TWITCH INTERPOLATION OF THE ELBOW FLEXOR MUSCLES AT HIGH FORCES

We investigated factors affecting maximal voluntary torque and the ass essment of the level of voluntary drive in the elbow flexor muscles. F irst, the effective compliance of the system was tested by using singl e, paired, and trains of four stimuli to measure voluntary activation. At high voluntary torques the responses to all these stimuli were ide ntical, suggesting that single stimuli are adequate for estimating vol untary drive. Second, the contribution of torque from synergist elbow flexor muscles was assessed. In attempted maximal voluntary contractio ns (MVCs), the voluntary activation of brachioradialis (median 91.5%, range 68.9-100%) was lower than for biceps brachii (median 99.1%, rang e 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 flex ors occurred during MVCs, due to slight shoulder movements. This would allow force to increase independently of an increase in voluntary dri ve. (C) 1998 John Wiley & Sons, Inc.