Mirror Visual Feedback Selectively Attenuates Crossover Fatigue in Distal Upper Limb Musculature: A Randomized Controlled Crossover Investigation Comparing Children and Adults

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: <i>n</i> = 17, 24.64 ± 5.38 years; children: <i>n</i> = 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], <i>p</i> = 0.987, dz =+ 0.003), whereas vision occlusion induced significant crossover fatigue (-3.37 kg [-4.40, -2.35], <i>p</i> < 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], <i>p</i> < 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 (<i>p</i> = 0.068) or extension (<i>p</i> = 0.156). Age Group × Condition × Time × Limb interactions were non-significant (all <i>p</i> > 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.