requiring a closed hand.

Lifting objects challenges postural stability and requires the generation of anticipatory (APA) and compensatory postural adjustments. This study investigated the combined effects of perturbation predictability and asymmetric load placement on postural control during a bimanual lifting task. Sixteen healthy adults completed a randomized crossover study involving repeated lifts of a box containing a load equal to 5% of their body weight that was positioned asymmetrically under three conditions: known, unpredictable, and predictable. Surface electromyography from trunk and lower limb muscles and center-of-pressure (COP) displacements were recorded. Electromyography integrals and COP displacements were analyzed separately for APA and compensatory postural adjustment phases using linear mixed-effects models. Explicit knowledge of load position resulted in larger and more direction-specific APAs, characterized by larger anticipatory trunk muscle activation and a proactive anterior shift of the COP (all p < .05). Predictability acquired through repeated exposure produced smaller APAs (p < .05). Under unpredictable conditions, participants exhibited reduced anticipatory COP shifts and larger compensatory responses for right-sided loads (all p < .05), whereas left-sided loads were characterized by a posterior COP shift before lift initiation. Postural adjustments during lifting are modulated by both explicit task information and experience-based predictability, and load position asymmetry alters anticipatory and compensatory control strategies. Enhancing information about load position may therefore improve postural stability and reduce reliance on compensatory mechanisms during manual material handling.

This study aimed to use nonlinear dynamical measures to examine the function of plantar tactile feedback during quiet standing without vision in order to elucidate the stimulation effects on postural sway. Detrended fluctuation analysis, sample entropy, and rambling and trembling analysis were applied to analyze the dynamics of postural sway in the anterior-posterior and mediolateral directions. Twenty young adults stood on a force plate with eyes closed for 35 s under three conditions: no plantar stimulation, plantar stimulation under the forefoot, and plantar stimulation under the midfoot (MS). The results showed that postural dynamics were affected by the placement of plantar stimulation in the absence of visual information. In the anterior-posterior direction, increased detrended fluctuation analysis values for the center of pressure and rambling displacements were revealed in the MS condition than in the plantar stimulation under the forefoot condition, indicating greater adaptability in postural sway. There were also increased detrended fluctuation analysis values in the rambling displacement in the MS condition compared with the no plantar stimulation condition. In the mediolateral direction, the sample entropy in the trembling displacement was significantly greater in the MS condition than in the no plantar stimulation condition, indicating more automaticity in postural sway. These results suggest that plantar stimulation may contribute to balance regulation in an axis- and component-specific manner during quiet standing without vision.

This study investigated the neural timing control characteristics of countermovement jumps between elite athletes and nonathletes using muscle synergy theory. Twenty male participants (high-level group [HLG, N = 10] and untrained group [UG, N = 10]) performed the countermovement jump while surface electromyography from eight lower limb muscles and three-dimensional force plate data were recorded. Nonnegative matrix factorization extracted three muscle synergy modules, and bootstrap resampling with bias-corrected 95% confidence intervals (CIs) analyzed temporal parameters. Results demonstrated: (a) HLG achieved significantly greater jump height (95% CI: [-0.139, -0.026]); (b) HLG exhibited delayed activation onset (95% CI: [-0.227, -0.015]), peak timing (95% CI: [-0.300, -0.028]), and offset (95% CI: [-0.216, -0.014]) in Synergy module 1, delayed onset (95% CI: [-0.225, -0.054]) and offset (95% CI: [-0.224, -0.072]) in Synergy module 2, and delayed offset in Synergy module 3 (95% CI: [-0.197, -0.017]); and (c) HLG showed narrower full width at half maximum in Synergy module 1 (95% CI: [0.009, 0.094]), with no group differences in amplitude parameters. These findings suggest that, compared with UG, HLG exhibit delayed synergy activation timing during the countermovement jump. This temporal pattern may be associated with more effective use of the eccentric phase and more coordinated interjoint timing during propulsion.

Muscular fatigue often induces compensatory adjustments-such as increased motor unit recruitment and synchronization-that can elevate kinematic variability. By contrast, mental fatigue alters top-down neural control, modulates corticospinal excitability, and changes prefrontal-motor cortical activity, indirectly influencing muscle activation patterns. These central and peripheral distinctions suggest that mental and muscular fatigue may differently affect movement variability and synergy organization, yet it remains unclear how the structure of variability diverges between the two. The purpose of this study was to examine the effect of mental and muscular fatigue on movement variability during dart throwing using the uncontrolled manifold method. This method decomposes joint kinematic into two components: components of joint variability that either stabilize (parallel variance) or destabilize (orthogonal variance) performance variable. Participants were 28 young individuals (19 females and nine males) aged 25-35 without regular experience in throwing darts. All participants threw darts under three conditions: mental fatigue, muscular fatigue, and nonfatigue. Throwing kinematics data was collected with a motion capture system and quantified movement variability using the uncontrolled manifold approach. We hypothesized that the structure of movement variability would differ across the three conditions-mental fatigue, muscular fatigue, and nonfatigue. Repeated-measures multivariate analysis of variance was used to test these hypotheses. The results showed that the main effect of fatigue has a significant effect on parallel variance (p = .034, partial η2 = .31) but not on orthogonal variance (p = .289, partial η2 = .23) and synergy index (p = .864, partial η2 = .14) at different times of the throwing cycle. Increasing parallel variance is an effective strategy for helping to be more flexible in using degrees of freedom to perform a task. These results suggest that an increase in parallel variance may reflect an adaptive mechanism to maintain stability.

