Human Movement Science

Targeted stepping requires coordinating limb transport, preparatory postural adjustments, and accurate foot placement while maintaining balance under competing speed and accuracy demands. Although Fitts' law robustly describes speed-accuracy trade-offs (SATs) in upper-limb movements, its applicability to whole-body stepping remains unclear due to additional postural and visuomotor constraints. This study investigates how instructional emphasis and spatial demands modulate SAT regulation and pre-step postural dynamics during stepping. Twenty-four healthy young adults will perform a multidirectional stepping task across two directions (0 degrees and 45 degrees) and two distances (45% and 80% of leg length) under three instruction conditions: Fast, Accurate, and Fast & Accurate. Kinematic and force-plate data will quantify movement time, foot-placement error, and pre-step center-of-pressure dynamics. We hypothesize that instructional emphasis and increased postural demands will systematically alter stepping strategies and produce deviations from classical Fitts-type scaling, particularly during diagonal stepping.

Motor imagery and overt action are proposed to share common planning mechanisms while relying on distinct processes during movement performance. To investigate how repetition affects physical and imagined movements, participants physically or mentally drew paths while avoiding barriers. The barrier orientations were either repeated or different across consecutive trials, and the modality (execution or imagery) of the trials either changed or remained the same for two consecutive trials. Reaction time and movement time were analysed as measures of movement planning and task performance, respectively. Imagined movements produced longer reaction times and movement times than physical movements. Repetition of orientation reduced reaction times in both physical and imagined trials, indicating that trajectory repetition facilitated movement preparation. In contrast, previous trial type influenced reaction times only for physical movements, which were only initiated faster when preceded by another physical trial. Different effects emerged for movement time. Physical movements were faster when the same trajectory was repeated no matter the previous trial type, whereas imagined movements were unaffected by orientation and instead showed shorter durations when preceded by another imagined trial. The findings indicate that physical and imagined movements rely on more similar trajectory representations during the planning stage, but diverge during task performance. While physical movements appear to benefit from the persistence of previously generated movement trajectories, imagined trials seems more influenced by modality-specific cognitive processes. These results provide behavioural support for the Motor-Cognitive model and contribute to understanding how movement repetition influences overt and imagined actions.

Objective: The aim of the current study was to evaluate the feasibility of a multi-channel distributed wearable ultrasound (D-WUS) system capable of simultaneously probing dynamic muscle activity across multiple anatomical sites to assess neuromuscular coordination during physical activity. Methods: Twenty NCAA Division I women athletes (8 with prior knee injury) performed countermovement jumps (CMJs) on dual force plates while wearing a four-channel D-WUS system targeting the vastus lateralis (VL) and vastus medialis oblique (VMO) in both limbs. Real-time M-mode ultrasound data were processed using a composite factorization approach integrating nonnegative matrix factorization and principal component analysis (CF-PCA) to extract dominant muscle-contraction patterns. Correlation and timing metrics between VL and VMO were compared across injury status. Results: CMJ performance metrics (jump height, RSImod) did not differ between groups (p > 0.05). However, D-WUS revealed significant differences in muscle coordination in the right limb: VL-VMO correlation (p = 0.029) and timing symmetry (p = 0.019) were reduced in participants with prior knee injury; left-limb differences were not significant. Right-limb timing symmetry was negatively correlated with SMFA score (rho = -0.48, p = 0.033). Functional survey scores (SMFA) also differed significantly (p = 0.001), while VISA-P scores did not. Conclusion: These preliminary findings provide feasibility-level evidence that distributed wearable ultrasound can detect limb-specific neuromuscular coordination differences not reflected in force-plate biomechanics, motivating larger prospective studies to establish reliability and clinical utility. Significance: With further validation, this approach may offer a scalable, objective means of monitoring muscle contraction during rehabilitation.

