The relationship between the soleus H-reflex amplitude and vibratory inhibition in controls and spastic subjects .1. Experimental results

The effect of continuous Achilles tendon vibration on the soleus H-reflex amplitude was quantified over the entire H-reflex recruitment trajectory in 30 controls and 33 patients with spasticity in the lower limbs. The results show that with increasing stimulus intensities, vibratory inhibition of the H-reflex initially increases, then subsequently decreases. This is probably a direct consequence of how the activation thresholds of the motoneurons are distributed over the motoneuron pool. In patients, vibratory inhibition of the H-reflex was less over the entire recruitment trajectory than in controls. The decrease in vibratory inhibition in spasticity is commonly attributed to a decrease in presynaptic inhibition or post-activation depression. However, the average H-reflex threshold was lower in the patients, suggesting a decrease of the motoneuron activation thresholds. A lower reflex threshold in spasticity, therefore, may contribute to the observed reduction of vibratory inhibition. Copyright (C) 1996 Published by Elsevier Science Ltd.