Figure 1.

Soleus H-reflex during standing and walking, soleus and tibialis anterior (TA) EMG activity and soleus H-reflex size over the step-cycle in a neurologically normal subject and in two subjects with chronic incomplete SCI. A: Soleus H-reflex sizes during standing and walking in each of 12 equally spaced step-cycle bins versus soleus EMG activity. Each standing H-reflex symbol (x) is the average of 4 responses and each walking H-reflex symbol (o) is the average of 5–10 responses. M-wave sizes are maintained the same within and between tasks. In the normal subjects, the H-reflex at a specific EMG level is clearly smaller during walking than during standing (Capaday and Stein 1986), whereas in subjects with chronic SCI, this task-dependent modulation of H-reflex size between standing and walking is impaired; and H-reflex size does not increase with background EMG level in standing or walking. B: Soleus H-reflex modulation during walking. In the normal subject, the pattern is similar to the soleus EMG pattern (C). In the subjects with SCI (middle and right), H-reflex modulation across the step-cycle is minimal, and hyperactivity of the H-reflex pathway probably contributes to the abnormally low TA activity during the swing phase (D) and to the resulting foot drop. C: Soleus EMG during walking. In the normal subject, soleus EMG activity gradually increases from heel contact to push off, then falls to near zero and remains low for the entire swing phase. In figure subjects with SCI, soleus EMG activity may (middle) or may not (right) be normally modulated. In the subject on the right column, the soleus remains active during the swing phase. D: TA EMG activity during walking. In the normal subject, TA activity typically shows two distinct peaks: one in the early swing phase and another during the swing-stance transition. In subjects with SCI (middle and right), TA activity is often minimal throughout the step-cycle.