BackgroundTrunk flexor and extensor measurement tools are useful for measuring progress throughout the identification, treatment and management of spinal pathologies. Handheld dynamometry is popular amongst clinicians due to its convenience. However, poor standardisation of the testing position can lead to suboptimal intra-rater reliability, which consequently reduces the validity of the results. The VALD ForceFrame Strength Testing System is a portable and standardized hybrid strength-assessment system that quantifies strength and power outputs. ObjectiveThis study aims to assess the test-retest reliability of isometric trunk flexion and extension strength using the ForceFrame testing technology. MethodsA total of 29 healthy participants undertook two testing sessions, each requiring them to perform maximal isometric trunk flexion and extension using the ForceFrame. Test re-test reliability was assessed using the Intraclass Correlation Coefficient (ICC), and the standard error of measure (SEM) was used to calculate the minimum detectable change. ResultsExcellent reliability was observed for both trunk flexion (ICC 0.92; 95% CI 0.83-0.96) and extension (ICC 0.93; 95% CI 0.86-0.97) tests, and the minimum detectable changes observed were 71.35N and 101.20N, respectively. ConclusionsThe results from this study suggest that the VALD force frame offers clinicians a reliable method of assessing change in trunk flexor and extensor strength.

Background: Safely walking through complex environments requires the use of visual information to plan and regulate steps, while managing competing cognitive demands. Age-related changes in visual control of gait and executive function may contribute to unsafe stepping behaviour, particularly under cognitive load. Route previewing may support visual motor planning, but its effects under dual task conditions in older adults are not well understood. Research Question: Does cognitive dual tasking influence gait and gaze behaviour during adaptive walking in older adults, and does previewing the walking route before gait initiation mitigate these effects? Methods: Thirty nine community dwelling older adults (Mage=73.4 years, SD=8.1; 14 male) completed four adaptive walking conditions, combining single and dual-task walking with and without previewing. Per condition, participants completed six trials across different 8-m long coloured walking routes. Gait characteristics were assessed using an instrumented walkway and video-analysis, while gaze behaviour was recorded using mobile eye tracking. Cognitive load was manipulated using a serial subtraction task. Gait and gaze outcomes were evaluated using linear mixed effects models. Results: Dual task performance significantly increased stepping errors, cross-steps, and double limb support, while reducing stride length (p < .001). It also significantly increased the number and duration of task irrelevant fixations (p < .001). After previewing, participants exhibited more efficient gaze behaviour, as evidenced by looking further ahead (larger forward gaze transitions) and fewer irrelevant gaze fixations (p < .002). Significant Task-by-Preview interactions revealed that previewing mitigated the negative effects of dual-tasking on stepping errors and cross steps (p < .001). Significance: Route previewing prior to gait initiation attenuates negative effects of cognitive load on adaptive gait in older adults, likely by supporting visual motor planning. These findings highlight the potential value of simple visual planning interventions for improving adaptive gait in older adults.

Background: School-based implementation of robot-assisted walking may support increased physical and social participation for children with physical disabilities, but evidence from an adequately powered trial is needed. This pilot study evaluated implementation strategies and study procedures to inform a future definitive trial. Methods: Participants were randomized to 12-weeks of robot-assisted walking or control, with subsequent crossover to the other condition. In the intervention arm, targeted dose was 75 minutes of robot-assisted walking, spread across three days/week. Implementation strategies for the child's classroom team included educational materials, centralized technical assistance, school champions, and on-going training. Implementation strategy evaluation focused on intervention protocol feasibility (i.e., weekly device use, troubleshooting) and fidelity (documentation of implementation strategy modifications). Preliminary estimates participation goal changes were obtained using the Canadian Occupational Performance Measure (COPM) 'Performance' scale (/10). Results: Eight children with neurodevelopmental physical disabilities participated (mean age 3.5, SD 0.5 years). Device use varied from 2.1 to 4.3 days/week (median 3.3) with a range of 48 to 96 minutes/week (median 80). Target use of >= 3 days/week was achieved for 66% of weeks and met the >=75-minute target for 33% of weeks. Troubleshooting occurred in 18.5% of sessions. Two implementation strategies were modified to improve feasibility. COPM mean change was 3.56 (SD 1.07) for robot-assisted walking and 2.11 (SD 2.41) for control (dz=0.74). Conclusions: This pilot study provided preliminary evidence regarding both the feasibility of robot-assisted walking implementation in schools and its potential effectiveness for children with physical disabilities. Findings support progression to a definitive trial.

