Unilateral strength training increases voluntary activation of the opposite untrained limb

Objective: We investigated whether an increase in neural drive from the motor cortex contributes to the cross-limb transfer of strength that can occur after unilateral strength training. Methods: Twitch interpolation was performed with transcranial magnetic stimulation to assess changes in strength and cortical voluntary activation in the untrained left wrist, before and after 4 weeks of unilateral strength-training involving maximal voluntary isometric wrist extension contractions (MVCs) for the right wrist (n = 10, control group = 10). Results: Wrist extension MVC force increased in both the trained (31.5 +/- 18%, mean SD, p < 0.001) and untrained wrist (8.2 +/- 9.7%, p = 0.02), whereas wrist abduction MVC did not change significantly. The amplitude of the superimposed twitches evoked during extension MVCs decreased by 35% ( 20%, p < 0.01), which contributed to a significant increase in voluntary activation (2.9 +/- 3.5%, p < 0.01). Electromyographic responses to cortical and peripheral stimulation were unchanged by training. There were no significant changes for the control group which did not train. Conclusion: Unilateral strength training increased the capacity of the motor cortex to drive the homogolous untrained muscles. Significance: The data show for the first time that an increase in cortical drive contributes to the contralateral strength training effect. (C) 2009 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.