Movement modulation of a short latency reflex linking the lower leg and the knee extensor muscles in humans.

Movement modulation of a heteronymous short latency spinal reflex was investigated with transcutaneous, low threshold, stimuli to the common peroneal nerve, during leg pedalling in humans. Electromyographic (EMG) electrodes on the skin over the quadriceps muscles revealed an excitatory response at a minimum latency of 22.8 msec. Samples were obtained at 5 phases in the cycle of pedal crank rotation, and for stimulus intensities to evoke both maximal, and submaximal, reflexes in quadriceps. Over the pedal cycle, the peak-to-peak magnitude of the reflex, in the 3 quadriceps muscles studied, was deeply modulated, including an areflexive phase. The reflex magnitude was linearly dependent on the ongoing EMG activity in quadriceps. It could be fully re-initiated if contractions were produced out of phase in pedalling. The slopes of the regression lines of reflex magnitude (as % maximum reflex) on contraction level (% maximum voluntary contraction) when pedalling (1.05 (r = 0.86, P less than 0.01], and with isometric contractions in the same subjects seated (1.03 (r = 0.69, P less than 0.01], were very similar, with similar thresholds. This contrasts with the inhibition of soleus H reflexes during human gait. The present heteronymous reflex, acting between limb segments, is modulated coincident with ongoing contraction level in the target muscle.