H-reflex changes during contractions of the ankle extensors in spastic patients.

Soleus H-reflexes during tonic contractions and isometric ramp contractions of the ankle extensors in spastic and healthy subjects were measured. During the tonic contractions, the H-reflex increased with the contraction level. The increase was highest in the patients (p < 0.001). The facilitation of the H-reflex during a ramp contraction is due to a static component resulting from the increased excitation level and a dynamic component resulting from the modulation of the H-reflex. The dynamic H-reflex facilitation during the ramp contraction was decreased in the patients (p < 0.05). The findings suggest that there is a decreased H-reflex control in spastic patients and this could be explained by a decreased presynaptic inhibition or by postsynaptic changes.