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Effect of Modulated TENS on Muscle Activation, Oxygenation, and Pain: Searching for a Physiological Mechanism

Objectives: To understand the possible physiological mechanisms of a modulated transcutaneous electrical nerve stimulation [TENS] device on the reported reductions in pain using quantitative electromyography to measure muscle activation and near-infrared spectroscopy to estimate muscular oxygenation levels. Methods: All subjects self-selected the intensity of the TENS stimulation current. Electromyography, near-infrared spectroscopy and a color analog scale of pain intensity to measure reported pain levels were collected both pre- and post-treatment. For Studies 1 and 2, measures of muscle activity and self-perceived pain were taken from 51 male and female subjects. For study 3, measures of muscle oxygenation and pain scores were taken from 12 different subjects. Results: Myoelectric activation level and pain scores of painful muscles were reduced after treatment [P < 0.001] with no change in activation or pain in the control muscles. The follow-up study revealed that muscle oxygenation was significantly increased in the treatment trials when compared to the control trials [P = 0.013], while the color analog scale of pain intensity pain scores decreased [P <. 05]. Conclusions: Pain reduction following TENS treatment has been reported before. The documented reduction in muscle activation suggests less vasoconstriction from prolonged contraction. Simultaneous documentation of increases in available oxygen suggests a tenable mechanism; reduced contraction level and less oxygen consumption, and/or more blood flow suggests an increased capacity to flush metabolites known to cause pain.