Changes in hip position modulate soleus H-reflex excitability in man.

The effects of hip flexion and extension on the ipsilateral soleus Hoffmann (H) reflex recruitment curve were studied in 11 healthy subjects. Hip flexion (50 degrees), but not hip extension (15-20 degrees), produced changes in the H-reflex. A maintained facilitation, peaking at intensities of stimulation producing a maximal H-reflex (Hmax), was observed in 6/18 sessions. Inhibition, peaking at intensities submaximal for Hmax, was seen in 7/18 sessions. In some of the latter experiments, there was also a facilitation at high intensities of stimulation (greater than Hmax). The remaining experiments were classified as showing no effect: 3 were unmodulated but 2 showed a facilitation at high intensities of stimulation (greater than Hmax). Since the knee was extended in the test position, a second series of experiments (n = 7) were carried out to determine the possible influence of stretch of the biarticular hamstrings muscle group on the soleus H-reflex by comparing the effects of hip flexion with the knee extended with those obtained when the knee was flexed, thereby relaxing the hamstrings. The results provided no evidence that the variability could be explained by differences in the relative degree of stretch on the hamstrings muscle group. There were, however, systematic variations in the shape of the corresponding control H-reflex recruitment curves between subjects: the mean slope of the rising limb of the recruitment curve was highest in those experiments showing an inhibition, intermediate in the ineffective experiments and lowest in those showing a maintained facilitation. These observations indicate that the reflex output studied was different in the three groups, possibly reflecting differences in the relative proportions of slow- and fast-twitch motor units contributing to the reflex response.