Observations on the variability of the H reflex in human soleus

H reflexes were evoked in human soleus by stimulating the tibial nerve at a constant intensity. Each trial was then assigned to one of three groups on the basis of the amplitude of its H reflex; all trials in each group were then full-wave rectified and reaveraged. There was a strong positive relati onship between the amplitude of the H reflex and the level of electromyogra phic activity in the muscle at the time of onset of the H reflex, which ref lects the activity of the motoneuronal pool when the afferent volley arrive d. Thus, much of the variability of the H reflex is due to small changes in the level of activation of the motoneuronal pool during repeated trials. T he steady torque preceding the H reflex was a poor predictor of the H-refle x amplitude, presumably because of the delay between the changes in the ele ctrical activity of motoneurons and the mechanical outcome thereof. (C) 199 9 John Wiley & Sons, Inc.