Aim. To evaluate the accuracy, run-to-run consistency, and agreement of large language models on the Ukrainian Integrated Licensing Examination Krok 2 for the Specialty of Physical Therapy. Materials and Methods. Five cloud-based LLMs (ChatGPT-5.5, Claude 5 Sonnet, Gemini 3.5 Flash, Grok 4, DeepSeek-V3) were evaluated using 150 Ukrainian-language multiple-choice questions from the official Krok 2 database. Each model completed two independent runs with identical inputs. Accuracy (with 95% confidence intervals, Wilson method), run-to-run consistency, and agreement were assessed. Statistical analysis included Cohen's {kappa}, Cochran's Q and pairwise McNemar tests with Holm-Bonferroni correction. Results. All models achieved >80% accuracy in both runs, exceeding the passing threshold of [&ge;]64%. In the first run, accuracy ranged from 80.00% (DeepSeek-V3) to 93.33% (Gemini 3.5 Flash); in the second run, from 82.67% (Claude 5 Sonnet) to 88.67% (Gemini 3.5 Flash). In the first run significant differences were observed between Gemini 3.5 Flash and Grok 4 (93.33% vs 84.00%; p=0.0022) and DeepSeek-V3 (93.33% vs 80.00%; p=0.0001). There were no statistically significant differences between models in the second run. R2R consistency exceeded 90% for all models with the highest in ChatGPT-5.5 (98.67%) and lowest in DeepSeek-V3 (90.00%). Cohen's {kappa} ranged from 0.64 to 0.94, indicating substantial to almost perfect agreement. The stability of LLMs' response correctness across two runs ranged from 77.33% (DeepSeek-V3) to 88.00% (Gemini 3.5 Flash). Conclusions. All LLMs demonstrated high performance on Ukrainian-language Krok 2 examination tasks. Gemini 3.5 Flash showed the highest accuracy in both runs and response correctness stability while ChatGPT-5.5 exhibited the greatest response consistency and agreement. Although the accuracy and stability of LLM responses may be interrelated, they represent distinct aspects of model performance. This highlights the need for a comprehensive approach to evaluating LLMs. The findings indicate the potential of LLMs as an auxiliary tool in the professional practice of physical therapy specialists.

Aims: Our primary aim was to determine the immediate effects of using a supported stepping device on the real-time locomotor behavior of pre-walking infants with Down syndrome (DS). Our secondary aim was to characterize the role of the caregiver in facilitating infant locomotor behavior by quantifying infant-initiated and caregiver-initiated walker movement, and to test whether walker movement encourages stepping behavior. Methods: Pre-walking infants with DS played with their caregiver in two conditions--with and without assistance from a wheeled suspension walker. To examine changes across development, we recorded infants across one to four sessions from 10 to 19 months of age. Results: Across developmental stages, we found that suspension walkers robustly increased infants' locomotor behavior: The walker increased upright time, percent time stepping, stepping rate, upright bout rate, percent time locomoting, and total bout rate. Most walker movement was initiated by caregivers, and infants stepped more while the walker was moving compared to stationary. Conclusions: Even without practice, pre-walking infants with DS were able to take advantage of the suspension walker to increase their locomotor behavior. Caregivers facilitated infants' activity by moving the walker. Future work will examine the potential of overground suspension walkers for home-based intervention.

The control of visually guided bimanual movements is thought to represent a compromise between the drive for synchrony that characterizes many bimanual movements and the competing demands that characterize the constituent unimanual movements. As a result, while visually guided bimanual movements are found to be ostensibly synchronized, they also demonstrate subtle asynchronies. Here, we conducted a fine-grained analysis of bimanual reaching movements focusing on their relative synchrony as movements unfold and the relationship with gaze behavior. Our novel analysis revealed that participants frequently shifted their gaze laterally several times during bimanual reaches and that these gaze shifts were systematically associated with changes in the speed ratio of the two limbs. More specifically, we observed a strong association between gaze shifts and the speed ratio shifting toward the limb on the attended side. This pattern of behavior was seen regardless of hand dominance and target difficulty, suggesting that it may reflect a common visuomotor strategy during bimanual reaching.

