Influence of static tilt on soleus motoneuron excitability in man.

The influence of whole head-body rotations with respect to gravity on the excitability of the human soleus motoneuron pool, was studied by mapping the H reflex amplitude as a function of static head displacements. In most subjects, the H reflex reached a minimum when the head was near the vertical position (90 degrees), and often increased as a sinusoidal function of tilt in the sagittal plane. These changes were attributed to the tonic labyrinthine reflexes which, unlike some findings in decerebrate cats, generally increase the extensor tone of the human lower limb when the head is displaced from the upright position.