Modulation of the soleus H reflex with different velocities of passive movement of the arm.

Modulation of the gain of the soleus H reflex pathway produced by passive movement of the elbow joint was investigated and compared to that produced by passively maintained displacement of the elbow joint position. Little modulation of the gain of the soleus H reflex pathway was observed during passive displacement of the elbow joint position. In contrast, passive movement of the elbow joint facilitated the gain of the soleus H reflex amplitude and the degree of modulation was large as the velocity increased. The degree of the facilitatory effect upon passive movement of the upper extremity ipsilateral to the leg under test was larger than that obtained upon passive movement of the upper extremity contralateral to the leg under test. The modulation may be caused by discharge at the primary endings of the spindles in the upper extremity which is likely to excite propriospinal interneurons connecting to motor neurons of the lower extremity.