Modulation of hamstrings reflexive responses during human gait

In humans, it is thought that both central commands and peripheral feedback from sensory receptors contribute to the control of locomotion. An important problem that exists in human locomotion research is the interactions and balance between the individual contributions of the central (CNS) and peripheral (PNS) nervous systems to the control of muscles during movement are not fully understood. Applying external perturbations such as stretches, tendon taps, and electrical stimulation to the neuromuscular system during walking can help us learn more about how the response to afferent information is modulated during locomotion. To date, most of the research looking at modulation of the response during walking has investigated the soleus and quadriceps muscles. Very little research has focused on the hamstring muscles, which are important during walking, particularly during late swing. One reason for this is that it is difficult to detect H-reflexes in hamstrings following electrical stimulation of the sciatic nerve. The purpose of this study was to demonstrate a new sciatic nerve stimulation technique and use it to study the modulation of the response to afferent feedback during walking.

This study consisted of two parts: 1) Establish the presence of an afferent mediated response (H-reflex) during prone lying in hamstrings muscles, and 2) Investigate the modulation of this afferent feedback during walking. Subjects underwent single and double pulse stimulations to the sciatic nerve during prone lying, followed by electrical stimulation at 12 different phases of the gait cycle. For each phase, stimulus response curves were created in which maximal direct (M-wave) and afferent mediated responses (H-reflex) could be determined. Maximal H-reflex (Hmax) was normalized to maximal M-wave (Mmax) to create an H:M ratio that was used to compare modulation of the responses between phases and subjects. Electrical stimulation of the sciatic nerve elicited detectable H-reflexes in biceps femoris during prone lying and walking. The modulation of the response to afferent feedback is not the same for all phases of the gait cycle, particularly in late swing when it has a higher amplitude than the rest of the gait cycle. This modulation was not simply related to background EMG as would be expected during isometric contractions. Thus, there must be both central and peripheral influences on the response. Understanding the control of human locomotion is important for developing rehabilitation programs for patients with lesions of the central nervous system such as stroke or spinal cord injury.