Effects of TENS and topical skin anesthesia on soleus H-reflex and the concomitant influence of skin/muscle temperature.

Abstract

The purpose of this study was to determine, in ten healthy subjects, the extent of soleus motoneuronal excitability during conditions of increased (transcutaneous electrical nerve stimulation [TENS]), decreased (Xylocaine [lidocaine]a anaesthesia) and normal (placebo anaesthesia) cutaneous inputs. Increased cutaneous activity was evoked using a TENS unit, with the two pairs of electrodes placed respectively over the Achilles (S2 dermatome) and tibialis anterior (L5 dermatome) tendons. Experimental and placebo topical anaesthesia were obtained after rubbing Xylocaine (5%) and Vaselineb ointment, respectively, on the skin surface overlying the Achilles tendon. Sets of ten H-responses (Hmax/2) were evoked at a frequency of 1 shock/30s and averaged at regular time intervals before, during and after the testing conditions. The results showed a gradual increase (up to 40% after 20 minutes) of H-reflex amplitude during TENS regardless of whether it was applied on the L5 or S2 dermatome. Furthermore, placebo anesthesia (Vaseline) caused the same gradual facilitatory response (up to 100% after 50 minutes) as that obtained during Xylocaine anaesthesia. Power spectral analysis of the H-responses obtained over time showed that the increase in the peak-to-peak H-response value was accompanied by a shift of the spectral content toward low frequencies. This shift occurred concomitantly with a cooling of the skin overlying the soleus muscle.