Eye movements have been shown to reveal the cognitive processes underlying spatial ability, but most studies investigating gaze behaviour in spatial cognition use a single, screen-based task. Therefore, it is unknown whether these gaze behaviours are preserved across naturalistic task environments, or whether their relationship with spatial performance is domain-general or task-specific. The present study tested whether established gaze behaviours are preserved and generalizable across diverse, naturalistic spatial tasks. N = 30 young adults completed three naturalistic spatial tasks varying in perceptual, cognitive, and visuomotor demands: Q-bitz, the Brick Building Task (BBT), and the Mental Rotation Test (MRT). Gaze was measured using the Pupil Labs Neon eye tracker. Within-task analyses were conducted to assess whether gaze behaviours were preserved, and cross-task analyses were conducted to assess the generalizability of preserved gaze behaviours. Within-task analyses identified source inspection and transition rate as gaze behaviours that were consistently sensitive to changing task demands. Cross-task analyses showed that source fixation duration was a domain-general predictor of performance, whereas the relationship between transition rate and performance differed by task. Exploratory sex-separated analyses also suggested a more consistent gaze-performance relationship in males. These findings show that source fixation duration may represent a common cognitive process underlying performance across naturalistic spatial tasks.

Background: Task challenge is recognized as a core principle of neurorehabilitation and is frequently mentioned within stroke rehabilitation guidelines. However, guidance for operationalizing challenge in clinical practice remains limited. Examining how therapists and people with stroke currently use challenge in practice may help address this gap. Objective: To examine how challenge is operationalized in routine stroke rehabilitation practice. Methods: This study used focused video analysis to examine eight video-recorded stroke rehabilitation sessions across physiotherapy, occupational therapy, and speech-language therapy contexts. Video data were supplemented by observational memos. Analysis was conducted iteratively through four stages: immersion, detailed transcription, in-depth episode analysis, and comparative analysis. These stages enabled examination of material, embodied, social, and contextual aspects of challenge-related interactions as they unfolded during rehabilitation. Results: Twenty individuals, comprising four patient-therapist dyads and four triads participated. Challenge was operationalized through three intersecting practices: (i) Structuring challenge-related work through temporal phases determined when challenge was introduced, experienced, and consolidated across the session and within tasks; (ii) Establishing the macro-architecture of challenge guided what and how it was operationalized by constructing a challenge plan, shaping an optimal bandwidth by tailoring challenge to the persons abilities and/or experiences, and facilitating its transfer beyond the session; (iii) Modulating challenge through micro-calibrations involved moment-to-moment adjustments (e.g., adapting physical assistance, communication style, environmental setup) to modulate active practice, navigate differing perspectives, and cultivate an atmosphere conducive to engaging with challenge. These overarching practices were dynamically enacted through a range of explicit and implicit practices. Conclusion: Challenge in stroke rehabilitation is not merely a task parameter; it is operationalized through an interplay of explicit and implicit practices that structure and enable its responsive implementation. Considering challenge through these practices may help therapists and people with stroke co-construct meaningful experiences of challenge, both within and beyond formal stroke rehabilitation sessions.

Markerless motion capture offers a practical approach to biomechanical assessment in clinical populations outside of controlled laboratory environments. The purpose of this study was to evaluate the utility of markerless motion capture for quantifying static postural control impairment in individuals with stroke. Participants with subacute stroke (n=26) and healthy controls (n=171) performed a 30-second quiet standing task across three data collection sites in Kingston, Ontario, Canada. Video data were processed using Theia3D, and pelvis center of mass (COM) trajectories were extracted. Five discrete COM parameters were computed and transformed into Z-scores using normative models developed from the healthy population. An overall Task Score was calculated by aggregating parameter Z-scores. Forty-six percent of participants with stroke were identified as impaired on the task (Task Score > 95th percentile of the healthy distribution). At the parameter level, impairments were more prevalent in the mediolateral (ML) than anteroposterior (AP) direction (root-mean-square distance: 50% impaired ML vs. 12% impaired AP; mean velocity: 46% impaired ML vs. 35% impaired AP). Task scores and several task parameters demonstrated significant correlations with Berg Balance Scale scores (r = -0.64 to -0.81) and Functional Independence Measure Motor Sub-Scale scores (r = -0.62 to -0.77), with poorer performance associated with lower clinical scores. These findings demonstrate the utility of markerless motion capture for quantifying static postural control impairment following stroke. Future work should extend this framework to evaluate impairment in more dynamic postural control and gait tasks.

