Muscle activation in the elbow-forearm complex during rapid elbow extension.

Normal human subjects performed extensions of the elbow from an initial position to a visually defined target as rapidly and accurately as possible at different angular amplitudes (9 degrees-54 degrees) and at a constant amplitude (36 degrees) under various additional instructions. Joint angle, angular acceleration, and electromyograms (EMGs) from agonist (triceps lateralis), antagonist (biceps), and antebrachial muscles were recorded. EMGs of the antagonistic muscles showed reciprocal activation of the first triceps, and then the biceps, which was followed by the second triceps activation and coactivation of both muscles. Movement amplitude changed the reciprocal activation, whereas the coactivation was unaltered. Additional instructions changed the coactivation in amplitude, initiation, and termination, whereas the reciprocal activation was relatively preserved. Under these conditions, EMGs of antebrachial muscles were always linked with the coactivation in amplitude and timing but not with the reciprocal activation. The linkage suggests that antebrachial muscle activation serves as an indicator of the coactivation. From the indicator, we infer that the coactivation initiates from the beginning of the rapid movements and changes during the dynamic movement phase.