Hip angle induced modulation of H reflex amplitude, latency and duration in spinal cord injured humans

Objectives: To investigate the modulation of the soleus H reflex in spinal cord injured (SCI) subjects resulting from imposed changes in hip angle and to establish whether changes in H reflex amplitude co-vary with changes in reflex latency and duration. Methods: H reflexes were recorded using conventional methods in 7 SCI subjects in the supine position. The right leg was secured by a leg brace and positioned at various angles of hip flexion (30°, 40°) and at 10° of hip extension. Results: We found that imposing 10° of hip extension resulted in a significant facilitation in the size of the soleus H reflex in all of the SCI subjects tested (200% of control reflex; recorded at 10° of hip flexion). In contrast, positioning the hip at 30° and at 40° of flexion resulted in a significant reduction of the H reflex in 6 of 7 SCI subjects tested. In the remaining subject, an increase in the H reflex amplitude was observed. Modulation of H reflex amplitude coincided with shifts in both H reflex latency and duration. The reflex latency was prolonged when the reflex amplitude was reduced following hip flexion, while hip extension shortened the reflex latency. In contrast, the H reflex duration was prolonged with hip extended and shortened with hip flexed. Conclusions: When changes in static hip joint position are imposed in SCI subjects, changes in afferent feedback from hip proprioceptors are capable of promoting a switch between excitatory and inhibitory pathways. Associated changes in H reflex latency and duration are consistent with the hypothesis that oligosynaptic inputs contribute to the hip angle-induced H reflex modulation. Possible mechanisms for these effects are discussed.