Observations on the control of human ankle position by stretch reflexes

Summary

Load compensation in human ankle muscles was investigated by applying disturbances which rotated the foot while the subject was endeavouring to maintain a constant position against a pre-existing force. Three separate stages of the force response were distinguished. First, for the initial 100 msec an 18% increase in force (expressed as a percentage of the total force required to correct the disturbance) which was attributed to the elastic resistance of muscles active prior to the disturbance. A short latency (SL) EMG response at 33 msec occurred in the soleus (SOL) muscle only when the foot was dorsiflexed, but no SL EMG response was observed in the tibialis anterior (TA) muscle following plantar flexion. It was difficult to attribute a part of the first increase in force to SL EMG activity in SOL. Second, after approximately 130–150 msec a medium latency (ML) increase in force which was preceded by an increase in EMG activity. After a dorsiflexion disturbance the ML EMG response at 120 msec in SOL was followed by an 18% increase in force. But, after a plantar flexion disturbance, the ML EMG response in TA at 100 msec was followed by ML EMG response in SOL at 130 msec, and a 2% increase in force. Third, a long latency increase in force accompanied EMG activity at 230 msec and was compatible with voluntary action which restored the foot to its original position. These results are consistent with previous results obtained in shoulder muscles (Allum, 1975) and support the hypothesis that the ML response is a test signal designed to inform the CNS of the current loading on the muscle.