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

The effect of continuous Achills 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 to a decrease in vibratory inhibition in spasticity is commonly attributed 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.