The cross-over effects of unilateral resistance exercise in trained males and females.

Purpose: To identify the cross-over effects on maximal contralateral force and muscle excitability following fatiguing unilateral resistance exercise of the lower and upper limbs in chronically trained participants.

Methods: Two crossover experiments using a within-subject, time-matched control design examined the contralateral responses on maximal force and muscle excitability in the elbow flexors (Experiment 1, resistance-trained, n = 20) and knee extensors (Experiment 2, endurance-trained, n = 23). Participants performed unilateral resistance exercise (four sets to task failure) while contralateral assessments were collected and compared to a no-exercise control condition.

Results: Contralateral elbow flexor, but not knee extension, force declined over time (p < 0.01, ηₚ² = 0.116), but was not influenced by visit (p = 0.563, ηₚ² < 0.01) or sex (p = 0.779, ηₚ² < 0.01), indicating no added effect of unilateral fatiguing exercise. Muscle excitability showed a significant effect for sex, with females maintaining higher excitability than males (p < 0.01, d = 0.420). Biceps brachii excitability showed a decline during the control visit only, while rectus femoris and vastus lateralis showed reduced excitability post-exercise compared to control (p < 0.05, d = 0.600-1.02).

Discussion: These findings suggest that acute unilateral resistance exercise does not compromise nor enhance contralateral force in trained individuals. However, alterations in maximal muscle excitability despite maintained force imply rapid contralateral neuromuscular adaptations within the input-output properties of muscle excitation and force. These results suggest differential susceptibility to cross-over effects across limbs and between sexes following unilateral resistance exercise.