Corticospinal and intracortical responses from both motor cortices following unilateral concentric versus eccentric contractions

Cross-education is the phenomenon where training of one limb can cause neuromuscular adaptations in the opposite untrained limb. This effect has been reported to be greater after eccentric (ECC) than concentric (CON) strength training; however, the underpinning neurophysiological mechanisms remain unclear. Thus, we compared responses to transcranial magnetic stimulation (TMS) in both motor cortices following single sessions of unilateral ECC and CON exercise of the elbow flexors. Fourteen healthy adults performed three sets of 10 ECC and CON right elbow flexor contractions at 75% of respective maximum on separate days. Elbow flexor maximal voluntary isometric contraction (MVIC) torques were measured before and after exercise, and responses to single- and paired-pulse TMS were recorded from the non-exercised left and exercised right biceps brachii. Pre-exercise and post-exercise responses for ECC and CON were compared by repeated measures analyses of variance (ANOVAs). MVIC torque of the exercised arm decreased (p < 0.01) after CON (-30 +/- 14%) and ECC (-39 +/- 13%) similarly. For the non-exercised left biceps brachii, resting motor threshold (RMT) decreased after CON only (-4.2 +/- 3.9% of maximum stimulator output [MSO], p < 0.01), and intracortical facilitation (ICF) decreased (-15.2 +/- 20.0%, p = 0.038) after ECC only. For the exercised right biceps, RMT increased after ECC (8.6 +/- 6.2% MSO, p = 0.014) but not after CON (6.4 +/- 8.1% MSO, p = 0.066). Thus, unilateral ECC and CON elbow flexor exercise modulated excitability differently for the non-exercised hemisphere. These findings suggest that responses after a single bout of exercise may not reflect longer term adaptations.