Modulation of soleus H reflex by lateral tilting in man.

Static vestibular influences on extensor tone of the lower limbs in man were studied by analyzing the changes in right soleus H-reflex (RSHR) area in relation to lateral tiltings. Eight normal adult volunteers were tested in an experimental situation designed to minimize all afferent inputs, except the vestibular ones. Each subject was seated on a chair which could be tilted laterally from the vertical to both sides. Lateral tiltings were applied at a random order from the vertical (0 degree, control position) to 4 degrees, 8 degrees, 12 degrees, 16 degrees, and 20 degrees of both sides (test positions). The results showed inhibition in SHR area of the leg ipsilateral to the tilting and facilitation of the contralateral SHR. These data indicate that, in man, as in the decerebrate cat, tonic labyrinth reflexes act asymmetrically and that, in static condition, the vestibular system modulates muscle tone of the lower limbs adequately to counteract lateral perturbation of upright position.