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.
PURPOSE:The present study explored the age-related influence of foot placement on balance control during quiet standing in tandem stance. Although tandem stance is widely used to assess postural control, foot dominance is rarely standardized in balance testing, even though it may influence stability outcomes. This study addressed this gap by examining whether dominant-foot placement affects postural control across the adult lifespan. METHODS:One-hundred participants (53 women) were separated into five age groups: 21-30, 31-40, 41-50, 51-60, and 61-75 years. They stood upright on a force plate with the dominant foot placed either in the front or back in two visual conditions (eyes open vs. eyes closed). Postural control was assessed by measuring center-of-pressure displacement. RESULTS:Analysis of variances indicated that center-of-pressure displacement increased across age groups (p < .01). For both visual conditions (eyes open or closed) and across all age groups, center-of-pressure displacement was reduced when the dominant foot was placed at the back during tandem stance (p < .05). CONCLUSION:This finding underscores the importance of considering foot dominance when assessing postural control during tandem stance. Standardizing foot placement relative to dominance may enhance the reliability and comparability of balance assessments across studies and age groups.
BACKGROUND:Prolonged standing demands continuous postural adjustments to redistribute weight and maintain comfort. People with Parkinson's disease (PwPD) may show impaired adaptability due to motor symptoms and rigidity. The effects of dopaminergic medication on postural control during prolonged standing are not well understood. This study aims to analyze the impact of dopaminergic medication on postural control during prolonged standing in PwPD, focusing on both global sway characteristics and discrete postural adjustments. METHODS:Twenty-two PwPD (Hoehn and Yahr II-III) performed a 15-min standing task on a force plate in randomized ON (medicated) and OFF (unmedicated) sessions. Global sway parameters (sway area, root mean square, velocity, and frequency) and discrete postural adjustments (shifts, fidgets, and drifts) were calculated from center of pressure data. Nonparametric and parametric tests compared ON and OFF states. RESULTS:In the ON state, participants showed significantly larger sway area, increased root mean square and velocity in anterior-posterior and mediolateral directions, and higher sway frequency (all p < .05). The number and amplitude of discrete adjustments were greater in the ON state, especially anterior-posterior drifts and mediolateral shifts. These changes indicate enhanced postural dynamics under dopaminergic medication. CONCLUSION:Dopaminergic therapy promotes a more dynamic and adaptable postural control strategy during prolonged standing in PwPD. Rather than reflecting instability, increased sway and frequent adjustments suggest adaptive behaviors facilitating comfort and stability over time. These findings underscore the value of prolonged standing tasks for assessing postural control and guiding rehabilitation strategies in PwPD beyond what short static tests capture.
Background : Age-related decline can impair older adults’ ability to perform tasks involving a mix of motor and cognitive goals in a dual-task (DT) paradigm. The amount of DT interference effects has typically been associated with the availability of attentional resources and the degree of balance automaticity. Older adults with mild cognitive impairment may lack sufficient sensorimotor capacity for “automatic” regulation of posture under demanding balance conditions, resulting in larger DT interference effects due to increasing attentional control. Research question : Does the degree of automaticity affect balance stability in older adults with mild cognitive impairment during dual tasking, and does this relationship vary with the difficulty of the balance task? Methods : Sixty-seven older adults, aged 60–80 years (23 mild cognitive impairmentss), were positioned barefoot on a single piezoelectric force plate in a double-support and tandem stance with eyes open. Each stance condition was tested as single task during performance of a mathematical counting task (i.e., DT). DT cost (DTC) scores of center-of-pressure sway velocity (DTCVcop) were calculated, and regression analyses were conducted to assess the unique contribution of baseline center-of-pressure sway entropy under single-task conditions to DTCVcop, with age, Montreal Cognitive Assessment scores, gender, and cognitive status included as covariates. Results : Baseline sway entropy accounted for only 0.25%–4% of the variance in DTC of Vcop. Gender and cognitive status accounted for 12%–20% of the variance under double-support but not in tandem stance. Significance : Our findings suggest that sway entropy has only minimal impact on DT interference while gender and cognitive status play a more substantial role, highlighting the importance of these factors in balance control of older adults.
