Biological sex influences bilateral transcortical reflexes during unilateral upper extremity exercise in healthy adults.

The transcortical long-latency reflex (LLR) is a rapid feedback response that is thought to help stabilize movement following sudden disturbances and is a critical mechanism enabling precise upper limb control during unexpected perturbations. It occurs 50-150 ms after limb displacement and integrates both spinal and supraspinal circuits. Despite extensive research on reflexes during movement, sex-specific differences in LLR responses remain underexplored. Importantly, while bilateral reflex responses have been documented when both arms contribute to a shared movement goal, cross-transcortical communication in relaxed, nonparticipating limbs during unilateral perturbations has not been investigated. This study examined sex differences in visuomotor tracking accuracy and transcortical reflex responses during exercise induced perturbations in 40 healthy participants (20 males and 20 females). Participants performed elbow flexion/extension tracking movements using their nondominant arm while the contralateral arm remained at rest. Unexpected perturbations were delivered by removing brake resistance during early elbow flexion, with surface electromyography recording muscle activity from bilateral triceps and biceps muscles. Results revealed distinct sex-specific neuromuscular control strategies: females employed mainly feed forward co-activation strategies, while males utilized reactive fast feedback LLR control strategies. Novel cross-transcortical responses were detected in the nonparticipating contralateral arm, with sex-specific bilateral motor control patterns emerging during unilateral perturbations. These findings advance our understanding of upper extremity movement control across sex during upper limb training and have implications for developing personalized skill training exercise programs for males and females.