Inhibition of the human flexion reflex by low intensity, high frequency transcutaneous electrical nerve stimulation (TENS) has a gradual onset and offset.

The present study examines the inhibitory effect of segmentally applied TENS on the nociceptive component of the flexion reflex elicited in various lower limb muscles, in an attempt to gain some insight into the underlying mechanism. The flexion reflex from 11 normal subjects was recorded electromyographically from the biceps femoris (BF), the tibialis anterior (TA), and in 2 subjects, the hip flexor (HF), in the manner described in a previous paper [9]. Amplitude and area values of the flexion reflex of each muscle were computerized prior to, during, and 50 min after the application of placebo or low intensity TENS at 100 Hz, for 30 min to the low back, at levels of segmental innervation (L4-S1) similar to those of the muscles under study. In the majority of subjects, we found that: Low intensity TENS caused a significant inhibition of the flexion reflex in proximal limb flexors. Thus, the BF measured 64% and 52%, and the HF 45% and 51%, of their respective mean control amplitude and area values at the time of maximum inhibition during TENS. Moreover, less reduction of the mean values of the flexion reflex was observed in the TA, a distal limb (ankle) flexor. It is noteworthy that in both the BF and HF, the time to peak maximum inhibitory effect took 30 and 20 min respectively after the onset of TENS, and the flexion reflex often did not return to control values even at 40-50 min after TENS. In contrast, placebo TENS application resulted in no significant change of the flexion reflex in all the muscles examined. These findings showed that prolonged stimulation of large diameter fibers by conventional TENS application to the lumbosacral level, exerts a progressive and long latency inhibitory influence on a number of lower limb flexor motoneurons. In keeping with functional demand, this effect was found to be more prominent on the proximal than distal limb muscles. Furthermore, a gradual onset and offset of this inhibitory action is consistent with the results of some investigators demonstrating the possible involvement of endogenous opioids.