Unilateral strength training increases voluntary activation of the opposite untrained limb

Abstract

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 homologous untrained muscles.

Significance: The data show for the first time that an increase in cortical drive contributes to the contralateral strength training effect.