Compensatory reactions in forward fall: are they initiated by stretch receptors?

Electromyographic activities from soleus and tibialis anterior muscles of 12 healthy subjects and 3 patients with vestibular syndrome, together with onset of movement of the head and leg were recorded during balance recovery. The disequilibrium was induced from initial forward inclination of the body. Soleus of the oscillatory foot showed brisk activity starting 59 msec (mean latency) after the perturbation and lasting 100-120 msec. Soleus of the stance foot showed similar activity, except that its duration was longer. The antagonist muscle activities started 5-20 msec later and showed similar time courses but of smaller amplitude. Ischaemia of the leg did not modify the latency. The onset of head movement was 10-20 msec after the perturbation, and those of leg were 70-90 msec. The results suggest that the early motor responses of the balance recovery were neither triggered by discharges from soleus group I and group II afferents, nor from vestibular cues, but could possibly originate in receptors located at the abdominal or lumbar level.