Hard-tissue surgery requires precise spatial control, while conventional computer-assisted navigation presents guidance separately from the operative field. Augmented reality can bring navigation information closer to the surgeon, but how best to translate this information into intuitive intraoperative guidance remains underexplored. We therefore developed an augmented reality navigation system (ANS) that translates real-time deviations from the planned trajectory into co-located holographic action cues. Evaluation comprised typodont and ex vivo porcine experiments, a randomized crossover study in 40 dental trainees, and eight clinical cases. In porcine mandibles, ANS showed lower angular deviation than static and dynamic navigation (1.78 vs 6.42 and 4.02 degrees). Among trainees, accuracy was comparable with dynamic navigation, while workload was lower (29.18 vs 33.15), usability higher (73.13 vs 68.88), and visual-focus ratings improved (3.55 vs 2.28). Clinical cases supported feasibility. These findings support co-located action guidance as an alternative interface paradigm for surgical navigation.

Objective: To examine changes in function and pain interference among patients utilizing a digital exercise therapy application (DETA) for knee and shoulder pain. Methods: We conducted a retrospective analysis of data from patients utilizing a DETA for non-operative knee and shoulder pain. Primary outcomes for the study were 8-week changes in physical function, measured using the PROMIS-Physical Function (knee pain) and PROMIS-Upper Extremity Function (shoulder pain); and pain interference, measured using the PROMIS-Pain Interference. Data on patient characteristics was used to examine outcomes among specific patient subgroups. Results: We observed a 3.5 point (SD: 7.8) increase in function and a 3.9 point (SD: 7.5) decrease in pain interference overall (p<0.001), with changes in function and pain interference surpassing the minimal clinically important difference for patients with knee and shoulder pain. Patients with knee pain experienced greater improvements in function than those with shoulder pain (p=0.02), as did patients who report moderate-high levels of exertion during exercise compared to those reporting low levels of exertion (p=0.05). We saw greater reductions in pain interference among those with higher education (p=0.03), those with more acute symptoms (p=0.01) and those reporting moderate-high levels of exercise exertion (p=0.02). Conclusion: We observed a clinically meaningful improvement in pain interference and function among patients utilizing a DETA for knee and shoulder pain. Patients with knee pain, higher levels of education, more acute symptoms, and those who reported higher levels of exertion with exercise experienced the greatest changes in function and pain. Impact Statement: This study indicates that self-guided interventions, such as those delivered through a DETA, may be beneficial for patients with knee and shoulder pain. These results may also inform the development of patient profiles that can be used to identify patients most appropriate to receive care through a mobile application.

Background: Paretic-arm nonuse and trunk compensation after stroke are often interpreted as impairment or learned behavior but may also reflect task-dependent optimization. We previously showed that gravitational shoulder effort and force reserve shape these behaviors. Here, we tested whether endpoint precision is an additional constraint. Methods: Twenty-two individuals with stroke and 22 matched controls performed a seated Proximal Arm Nonuse reaching task toward small (2 cm) and large (12 cm) targets. Stroke participants were tested at normal arm weight and with the paretic arm mechanically lightened; controls were tested at normal arm weight and while holding a dumbbell corresponding to 85% of maximal voluntary shoulder force. Results: In stroke participants, trunk compensation was 15% lower for the large target than for the small target, independently of arm weight condition (F(1,13)=8.166, p=.013, pes=.39). Target size did not significantly affect trunk compensation in controls or anterior deltoid activation in either group. In stroke participants, movement time was 24% longer for the small target than for the large target (F(1,13)=8.533, p=.012, pes=.40). In controls, the small target increased movement time only when the arm was weighted (F(1,18)=14.9, p=.001, pes=.45), not at normal arm weight (F(1,18)=0.158, p=.695, pes=.01). Conclusions: Endpoint precision altered reaching after stroke partly independently of gravitational shoulder effort. Trunk compensation after stroke was therefore modulated by task demands rather than fixed by impairment, a pattern consistent with, although not a direct demonstration of, an optimal-control account. Rehabilitation should evaluate compensation by its functional role and the cost of suppressing it, not only by its deviation from typical kinematics, and should treat target size as an adjustable parameter of the task. Clinical Trial: NCT04747587. https://clinicaltrials.gov/study/NCT04747587

