Is human galvanically induced triceps surae electromyogram a vestibulospinal reflex response?

Interest in understanding the human vestibulospinal reflex has increased enormously over the past three decades, because this reflex is the primary effector of maintenance of posture and balance. On a posture platform, forces exerted by the triceps surae (TS) and tibialis anterior muscles are measured to calculate center of mass sway. We wished to determine whether the TS response is a direct component of the vestibulospinal reflex. Ten healthy human beings were stimulated with sinusoidal galvanic currents delivered over their mastoid processes. Sway response on a posture platform and TS electromyogram (EMG) were recorded for the following conditions: (1) standing unrestrained; (2) standing completely restrained above the leg; and (3) sitting unrestrained. Results were similar for all subjects. Computer-aided analysis for case 1 reveals that TS EMG and horizontal body sway responses are generated at the same frequency as the stimulating current, with a phase lag of 90 degrees. For case 2, body sway response and any component of the TS EMG over the unstimulated condition were absent in all subjects. For case 3, body sway persisted, but no TS EMG above the unstimulated condition was recorded. As the TS EMG disappears when the standing subject is restrained from swaying or in the unrestrained seated subject, we conclude that the TS EMG response is compensatory to motion of more superior portions of the musculoskeletal system; it is not part of the vestibulospinal reflex.