Periodicity in the Hoffmann reflex recovery curve

Hoffmann (H) reflex recovery curves were recorded from the soleus muscles of 10 healthy adult subjects with a view to further elucidating the pattern of facilitation of the motoneuron pool. Specific consideration was given to the possible existence of periodicity in facilitation of the motoneuron pool following a subthreshold conditioning H-reflex stimulus. The reliability of the recovery curves was also examined. The recovery curves, which utilized a 50% maximal test response, revealed the well established early facilitation (peak at 10 ms) followed by a later facilitation commencing 50 to 70 ms after the conditioning stimulus. In addition, there was evidence of peaks in excitability of the motoneuron pool occurring at 70 to 75 ms, 125 to 150 ms, and about 250 ms. The reliability of the periodicity in the recovery curve was established by examining the subjects on two separate occasions and investigating the effect of the number of measurements made within each day. Although there was considerable inter- and intrasubject variability in the form of the recovery curves, both within days, and between days, the overall group data showed a remarkably consistent periodicity for the first 350 ms following the conditioning stimulus. Methodological considerations that are important for observing the periodicity in the recovery curve were identified. These included an adequate number of measurements (test reflexes) per data point in the recovery curve, adequate sampling rate, and conditioning and test stimulus intensity. The clinical significance of these results resides in the possible contributions of the peripherally triggered periodic facilitation to rhythmic phenomena such as clonus and physiologic tremor.