Computer mouse weight has been proposed to influence human motor performance, but existing evidence comes largely from small, short-duration controlled studies, leaving its relationship to performance during naturalistic gameplay unresolved. We analyzed one-week skill improvement in Aimlabs, a popular first-person shooter aim-training and assessment platform, among 9,067 players whose telemetry recorded a mouse-model change (switchers) during self-directed game use, using a 21-day baseline and 7-day follow-up; 4,991 players with a single mouse model (non-switchers) observed over matched pseudo-anchor intervals provided a secondary contextual comparison. Post-switch mouse weight was represented by eight bands spanning 35-124 g. The primary multivariable generalized additive regression model accounted for baseline skill, pre- and post-switch practice intensity, and pre- and post-switch mouse sensitivity, and allowed the association between weight band and improvement to vary with baseline skill, post-switch practice, and post-switch sensitivity. For each weight band, predicted mean one-week improvement was standardized to the observed distribution of these covariates among switchers and reported in percentile-rank points. Statistical significance was evaluated using 1,000 permutations in which observed mouse weights were randomly reassigned among switchers without replacement and the model was refitted to generate empirical null distributions for the weight-related test statistics. Predicted improvement differed significantly across the eight weight bands (p = 0.001). Pairwise comparisons identified significant differences between bands (p < 0.05), but no single band was associated with significantly greater predicted improvement than all seven others. In this large naturalistic cohort, predicted improvement was highest for players with 35-44 g mice, followed by 45-54 and 55-64 g, and several pairwise comparisons showed significantly greater improvement in lighter than in heavier weight bands. Across the examined baseline-skill profiles, the lowest predicted improvements occurred in either the 85-99 or 115-124 g band. Predicted improvement differed across weight bands at the 25th-percentile and median baseline-skill values (p = 0.002 and p = 0.003) but not at the 75th percentile (p = 0.153). However, the weight-band x baseline-skill interaction was not significant (p = 0.181), indicating that the analysis did not establish that weight-related differences varied by skill level.

We explored the role of proprioceptive feedback from Ia afferents in intra-muscle synergies stabilizing muscle force with the help of muscle vibration, which is known to increase presynaptic inhibition of those afferents to alpha-motoneurons. Healthy young adults performed an accurate three-finger force production task by pressing with the index, middle and ring fingers of the dominant (right) hand. This was done without vibration and with vibration applied to the extrinsic finger flexors, extensors, both muscles, the palm, or biceps brachii. High-density electromyography was used to identify motor units in both finger flexors and extensors. The raster plots of motor units were converted into firing frequency functions and then subjected to principal component analysis to extract stable motor unit groups (MU-modes). The uncontrolled manifold (UCM) analysis was used to quantify the variance components affecting and not affecting force, and the synergy index in the MU-mode space. Contrary to our expectations, the synergy index did not drop under any vibration condition and showed an increase during vibration applied simultaneously to both flexor and extensor muscles. This was primarily associated with a drop in the component of variance affecting the force magnitude. The results speak against an important role of monosynaptic Ia-afferent projections to alpha-motoneurons in the formation of intra-muscle synergies. They suggest that muscle vibration can play the role of a noisy signal decreasing performance-affecting variance via stochastic resonance effects.

