MOVEMENT-INDUCED MODULATION OF SOLEUS H-REFLEX WITH ALTERED LENGTH OFBIARTICULAR MUSCLES

Passive pedaling movements of the leg results in the phasic modulation of the soleus H reflex of that leg. in contrast, the H reflex of the contralateral leg is attenuated tonically. The phasic modulation of th e reflex ipsilaterally can be attributed to the afferent discharge ass ociated with the cyclic lengthening of the extensor muscles. We hypoth esized that the tonic attenuation of the contralateral reflex could be explained if the afferent feedback arising from the lengthening of th e biarticular muscles had an increased importance in regulating the am plitude of the contralateral reflex. To test this, the passive pedalin g movements were reduced to those about either the knee or hip alone. Despite the alteration in the pattern of stretching of the biarticular muscles, the contralateral soleus H reflex was tonically attenuated d uring both forms of single joint movements. We suggest that the same p hasic afferent discharge responsible for the modulation of the ipsilat eral soleus H reflex initiates the tonic attenuation contralaterally, but that the signal undergoes a complex transformation in crossing the cord. These results do not rule out the possibility that the stretchi ng of the biarticular muscles contributes to the attenuation of the ip silateral soleus H reflex, which is subsequently masked by a powerful influence from the stretching of the uniarticular extensor muscles. To test this possibility, a second experiment manipulated the lengths of the muscles of the leg by altering the positions of the static joints during isolated rotation of either the knee or hip and measuring the amplitude of the ipsilateral soleus H reflex. From the results, it was clear that stretching the uniarticular extensor muscles produced the most dramatic effects. However, the stretch of the biarticular muscles yielded mild inhibitory influences if these muscles were near their m aximal lengths. (C) 1998 Elsevier Science B.V. All rights reserved.