BACKGROUND:The great toe contributes to healthy gait and balance but may be impacted by exposure to cold. The purpose of this study was to examine the effects of 60 min of cold-water immersion on great toe flexor force and single-limb balance. METHODS:Forty-four healthy adults participated in one of two experiments. All participants placed one foot in water that was maintained at 10 °C and the other foot in water maintained at body temperature (37 °C) for 60 min. Both feet were submerged to a depth just above the malleolus. For the first experiment (n = 25), maximum voluntary great toe flexion force and control of submaximal force were measured while standing using a custom designed apparatus both before and immediately following immersion. For the second experiment (n = 19), postural sway was assessed before and after immersion while standing on an unstable surface with a single limb and eyes open. RESULTS:Cold-water immersion resulted in a significant decrease in maximal flexor force of the great toe (332.6 ± 111.5 N vs. 235.1 ± 108.2 N, p < .001, Cohen's d = -0.89), but it did not impact the amount of error measured during control of submaximal flexor force (13.5 ± 4.4 N vs. 14.4 ± 5.6 N, p = .368). Further, exposure to cold resulted in significant increases to center of pressure rambling in the mediolateral direction (4.0 ± 0.9 mm vs. 4.8 ± 1.0 mm, p < .001, Cohen's d = 0.79) and trembling (3.2 ± 0.9 mm vs. 3.9 ± 1.4 mm, p = .002, Cohen's d = -0.63) and range of motion (49.6 ± 15.3 mm vs. 61.8 ± 21.9 mm, p = .007, Cohen's d = -0.62) in the anteroposterior direction. CONCLUSION:Cold-water immersion can impact force at the great toe and may adversely affect balance.
Despite the main clinical criteria for Autism Spectrum Disorder (ASD) diagnosis being the dysfunction of socio-communicative interaction abilities and the presence of restricted interests and repetitive behaviors, sensory-motor dysfunctions are also frequently observed in this population. Moreover, among sensory-motor issues, both postural control and visual processing may be impaired. The main aim of this scoping review is to synthesize the evidence on the relationship between visual behavior and postural control dysfunctions in children and adolescents with ASD. This scoping review was conducted using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Protocol, Extension for Scoping Reviews guidelines, and was registered a priori on the Open Science Framework. PubMed, CINAHL, Embase, PsycINFO, Scopus, and Web of Science databases were consulted. Primary studies with any study design were included. No time, geographical, or study setting limitations or language restrictions were applied. A total of 646 articles were found in the initial search, but only 14 studies met the full inclusion criteria. Findings highlighted that studies on the relationship between vision and posture in ASD children and adolescents are scarce, and their results are contrasting because of the significant heterogeneity in the methods. This scoping review carried out a relevant survey of the literature considering the relationship between vision and posture in ASD. Nonetheless, the characteristics of the included sample and the methodology used in the analyzed studies were highly variable. Thus, rigorous study methods with population-specific objective outcome measures are needed to draw generalizable conclusions.
Fatigue experienced by individuals with multiple sclerosis (MS) during activities of daily living affects their balance and increases the risk of falls. The aim was to assess the effect of fatigue induced by a functional task on balance control in individuals with MS. The study involved 10 individuals with MS and 10 gender-matched healthy volunteers who performed a functional sit-to-stand task using a standardized chair and metronome until they reported an inability to continue. Motor Control and Sensory Organization tests were implemented using Dynamic Posturography three times: before, immediately after fatigue, and after a 30-min rest period. The Motor Control Test revealed that individuals with MS, compared to healthy controls, showed significantly longer latencies during small, medium, and large amplitudes of backward perturbation. Longer latencies were observed in the MS group after fatigue, specifically during small and large amplitudes of forward perturbations. The Sensory Organization Test discovered that following fatigue, the MS group demonstrated significantly lower equilibrium scores compared with healthy controls in the following conditions: eyes closed (p < .001), sway-referenced visual surround (p = .02), and sway-referenced support surface (p = .03). Functional fatigue induced by sit-to-stand tasks significantly impacted outcomes of both the Motor Control and Sensory Organization tests of balance control in individuals with MS. These findings lay the groundwork for future investigations into the role of fatigue in balance control in individuals with MS.
Different attentional focus strategies are commonly employed in sports training, with the external focus of attention (EF) strategy shown to enhance performance in the standing long jump. However, the underlying muscular dynamics responsible for this improvement remain unclear. In this study, we utilized a Latin square design to recruit 12 healthy male participants, each undergoing standing long jump testing under EF, internal focus (IF), and control conditions. During the take-off phase, we synchronously recorded kinematic and dynamic data. Inverse dynamics calculations were performed on the dynamic and kinematic data. Compared with IF and control, the EF condition exhibited longer jump distances, lower projection angles, and higher peak horizontal take-off velocities and impulses alongside increased center of mass displacement during the eccentric phase and decreased center of mass displacement during the concentric phase (p < .05). The peak muscle force and rate of muscle force development of the vastus lateralis, vastus medialis, vastus intermedius, and gluteus maximus during the take-off phase were also higher in the EF group (p < .05). These results indicate that the EF strategy optimizes standing long jump mechanics by enhancing the efficiency of eccentric muscle contractions, thereby increasing stored elastic potential energy in the muscles. Consequently, this leads to greater muscle force and rate of muscle force development during the concentric phase, ultimately resulting in increased jump distances.