Background: Knee osteoarthritis (OA) is characterized by progressive joint degradation and altered gait biomechanics. Inter-limb load asymmetry during daily living tasks may accelerate joint deterioration, particularly when combined with physical fatigue. This study investigates the relationship between daily physical fatigue and inter-limb load asymmetry patterns during functional activities in individuals with unilateral knee OA. Objective: To evaluate the impact of fatigue on inter-limb load asymmetry and joint loading rates during daily living tasks in patients with unilateral knee osteoarthritis, and to identify biomechanical and clinical factors associated with load distribution patterns. Methods: One hundred eighteen participants with unilateral knee OA (mean age 62.6 (SD 7.4) years, BMI 29.9 (SD 4.2) kg/m2) completed biomechanical assessments during non-fatigued and fatigued conditions. Ground reaction force measurements were obtained during functional tasks including stair climbing, sit-to-stand transitions, and level walking. Load asymmetry was quantified as the inter-limb difference in peak vertical ground reaction forces. Statistical analysis was conducted using Python (v3.9), including paired t-tests, analysis of variance, and linear regression modeling. Results: Participants demonstrated significantly increased load asymmetry under fatigue conditions (mean difference 8.7% (SD 12.3) %, p < 0.001). Fatigue-induced asymmetry was task-dependent, with stair negotiation showing the greatest increase (11.2% (SD 14.1) %). Peak loading rates on the affected knee were inversely correlated with time-to-peak load during fatigued conditions (r = -0.68, p < 0.001). Disease severity (WOMAC score) and age significantly predicted asymmetry changes (R2 = 0.52, p < 0.001). Conclusions: Physical fatigue exacerbates inter-limb load asymmetry in unilateral knee OA, potentially accelerating joint degeneration. Task-specific interventions targeting fatigue management and load redistribution may improve long-term outcomes. Future longitudinal studies should examine the temporal relationship between acute biomechanical changes and progressive structural joint damage. Keywords: Knee osteoarthritis, load asymmetry, fatigue, biomechanics, ground reaction forces, functional tasks.

Corticospinal-motoneuronal connections are more prominent in distal than proximal upper-limb muscles, but whether this anatomical specialization influences the capacity for activity-dependent plasticity remains unknown. Here, we tested whether Hebbian stimulation targeting corticospinal-motoneuronal connections, induces muscle-specific plasticity in proximal and distal arm muscles in healthy humans. We applied a spike-timing-dependent stimulation protocol comprising 180 paired stimuli, with transcranial magnetic stimulation timed to evoke corticospinal volleys 1-2 ms before motoneurons were antidromically activated by electrical stimulation of the brachial plexus for the biceps brachii or the ulnar nerve for the first dorsal interosseous (FDI). Corticospinal excitability was assessed by measuring motor-evoked potentials (MEPs) before and 30 min after stimulation. Hebbian stimulation increased MEP amplitudes in both muscles, but facilitation was greater in the FDI than in the biceps. Because resting motor threshold was higher for the biceps than for the FDI, in a control experiment we matched stimulation intensities between muscles. Under these conditions, Hebbian stimulation continued producing greater facilitation in the FDI than in the biceps, indicating that the difference was not attributable to stimulation intensity. Moreover, changes in MEP amplitude were associated with maximal MEP size in the FDI but not in the biceps, whereas no muscle-specific association was observed with peripheral responses (M-max), supporting a stronger relationship between corticospinal drive and Hebbian plasticity in the distal than in the proximal arm muscle. Together, these findings identify a distal-to-proximal gradient in Hebbian plasticity across arm muscles, which may contribute to optimizing targeted neuromodulation approaches for upper-limb rehabilitation.

Serial dependence, whereby current perception is systematically biased toward the most recent sensory experience, has been demonstrated across multiple sensory modalities. However, whether serial dependence occurs during active haptic perception remains unknown. Moreover, because haptic perception of object geometry depends on hand movements for sensory acquisition (i.e., active touch), it remains unclear whether serial dependence, if any, originates from perceptual processing, movement, or both. Here, we investigated whether active haptic perception exhibits attractive serial dependence using a newly developed haptic device that let participants explore virtual slopes of different heights. Participants reported perceived slope heights, and we recorded their kinematics. Perceptual judgments were systematically attracted toward the most recently experienced slope, demonstrating serial dependence during active haptic perception. By contrast, none of the kinematic parameters exhibited corresponding serial dependence. These findings show that serial dependence extends to the somatosensory perceptual domain and suggest that the observed perceptual bias was not mediated by systematic changes in hand movements.