Assessment of voluntary activation by stimulation of one muscle or two synergistic muscles

In order to test the hypothesis that suboptimal activation of synergistic muscles explains the nonlinear nature of the interpolated twitch-voluntary torque relationship, we evaluated differences between stimulating a single muscle [biceps brachii (BB)] and two synergists simultaneously [BB and brachioradialis (BR)] on variables related to the estimation of voluntary activation. Ten subjects performed maximal and submaximal voluntary contractions while electrical stimulation was applied either to the BB, or BB and BR simultaneously, to assess voluntary activation. Simultaneous stimulation of the two synergistic muscles produced significantly greater torque at rest and a significantly greater activation index (95% vs. 91.7% for stimulation of BB only). However, simultaneous stimulation of BB and BR did not consistently elicit greater additional torque during contraction, nor did it improve the linearity of the interpolated twitch-voluntary torque relationship. The greater activation index observed for the simultaneous stimulation of BB and BR compared to BB is explained by the greater torque elicited at rest. We conclude that the nonlinear relationship between interpolated and voluntary elbow flexion torque is not explained by suboptimally activated synergistic muscles.