H reflexes were elicited in the small muscles of the foot in the canine and human during passive locomotor-like rotation of the shank about the ipsilateral knee. The movement-induced effect was similar in the two species. In the anesthetized dog, the reflex gain was reduced by 36 +/- 8.4% (mean +/- SE) on average, compared with appropriate stationary controls. Reflexes in the human were reduced during movement to 45 +/- 3.5% of their stationary control values. 2. H reflexes were elicited in the anesthetized dog during passive locomotor-like rotation about the knee and were compared with reflexes obtained with the limb stationary. Populations of mechanoreceptors were then systematically removed to ascertain which group or groups provided the sensory input that leads to the decrease in reflex gain during movement. We hypothesized that the majority of the reflex attenuation could be attributed to muscle mechanoreceptors. 3. Reflexes continued to be significantly reduced (P < 0.05) during passive movement about the knee until the muscle mechanoreceptors of the quadriceps muscle group were deactivated. The removal of input from joint receptors or cutaneous receptors did not eliminate the gain reduction induced by the passive movement. 4. It is concluded that muscle mechanoreceptors of the quadriceps muscle group provide an inhibitory input to the H reflex pathway of the dog plantar muscle when the knee is passively moved in a locomotor-like fashion. This source of inhibition likely also contributes to the soleus H reflex gain reduction in humans.
