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.