Differences in stretch reflex responses of elbow flexor muscles during shortening, lengthening and isometric contractions.

Stretch reflexes were evoked in elbow flexor muscles undergoing three different muscle contractions, i.e. isotonic shortening (SHO) and lengthening (LEN), and isometric (ISO) contractions. The intermuscle relationships for the magnitude of the stretch reflex component in the eletromyographic (EMG) activities of two main elbow flexor muscles, i.e. the biceps brachii (BB) and the brachioradialis (BRD), were compared among the three types of contractions. The subjects were requested to move their forearms sinusoidally (0.1 Hz) against a constant pre-load between elbow joint angles of 10 degrees (0 degrees = full extension) and 80 degrees during SHO and LEN, and to keep an angle of 45 degrees during the ISO. The perturbations were applied at the elbow angle of 45 degrees in pseudo-random order. The EMG signals were rectified and averaged over a period of 100 ms before and 400 ms after the onset of the perturbation 40-50 times. From the ensemble averaged EMG waveform, the background activity (BGA), short (20-50 ms) and long latency (M2, 50-80, M3, 80-100 ms) reflex and voluntary activity (100-150 ms) components were measured. The results showed that both BGA and reflex EMG activity of the two elbow flexor muscles were markedly decreased during the lengthening contraction compared to the SHO and ISO contractions. Furthermore, the changes of reflex EMG components in the BRD muscle were more pronounced than those in the BB muscle, i.e. the ratios of M2 and M3 magnitudes between BRD and BB (BRD:BB) were significantly reduced during the LEN contractions. These results would suggest that the gain of long latency stretch reflex EMG activities in synergistic muscles might be modulated independently according to the model of muscle contraction.