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 90 degrees ) during lengthening and shortening (ankle angular velocity 5 degrees 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 Ia-excitatory effect is modulated as the soleus muscle is voluntarily activated.