Analgesic effects induced by TENS and electroacupuncture with different types of stimulating electrodes on deep tissues in human subjects

Effects of conditioning peripheral nerve stimulation with different types of stimulating electrodes on pain thresholds in various deep tissues were measured in human subjects. Cone-shaped metal (phi 13 mm), rubber (phi 13 mm), and large soft surface electrodes (50 x 150 mm) were used for transcutaneous electrical nerve stimulation (TENS), and insulated and non-insulated acupuncture needles (diameter: 240 microns) were used for electroacupuncture (EA). Two pairs of electrodes were placed around the point of deep pain measurement. Symmetrical positive and negative square pulses (0.1 msec at 100 Hz) of just below the pain tolerance intensity were used for both TENS and EA. Deep pain thresholds were measured at the center of the thigh with a pulse algometer and insulated needle electrodes. Pain thresholds of deep tissues were in the order periosteum < fascia < skin (including subcutaneous tissues) < muscle. TENS with surface electrodes significantly increased pain thresholds of skin and fascia but not those of muscle or periosteum. The shape, material and size of the surface electrodes hardly affected the degree of analgesic effect, except in the fascia by large soft electrodes. In contrast, EA with non-insulated needles induced a greater increase in pain threshold in skin, fascia and muscle, although statistically significant results were obtained in only the first two tissues. EA with insulated needle electrodes was the only technique with which we obtained a significant increase in pain threshold in muscle and periosteum. These results suggest that the choice of electrode and stimulus parameters is important for the production of sufficient analgesic effects in different somatic tissues and that insulated needle electrodes are useful for pain relief in deeper tissues such as muscle and periosteum.