Background and Objectives: This study investigated whether crossover neuromuscular fatigue following unilateral handgrip exercise exhibits directional asymmetry, testing whether dominant-limb fatigue produces greater contralateral performance decrements than non-dominant-limb fatigue in adults and pre-peak-height-velocity children. Materials and Methods: Thirty-three healthy, right-handed males (16 adults: 22.5 ± 1.6 years; 17 pre-peak-height-velocity boys: 11.2 ± 0.8 years, maturity offset −2.2 ± 0.4 years) completed three counterbalanced experimental sessions (48–72 h apart): dominant-arm fatigue, non-dominant-arm fatigue, and control. The fatigue protocol consisted of 20 consecutive 6 s maximal voluntary isometric handgrip contractions. Primary outcomes were percentage changes in maximal voluntary isometric contraction of the contralateral limb across handgrip, elbow flexor, and elbow extensor muscle groups. Results: The experimental condition explained approximately 64% of crossover variance in adults (η_p² = 0.650, ηG² = 0.421) and children (η_p² = 0.638, ηG² = 0.448; both p < 0.001). Dominant-limb fatigue elicited substantially greater contralateral decrements than non-dominant-limb fatigue in adults (−11.00% vs. −3.92%, dz = −1.07) and children (−12.71% vs. −3.08%, dz = −1.33), representing 2.5- to 3.5-fold greater transfer efficiency (both p < 0.001). Age-group comparisons revealed no differences in crossover susceptibility (p = 0.627, η_p² = 0.008), with equivalence testing confirming developmental invariance. Crossover effects extended to heterologous proximal muscles without magnitude differences (p > 0.13). Conclusions: Crossover fatigue (contralateral performance decrement following unilateral exercise) exhibited directional asymmetry, with dominant-limb protocols eliciting 2.5- to 3.5-fold greater contralateral decrements. This pattern aligns with asymmetric transcallosal inhibitory projections demonstrated in prior transcranial magnetic stimulation studies, though direct neurophysiological confirmation was not obtained. Functional equivalence between pre-peak-height-velocity children and adults indicates that interhemispheric transfer mechanisms achieve operational maturity before peak height velocity. Extension to heterologous muscles implicates supraspinal mechanisms. The findings establish normative parameters for clinical populations with compromised transcallosal integrity.

This investigation examined whether mirror visual feedback modulates crossover fatigue magnitude during unilateral handgrip exertion and whether efficacy demonstrates age-dependent and muscle-group-specific characteristics. Thirty-three participants stratified by developmental stage (adults: n = 17, 24.64 ± 5.38 years; children: n = 16, 11.87 ± 0.79 years) completed a randomized controlled crossover protocol incorporating three visual feedback conditions: mirror reflection of the exercised limb, occluded vision (no-mirror), and passive rest control. Participants performed unilateral dominant handgrip fatigue induction (20 × 6 s maximal voluntary isometric contractions) while bilateral force production was quantified pre-intervention and post-intervention across handgrip, elbow flexion, and elbow extension domains. Linear mixed-effects models with participant-specific random intercepts and slopes quantified Condition × Time × Limb interactions. In the non-exercised contralateral limb, linear mixed-effects models demonstrated that under the mirror condition, non-dominant handgrip force was maintained at rest-equivalent levels relative to control (+0.02 kg, 95% CI [−1.15, +1.17], p = 0.987, dz =+ 0.003), whereas vision occlusion induced significant crossover fatigue (−3.37 kg [−4.40, −2.35], p < 0.001, dz =− 1.16). All contrasts represent within-subject difference-of-differences in non-dominant limb change score (Post − Pre) extracted from the full factorial LMM via emmeans within the Limb = Non-dominant stratum pooled across age groups. The mirror versus no-mirror comparison yielded +3.38 kg [+2.43, +4.34], p < 0.001, dz =+ 1.26. Age-stratified analyses confirmed comparable effect magnitudes (adults: dz =+ 1.40; children: dz =+ 1.33). Muscle-group specificity emerged for handgrip but not elbow flexion (p = 0.068) or extension (p = 0.156). Age Group × Condition × Time × Limb interactions were non-significant (all p > 0.16), providing no evidence of age moderation within the tested developmental range. Mirror visual feedback constitutes an effective countermeasure against crossover fatigue in distal upper limb musculature. The magnitude of mirror-induced attenuation did not differ between children (aged 10–13 years) and adults within our sample, with no statistically detectable age moderation within the tested developmental range; formal equivalence testing was not conducted. Effects demonstrated anatomical selectivity, favoring hand musculature over proximal elbow musculature.

This study aimed to investigate the long-term effects of a six-week dynamic stretching warm-up protocol on sprint performance, agility, and dynamic balance in adolescent volleyball athletes. Thirty male volleyball players, aged 16 to 17 years, were randomly assigned to an experimental group (n = 14) performing dynamic stretching or a control group (n = 16) undertaking a conventional warm-up without stretching. The intervention lasted 6 weeks and comprised 5 sessions each week. Evaluations performed prior to and after the intervention included a 5-m-sprint test, a modified T-test for agility, and a Y-Balance-test for dynamic balance. The results indicated a significant interaction between time and the group (p < 0.01, ηp2: 0.32–0.47 [large]) only in the Y-balance test in the anterior and posterolateral directions. The experimental group demonstrated a significant improvement in dynamic balance compared to the control group in both the anterior (p = 0.018, d = 1.15 [large]) and posterolateral (p = 0.001, d = 1.72 [large]) directions of the Y-balance test. No significant improvements in agility performance were seen (p = 0.469, ηp2 = 0.03 [small]). Sprint performance declined in both groups, demonstrating a significant main effect of time (p < 0.001, ηp2 = 0.59 [large]), whereas no significant group effect was seen (p = 0.086, ηp2 = 0.16 [large]). Therefore, a six-week dynamic stretching warm-up exercise program enhanced teenage male volleyball players' dynamic balance significantly, particularly in the front and posterolateral directions. The training intervention decreased short-distance (5 m) sprint performance and did not improve agility. Although dynamic stretching can improve some aspects of neuromuscular control, the findings suggest that it should be paired with sport-specific training to optimize overall volleyball performance and injury prevention techniques.