Twitch interpolation of the elbow flexor muscles at high forces.

We investigated factors affecting maximal voluntary torque and the assessment of the level of voluntary drive in the elbow flexor muscles. First, the effective compliance of the system was tested by using single, paired, and trains of four stimuli to measure voluntary activation. At high voluntary torques the responses to all these stimuli were identical, suggesting that single stimuli are adequate for estimating voluntary drive. Second, the contribution of torque from synergist elbow flexor muscles was assessed. In attempted maximal voluntary contractions (MVCs), the voluntary activation of brachioradialis (median 91.5%, range 68.9-100%) was lower than for biceps brachii (median 99.1%, range 78.5-100%; P < 0.01). This suggests extra torque may be generated by brachioradialis during elbow flexion, beyond the torque where biceps brachii is maximally activated. Finally, lengthening of the elbow flexors occurred during MVCs, due to slight shoulder movements. This would allow force to increase independently of an increase in voluntary drive.