A core feature of joint action is task-sharing-the dynamic coordination of individual efforts to produce and stabilize a shared outcome. Yet, it remains unclear which features of task-sharing dynamics relate to stable performance. In this study, we address this gap by identifying behavioral markers that capture the organization and robustness of task-sharing in a joint force-production task. Thirty dyads performed a continuous force-production task in which each partner independently applied force to jointly match a target total force. Real-time visual feedback was provided on their combined output. The task was completed under low-force (10% maximum voluntary contraction) and high-force (30% maximum voluntary contraction) conditions to manipulate difficulty. Trials revealed qualitatively distinct patterns of force contribution, including cases in which a single partner carried most of the output, patterns in which partners contributed with distinct temporal roles, and fully synergistic task-sharing in which both partners continuously shaped total force fluctuations. We used within-trial uncontrolled manifold (UCM) analysis to characterize the dominant, synergistic mode along two dimensions. The first is the degree of variation in task-sharing patterns that leaves performance unchanged (VUCM). The second is the degree of irregularity-computed as sample entropy-of these patterns (SampEnUCM). Dyads with lower VUCM and lower SampEnUCM demonstrated greater performance stability. VUCM and SampEnUCM thus serve as indices of robust task-sharing patterns, offering behavioral access to the control mechanisms that enable stable joint performance, particularly under more challenging conditions.

Single-effort isometric handgrip strength tests are widely used as a proxy of overall muscular strength. While using peak force (PF) is practical and meaningful, additional force-time characteristics could provide insight into muscle function. The purpose of this study was to assess test-retest reliability of force-time characteristics for maximal voluntary handgrip contractions (Analysis 1) and assess the influence of fatigability and time-of-day (Analysis 2). Nineteen participants completed Analysis 1 (23.5 ± 5.7 years, 168.7 ± 8.3 cm, 75.4 ± 14.0 kg) and 18 participants completed Analysis 2 (23.4 ± 5.9 years, 168.7 ± 10.4 cm, 77.6 ± 14.6 kg). For both analyses, participants completed three 4-s maximal voluntary handgrip contractions with 1-min rest periods. For Analysis 2, participants completed the maximal voluntary handgrip contractions before and after a fatiguing handgrip intervention at three separate times-of-day. Variables included PF, time to PF, time to 90% PF (TT90), area under the curve deficit (AUC Deficit), plateau force coefficient of variation, and plateau derivative root mean square. Using point estimates, all dependent variables demonstrated moderate-excellent relative reliability. A Time-of-day × Fatigability interaction was observed for TT90, AUC Deficit, and force coefficient of variation. Main effects of fatigability were observed for all variables, excluding plateau derivative root mean square. Time-of-day did not demonstrate an impact to any dependent variable. While task specific, an adequate warm-up may mitigate time-of-day impact to performance previously reported. TT90 and force maintenance variables, such as AUC Deficit, and force coefficient of variation, appear to be sensitive to the interaction between time-of-day and fatigability. Force maintenance variables may provide further insight beyond PF when examining maximal effort handgrip strength.

Artificially restricting the center of mass (COM) motion during quiet standing through multisegment constraints has previously been shown to alter center of pressure (COP) dynamics. Increases in COP motion during such mechanically restricted stances have been interpreted as potential markers of exploratory behavior, whereby the central nervous system may generate movements to gather sensory input that would otherwise be limited by the imposed constraints. However, past studies relied on a plank-based apparatus that places participants in a nonecological posture, potentially limiting interpretations. This study compared the effects of the plank-based setup and a more ecologically valid belt-based apparatus that restricts COM movement without fully constraining the body. The analysis focused on both traditional measures of COP (i.e., variability) and dynamic measures (i.e., regularity and frequency components). Participants performed standing trials with both apparatuses, with eyes open and closed. Results revealed distinct COP behaviors between the two systems: compared with the unrestricted condition, the restricted condition with the plank-based apparatus yielded higher COP variability and regularity, whereas the belt-based system yielded lower COP variability and increased irregularity. These effects were not strongly influenced by visual input. The persistent COP motion observed under both conditions suggests that COP displacement may serve functions beyond merely stabilizing the COM, possibly reflecting exploratory behaviors. Overall, our findings support the use of belt-based systems as a more ecological alternative for restricting the COM motion, allowing for the study of the role of the COP in sensorimotor regulation without the limitations of a multisegment constraint.

The objective of this study is to examine the relationship between the spin-related variables during real four-seam fastball pitching in baseball and the finger force production capability and its coordination in the laboratory test. Consequently, the sets of variables were derived from two distinct experiments: a field task and a laboratory task. The field task was designed to quantify the spin-related variables during actual pitching performance, and the laboratory task was designed to quantify the strength of the two fingers and the coordination-related variables of the index and middle fingers during the cyclic net finger force production task. The statistical test was implemented with the primary objective of ascertaining the significant correlation between the two metrics from the field and laboratory tests. The findings indicated a negative correlation between finger forces and spin-related variables (p < .05), suggesting that those who have greater finger force do not have a positive effect on spin-related variables. The strength of the synergy indices of moment stabilization was associated with a positive effect on spin-related variables (p < .05). The present experiment demonstrated that the stabilization of mechanical variables by the force production of the two fingers, that is, the net moment, indicates that the rotational effect of finger force production plays a critical role in determining the superior spin qualities of a fastball pitch in baseball.