REASSESSMENT OF H-REFLEX RECOVERY CURVE USING THE DOUBLE STIMULATION PROCEDURE

We conducted two types of experiments to assess the validity of the H- reflex recovery test, using double stimulation to test soleus motoneur on pool excitability in healthy and spastic subjects, One type dealt w ith the mechanical effect of the conditioning H reflex on the ankle jo int; the other type with the effect of change in reflex size, The mech anical effect was tested both with the ankle joint fixed (FX) and free to move (FR). Differences between FX and FR conditions commenced with relaxation of soleus muscle contraction by the conditioning H reflex. In the FR condition, abrupt facilitation occurred, and changed to mar ked depression, We conclude that specific facilitation and inhibition in the FR condition were secondary effects of group la inflows caused by the ankle extensor muscle stretching on relaxation. In some spastic patients as well as in controls, facilitation due to the mechanical e ffect in the FR condition was observed despite the FX condition. The e ffects of systematic changes on soleus H-reflex size were investigated at conditioning-test intervals of 80 ms, so as to avoid mechanical ef fects. When conditioning and test reflexes were the same size, the amo unt of recovery increased as the H-reflex size increased. Comparison o f the relation between amount of recovery and H-reflex size, expressed as a percentage of Mmax, showed no significant difference between the two groups. We speculate that the stronger recovery of spasticity men tioned in previous literature may have resulted from the fact that rel atively greater H reflexes were tested in those studies. In conclusion , the present study indicates that double stimulation is not appropria te for assessing spinal motoneuron pool ''excitability increase'' in s pasticity. (C) 1998 John Wiley & Sons, Inc.