Variations in the soleus H-reflex as a function of activation during controlled lengthening and shortening actions

The effect of soleus activation on the soleus H-reflex was investigated during controlled lengthening and shortening of the plantar flexor muscles. Maximal H-reflexes and M-waves were evoked at the same muscle length (ankle angle 90degrees) during lengthening and shortening (ankle angular velocity 5degrees s(-1)) with soleus either passive or with its electromyographic activity at 10, 20 and 30% of that during a maximal voluntary isometric plantar flexion. In passive trials, the H-MAX:M-MAX ratio during lengthening was lower than during shortening. In active trials at 10 and 20%, the H-MAX:M-MAX ratio tended to be lower during lengthening than shortening. Within the active trials, H-MAX:M-MAX ratios were not different between the three levels of soleus activation, neither for lengthening nor shortening actions. When all active trials were pooled, the lengthening H-MAX:M-MAX ratio was significantly lower than the shortening one. In lengthening, the H-MAX:M-MAX ratio increased in the active with respect to the passive condition, whereas no change occurred in active with respect to the passive shortening. These results indicate action type specificity in the way the la-excitatory effect is modulated as the soleus muscle is voluntarily activated. (C) 2002 Elsevier Science B.V. All rights reserved.