Reciprocal Ia inhibition in patients with spinal spasticity.

Reciprocal Ia inhibition from ankle flexors to extensors was studied in five patients with spasticity due to incomplete traumatic spinal cord lesions. Nine healthy subjects were tested as controls. Excitability of the soleus motoneuron pool was estimated by H-reflex testing in the resting state. Ia inhibition was activated by conditioning stimuli to the peroneal nerve. Ia inhibition was detected in all patients tested, the amount of inhibition ranging from 8% to more than 50% of the test H-reflex size. In the control subjects only weak Ia inhibitory effects were present. These findings indicate increased excitability of the Ia inhibitory pathway to ankle extensor motoneurons in patients with spasticity due to spinal cord injury.