MUSCLE ACTIVATION IN THE ELBOW-FOREARM COMPLEX DURING RAPID ELBOW EXTENSION

Normal human subjects performed extensions of the elbow from an initia l position to a visually defined target as rapidly and accurately as p ossible at different angular amplitudes (9 degrees-54 degrees) and at a constant amplitude (36 degrees) under various additional instruction s. Joint angle, angular acceleration, and electromyograms (EMGs) from agonist (triceps lateralis), antagonist (biceps), and antebrachial mus cles were recorded. EMGs of the antagonistic muscles showed reciprocal activation of the first triceps, and then the biceps, which was follo wed by the second triceps activation and coactivation of both muscles. Movement amplitude changed the reciprocal activation, whereas the coa ctivation was unaltered. Additional instructions changed the coactivat ion in amplitude, initiation, and termination, whereas the reciprocal activation was relatively preserved. Under these conditions, EMGs of a ntebrachial muscles were always linked with the coactivation in amplit ude and timing but not with the reciprocal activation. The linkage sug gests that antebrachial muscle activation serves as an indicator of th e coactivation. From the indicator, we infer that the coactivation ini tiates from the beginning of the rapid movements and changes during th e dynamic movement phase.