Inter-trial variation of soleus H reflex in humans: implication for supraspinal influence.

Inter-trial variation in soleus H reflex amplitude for different effort modes, elicited at different test-stimulus intensities, was studied in fifteen healthy subjects. Regardless of test-stimulus intensity, weak isometric plantarflexion and dorsiflexion at 20% of maximum voluntary contraction caused significant potentiation and depression of the H reflex amplitude. In addition, both facilitation and inhibition of the soleus H reflex was associated with considerable inter-trial variability of the H reflex amplitude that varied proportionally with test-stimulus intensity. As inter-variability of the H reflex was independent of that of M response, the results indicate that: 1) supra-spinal input influences loop-gain fluctuations of the monosynaptic-reflex arc; and 2) smaller test stimuli may be more efficacious for exploration of motoneuronal excitability because of reduction in variability in H reflex amplitude.