Static vestibulospinal influences in relation to different body tilts in man

In this study, H-reflex testing was used to identify possible static vestibular influences on soleus alpha motoneuron excitability in man in relation to different body tilts. Ten normal adult volunteers and one labyrinthine-defective patient were tested in an experimental situation designed to minimize all afferent inputs except the vestibular ones. Each subject was fixed to a rigid platform inclinable from the horizontal (0 degree) to the vertical (90 degrees) position and vice versa. On each subject, 10 consecutive H reflexes were recorded for each of the following positions: 0 degree, 15 degrees, 30 degrees, 45 degrees, 60 degrees, 75 degrees, 90 degrees and 90 degrees, 75 degrees, 60 degrees, 45 degrees, 30 degrees, 15 degrees, 0 degree. In all normal subjects, we observed a significant increase in amplitude of the H reflex as the body was tilted from horizontal to vertical, and a significant decrease when the body was returned to the original position. These progressive changes in spinal motoneuron excitability were linearly related to the tilt angles. By contrast, no significant change in H-reflex amplitude was observed in the labyrinthine-defective patient. These results would seem to indicate that the vestibular system exerts some influences on antigravity muscles in static tilts and that these influences are behaviorally adequate to provide for the postural stability.