Muscle activation in unilateral and bilateral efforts assessed by motor nerve and cortical stimulation.

Voluntary muscle activation was measured with twitch interpolation in 11 subjects during attempted maximal voluntary contractions of the right thumb adductor muscles either in isolation ("thumb alone") or as the subjects simultaneously performed maximal voluntary contractions of the left thumb adductors or left elbow flexors ("both thumbs" or "thumb and elbow", respectively). During thumb alone contractions, median voluntary activation of the right thumb adductors was 90.3%, and subjects fully activated the thumb adductors on 22% of all contractions. Transcranial magnetic stimulation of the cortex during maximal voluntary efforts produced small twitchlike force increases, suggesting that at least part of the voluntary activation failure was attributable to suboptimal corticospinal drive. Maximal voluntary force produced by the right thumb adductors in the three conditions differed by < 2% (P = 0.21), and the ability to activate the thumb adductors in the both thumbs condition was only marginally less than during thumb alone contractions (median 88.6%; P = 0.004). Thus subjects are usually unable to fully activate their thumb adductors with maximal voluntary efforts, and simultaneous maximal contractions of contralateral muscle groups have little effect on this ability.