Anterior cruciate ligament reconstruction (ACLR) can result in persistent deficits in postural stability particularly under dual-task conditions involving cognitive demands. This study investigated the effects of cognitive tasks from different domains on postural control among individuals with ACLR compared with healthy controls. Twenty-five individuals with ACLR and 25 asymptomatic matched controls performed three cognitive tasks of backward counting (working memory), Stroop Color-Word Test (executive function), and Benton's Judgment of Line Orientation (visuospatial perception) under single- (seated) and dual-task (single-leg stance on rigid and foam surfaces) conditions. Postural stability was quantified by measuring center-of-pressure sway area and sway velocity in anterior-posterior and medial-lateral directions. Cognitive performance was evaluated by counting task errors. Results indicated significantly greater postural sway in the ACLR group compared with controls across all tasks (p < .01), with the Stroop Color-Word Test significantly increasing mean sway velocity compared with the no-task condition (p < .01) in only the ACLR group. ACLR participants also demonstrated consistently greater cognitive errors under all conditions. However, cognitive errors were not significantly affected by increased postural difficulty. Findings suggest that executive function tasks uniquely exacerbate postural control deficits following ACLR, highlighting the clinical importance of incorporating targeted cognitive-postural training focusing on executive control into rehabilitation protocols.
BACKGROUND:Individuals with stroke have difficulty adapting their walking to their environment; the upper extremity on the paretic side is particularly prone to colliding with surfaces when walking through openings. OBJECTIVES:We aim to investigate how individuals with stroke with different levels of upper extremity motor paralysis rotate each part of their upper body for safe obstacle avoidance. METHODS:Participants included eight, 17, and 19 individuals in the moderate (MDR), mild (MLD), and control groups, respectively. Participants were asked to walk through door-like openings of four different widths without colliding with them. Each participant performed the task by entering either from the paretic side or the nonparetic side. To examine the effects of the group, entering direction, and opening width on rotation angles we conducted a statistical analysis using a Generalized Linear Mixed Model. RESULTS:For the head angle, the main effect of group was significant (p = .011). Post hoc multiple comparisons showed that the angle was significantly larger in the MDR than that in the MLD and control groups (both p < .001). Shoulder and hip rotation angles did not differ significantly between groups. Although some interaction effects between the group and opening width were observed, no consistent interaction patterns were found across body segments. CONCLUSIONS:The results were interpreted in terms of motor difficulty and visual attention. Therefore, individuals with stroke who had moderate upper extremity paralysis may have rotated their heads at higher angles due to difficulty with movement adjustments and increased visual attention demands.
This study examined the influence of task type, knee angle, contraction intensity, and foot positioning on the motor neuron discharge characteristics of knee extensors during single-joint knee extensions and multijoint leg extensions. Ten male participants performed isometric contractions at 10% and 30% of maximum voluntary contraction at 90° and 135° knee angles with identical hip angles. For the 90° leg press, the influence of foot positioning was explored additionally. Motor unit activation was assessed through decomposition of high-density surface electromyography signals to analyze discharge rate, recruitment threshold, and derecruitment threshold. The results showed no significant differences between the vastus medialis and vastus lateralis muscles across all conditions. Knee angle did not affect motor unit characteristics, and foot positioning (internal or external rotation) in the leg press did not alter discharge characteristics, except for neutral foot positioning at 30% maximum voluntary contraction. Significant differences were found between single- and multijoint tasks in all motor unit activation characteristics. Additionally, significant differences in discharge rate were observed between 10% and 30% maximum voluntary contraction. These findings suggest that task type and contraction intensity influence motor unit activation, but knee angle and foot rotation do not have a significant impact on the motor neuron discharge characteristics of knee extensors.
OBJECTIVE:The purpose of our study was to investigate the relationships among muscle strength, proprioception, and postural stability, as well as sex differences in these relationships. METHODS:A total of 189 older participants were recruited from local communities, and muscle strength, proprioception, the Berg Balance Scale, and the Timed Up and Go test were measured. Structural equation modeling was performed to examine the interrelationship between proprioception, muscle strength, and stability, with multiple-group analysis used to explore the sex difference. RESULTS:Compared with males, females exhibited lower levels of muscle strength, proprioception, and Berg Balance Scale (p < .05). Proprioception was negatively associated with stability, with a standard path coefficient (βr) of -0.219 (p = .009), and muscle strength was positively associated with stability (βr = 0.635, p < .001). Muscle strength showed a positive correlation with stability in both males (βr = 0.660, p < .001) and females (βr = 0.796, p < .001). Proprioception had a negative impact on stability only in males (βr = -0.270, p = .006). The association between muscle strength and stability was stronger in females than in males (p < .05). CONCLUSION:Both strength and proprioception affect stability. Proprioception negatively affects stability only in males, and the contribution of strength to stability is greater for older females than for older males.
