Comparison of amplitude of human soleus H-reflex during sitting and standing.

The modulation of the H-reflex in the human soleus muscle under conditions of different length or of background EMG activity was compared in 7 healthy subjects under three conditions: sitting, standing with support, and standing without support. The amplitude of the H-reflex increased when the muscle was shortened in both the sitting and standing conditions. The degree of increase in H-reflex was smaller during standing than sitting for the same change in muscle length. The H-reflex was augmented according to the increase of the background EMG. The "reflex gain", the ratio of the increase in amplitude of the H-reflex to soleus muscle EMG activity, decreased on sitting, standing with support and standing without support, ranked in that order. From these observations, it is concluded that the H-reflex is modulated by both muscle length and the degree of postural stability. The modulation of the reflex could be interpreted in terms of gain compensation and would serve to stabilize posture. A decrease in reflex gain may be appropriate in stabilizing the spinal reflex feedback loop during standing, especially without support.