Spinal reflexes in ankle flexor and extensor muscles after chronic central nervous system lesions and functional electrical stimulation.

OBJECTIVE: Spinal reciprocal inhibitory and excitatory reflexes of ankle extensor and flexor muscles were investigated in ambulatory participants with chronic central nervous system (CNS) lesions causing foot drop as a function of time after lesion and stimulator use. METHODS: Thirty-nine participants with progressive (eg, secondary progressive MS) and 36 with generally nonprogressive (eg, stroke) conditions were studied. The tibialis anterior (TA) and soleus maximum H-reflex/M-wave (Hmax/Mmax) ratios and maximum voluntary contractions (MVC) were measured and compared with those in age-matched control participants. Reciprocal inhibition was measured as a depression of the ongoing electromyographic (EMG) activity produced by antagonist muscle-nerve stimulation. RESULTS: Participants with CNS lesions had significantly higher soleus Hmax/Mmax ratios than control participants, and reduced voluntary modulation of the reflexes occurred in both muscles. Reciprocal inhibition of soleus from common peroneal (CP) nerve stimulation was not significantly different from controls in either group. Inhibition of the TA by tibial nerve stimulation decreased and was eventually replaced by excitation in participants with nonprogressive disorders. No significant change occurred in progressive disorders. Use of a foot drop stimulator increased the TA, but not the soleus MVC overall. H-reflexes only showed small changes. Reciprocal inhibition of the TA increased considerably, while that of the soleus muscle decreased toward control values. CONCLUSIONS: Disorders that produce foot drop also produce reflex changes, some of which only develop over a period of years or even decades. Regular use of a foot drop stimulator strengthens voluntary pathways and changes some reflexes toward control values. Thus, stimulators may provide multiple benefits to people with foot drop.

Quantifying the effects of voluntary contraction and inter-stimulus interval on the human soleus H-reflex.

The human soleus H-reflex is commonly tested as an indicator of the reflex excitability of the calf muscles with infrequent stimuli to a subject seated and at rest. However, the reflex varies widely with the level of voluntary contraction and with the time history of stimulation. We studied two aspects of this variation. Antagonist (tibialis anterior) activation decreases the response, while increasing agonist (soleus) activation increases the H-reflex to a peak after which it declines. In subjects with large H-reflexes at rest, the reflex peaked at low levels of contraction. In contrast, in subjects with small H-reflexes at rest, the reflex peaked at higher levels of contraction for reasons that were elucidated using a realistic computer model. A parabolic curve fitted the maximum amplitude of the H-reflex in the model and over the entire range of contractile levels studied. The second aspect studied was post-activation depression or homosynaptic depression (HD), which has been described previously as a reduction of a second H-reflex elicited shortly after an initial reflex. We confirmed the presence of HD in resting, seated subjects for intervals up to 4 s. However, by voluntarily activating the soleus muscle, HD was drastically reduced when seated and abolished when standing. This suggests that HD may be absent in normal, functional movements and perhaps in clinical conditions that alter H-reflexes. Meaningful, quantitative measurements of reflex excitability can only be made under voluntary activity that mimics the condition of interest.