The excitability of a motoneuron pool assessed by the H-reflex method is correlated with the susceptibility of Ia terminals to repetitive discharges in humans.

The correlation of the degree of the Hoffmann (H-) reflex depression induced by a passive stretch of the soleus muscle (Sol) and the excitability of the motoneuron (MN) pool was investigated in 24 healthy human subjects. The excitability of the Sol MN pool was determined by the ratio of the slope of the H-reflex (Hslp) and M-response recruitment curves (Mslp). The Hslp/Mslp ratio was decreased by a passive stretch of the Sol given 1 s before the H-reflex stimulus, and was significantly correlated with the amount of the H-reflex depression. These results suggest that the excitability of the MN pool assessed by the H-reflex method depends on the presynaptic mechanism involved in the efficiency of transmission across the synapses of Ia afferents in response to repetitive discharge. Copyright 1999 Elsevier Science B.V.