Motor compensatory reactions following a forward fall in subjects with unilateral abolition of the triceps-surae H reflex.

The electromyograms of the right and left soleus and tibialis anterior muscles of 6 subjects with unilateral abolition of Achilles tendon reflex due to S1 radiculitis were recorded during a forward fall involving stepping to recover balance. Each subject took part in two series of experiments, one in which the step was performed with the unaffected leg and a second in which the affected leg was used. A unilateral deficiency of peripheral proprioceptive afferents affected ankle muscles EMG activities bilaterally, except for the EMG activity of the soleus of the starting foot. The tibialis anterior of both the affected and the unaffected side, showed either a normal pattern (i.e. a phasic contraction after soleus contraction stopped), or an early contraction. On the stance side, the early contraction was associated with a depressed soleus EMG activity. Some abnormal motor patterns could be due to the ipsilateral deficiency of the Ia inhibitory projection from soleus to tibialis anterior. The presence of abnormal patterns on the unaffected side indicates that the motor activity in one lower limb can be modified by a loss of peripheral afference in the contralateral limb. This suggests that crossed pathways between lower limbs are involved in balance recovery movement.