Static and dynamic changes in body orientation modulate spinal reflex excitability in humans.

In the present study, we investigated the modulation pattern of the soleus H reflex in healthy subjects in response to imposed static and dynamic changes in body angle, referenced to the vertical plane. Soleus H reflexes were recorded using conventional methods with subjects either supine or while they were erect. Changes in body angle were initiated with subjects lying supine on a tilt table. Table position was controlled via a motor and could move from the horizontal to the upright position and beyond. Elastic bands around the trunk (upper and lower part) and around the thigh and shank secured subjects' position. In the vertical position, the soleus H reflex exhibited a strong depression in all subjects tested, reaching amplitudes as low as 40+/-8.1% of the control reflex (Ho). With subjects supine, positioning the body at 10 degrees, 20 degrees, 40 degrees, 60 degrees, 90 degrees, -50 degrees and -20 degrees all resulted in a significant facilitation of the soleus H reflex. The reflex magnitude at these angles ranged from 140+/-17.2% to 180+/-10.9% of the Ho. Reflex facilitation was also observed following dynamic tilt of the body in the sagittal plane (at 1.8 degrees /s) with the H reflex reaching amplitudes as high as 300+/-18.3% of Ho. Our findings indicate that changes in body orientation induced a significant facilitation of the H reflex magnitude in soleus motoneurones that were essentially independent of angular change in body orientation or of movement direction. In addition, they highlight the potent modulatory effects that natural stimulation of the vestibular system can have on reflex excitability. The implications of our findings are discussed in relation to the maintenance of body posture.