Stretch responses to ankle rotation in multiple sclerosis patients with spasticity

In 13 spastic patients with multiple sclerosis and 10 control subjects, electromyographic (EMG) and mechanical responses to stretch of the ankle extensors and ankle flexors during maintained contraction were measured. The reflex EMG responses in the extensors were divided into a phasic response (40-140 msec after onset of stretch) and a tonic response (200-400 msec after onset of stretch). In the control subjects, both the onset and peak latency of the phasic EMG response decreased with the contraction level (0.01 < P < 0.02 and P < 0.002 respectively) in the extensors. In the patients the latency of the phasic EMG response in the extensors was independent of the voluntary contraction level. This could be attributed to a disruption of the normal recruitment of the motor units according to the size principle. The phasic EMG response was larger in the patients than in the control subjects (P < 0.01). The tonic EMG response was of equal size in the two groups. The larger phasic EMG response in the patients was not followed by an increase in the reflex mediated mechanical stretch response. This shows that proposed changes in the muscle function in spastic patients based on changes in EMG stretch responses must be made with caution. In the ankle flexors all patients had reduced or absent EMG responses to stretch, consistent with earlier findings of an absent mechanical reflex mediated response.