H-reflex modulation during reverse passive pedalling

During passive pedalling or walking movements of the human leg, the gain of the soleus H reflex is depressed in proportion to the rate of the movement, with the depression maximal at the end of leg flexion. In this phase, maximum flexion of the hip and knee occur at different positions. The question was raised as to whether the gain changes still occur when the direction of movement is reversed, thus reversing those positions. In four subjects, reverse passive pedalling movements of the legs were studied at two velocities, 10 and 30 rpm. Ten H reflexes per subject were elicited from popliteal fossa stimulation at eight equispaced positions around the cycle and M waves were used as an indicator of stimulation stability. For each subject, reflex attenuation occurred in the flexion phase, but unlike forward movement, peak inhibition occurred before full flexion of the knee. Movement velocity continued to determine the degree of inhibition. The reverse vs. forward difference in peak inhibition most likely reflects differences in conditioning receptor discharge from hip and knee extensor muscles, due to the differing kinematic profiles for the two movements. Therefore the spinal gain modulation appears to receive significant input from specific somatosensory discharge consequential to the movement.