Differential angle-dependent modulation of the long-latency stretch reflexresponses in elbow flexion synergists

We determined the effect of elbow joint angle on the short-(M1) and long-la tency stretch reflex (M2 and M3) responses of the elbow flexion synergists, the brachioradialis (BR), and the biceps brachii (BB), during weak isometr ic elbow flexion tasks. The elbow joint angle was 35,75 and 115 degrees (fu ll-extension angle was 0 degrees), and the muscle contraction level was 0,3 and 6% of maximum voluntary contraction (MVC) of the BR. In BR, the M1, M2 and M3 responses were significantly greater at 75 and 115 degrees than at 35 degrees. On the other hand, in BB, the M2 response was significantly gre ater at 35 degrees than at 75 and 115 degrees, while the M1 and M3 response s were not significantly different among the elbow joint angles. These resu lts indicated that the stretch reflex responses of BR might be dependent on the changes of muscle length in stretch stimulus, while the M2 response of BB might not be dependent on the actual stimulus intensity. Therefore, we concluded that the M2 of BB might be modulated selectively by a higher refl ex center in accordance with relationships of the mechanical advantages bet ween synergistic muscles. (C) 2000 Elsevier Science Ltd. All rights reserve d.