Stretch reflex gain and threshold changes as a function of elbow stretch velocity in hemiparetic stroke

The main emphasis of this study is to determine the effect of stretch velocity on stretch reflex gain and threshold during elbow extension in the paretic upper limb of 13 hemiparetic stroke subjects. Ten velocities ranging from 6 to 150°/s were applied in random fashion. The resulting reflex torques were subjected to two different quantitative approaches following subtraction of passive torques obtained at the 6°/s velocity to determine the reflex threshold and gain. Both approaches determined changes in reflex threshold and gain as a function of stretch velocity in all subjects tested. The relative effects of threshold versus gain changes on stretch reflex induced flexion torques were tested for a 60°/s increase in stretch velocity from 45°to 105°/s. In all but one of the 13 subjects the effect of reflex gain became the predominant contributor to the reflex torque before the end of the stretch. The neurophysiological and clinical implications of these findings are discussed in the paper.