Modulation of the soleus H-reflex during pedalling in normal humans and in patients with spinal spasticity

Soleus H-reflexes were recorded in 10 normal subjects and seven patients with spasticity caused by incomplete spinal cord injury while they pedalled on a stationary bicycle which had been modified to trigger electrical stimuli to the tibial nerve at eight precise points in the pedal cycle. Stimulus strength was adjusted to yield M-waves of constant amplitude at each pedal position. During active pedalling, all normal subjects showed modulation of the H-reflex with the amplitude being increased during the downstroke portion of the pedal cycle and the reflex suppressed or absent during the upstroke. This modulation was not present during passive pedalling, with the experimenter cranking the pedals by hand, or when the pedals were locked at each of the eight positions. In five of the seven patients with spasticity, there was reduced or absent modulation of the H-reflex during active pedalling and the reflex remained large during pedal upstroke. It is concluded that descending motor commands that produce patterned voluntary activity during pedalling normally cause cyclical gating of spinal reflexes by either presynaptic or postsynaptic inhibitory mechanisms. Loss of supraspinal control over these spinal inhibitory systems could result in failure to produce appropriate suppression of reflexes during patterned voluntary movements such as pedalling or walking, and may be an important factor contributing to the functional disability in spasticity.