On the soleus H-reflex modulation pattern during walking.

In a recent paper it was claimed that in the majority (9/15) of subjects studied the soleus H-reflex increases progressively during the swing phase of walking. This pattern was at odds with our numerous observations made since 1986, as was the very large proportion of subjects reported to exhibit this pattern. We therefore reinvestigated the issue in an extensive series of experiments and detailed subsequent analysis on 21 subjects. In most subjects (13/21) the soleus H-reflex was completely inhibited during most or all of the swing phase (group A). In 8/21 subjects (group B) there was a small H-reflex mean 16% (SD=10.6%) of the value in quiet standing present during most or all of swing, but there was no systematic modulation pattern; the reflex amplitude fluctuated in a seemingly random manner. The difference between the two somewhat arbitrary groups could not be explained on the basis of greater electromyographic activity in the tibialis anterior (TA) during the swing phase or at the time of heel contact. However, by normalizing the mean level of TA activity to the peak level, the ratio was significantly greater for the group A subjects. This highlights the importance of reciprocal inhibition in accounting for the suppression of the soleus H-reflex in swing. In the discussion we emphasize that the presence of a small H-reflex during swing in the group B subjects is unlikely to have any functional role. What is of functional importance is the strong inhibition of the H-reflex during swing which reflects the ensemble of neural mechanisms at play to prevent the unwanted activation of the powerful ankle extensor muscles.