Exercise-induced Hypoalgesia After Comparative Forms of Anaerobic Training in Healthy Adults

Known changes of the hypothalamic-pituitary-adrenal (HPA) axis, as well as other neurotransmitter systems, lead to alteration of peripheral and central nociception during acute and chronic stress. Exercise is one example of such physiologic stress. Exercise, in various forms, has been shown to exert effects on pain attenuation. Several studies have demonstrated post-exercise reduction in pain ratings, increased pain threshold as well as attenuation of pain-related increases in motor evoked potentials. This phenomenon is referred to as exercise-induced analgesia (EIA) or, perhaps more accurately, exercise-induced hypoalgesia (EIH).

Exercise at higher intensities and longer durations have the most robust evidence supporting their role in EIH. This is true of both long-duration aerobic training as well as high-intensity anaerobic training. This type of activity, however, may not be the most practical type of activity prescribed in a clinical population, especially in people with painful conditions, due to decreased conditioning and tolerance. Beyond long-duration and/or high-intensity aerobic exercise, there is growing evidence supporting the use of other forms and intensities of exercise that may induce EIH. Alternative forms of exercise, such as isometric stabilization, have been shown to decrease pain in a population of patients with low back pain. Koltyn and Arbogast investigated acute effects of exercise on pain perception and found that a single bout of resistance exercise at 75% of maximum intensity produces significant increases in pain thresholds and reductions in pain ratings.11 EIH can also occur during and following short duration (e.g., 1.5-5 min) and lower intensity (e.g., 15-50% max) isometric exercise. Along with high-intensity training, long-duration/low-intensity isometric contractions also have been found to produce an analgesic response. Studies have suggested different post-exercise pain responses in chronic pain populations compared to healthy controls, however questions still remain over the influence of various exercise intensities, protocols or types of contractions. Slater, et al examined the effects of eccentric exercise versus combined concentric-eccentric exercise at an intensity of 30% of max voluntary contraction (MVC) showed a significant increase in pain thresholds for both exercise protocols. Their study highlighted the need for further research to examine if changes also occur in response to single exercise modes. To date, no studies have compared EIH produced by isometric and concentric exercises at similar intensities and duration. Furthermore, little research in EIH has compared various forms of exercise to non-exercising controls.

Similar to EIH induced by aerobic exercise, the relationship of anaerobic exercise with hypoalgesia indicates a common mechanism that may be related to the augmentation of the central and peripheral nervous system through modulation of descending inhibition as well as the release of pain-relieving substances. There is possibly an orchestra of overlying and inter-related physiologic, social and psychologic mechanisms that contribute to hypoalgesia response to exercise. Because of confounding effect of various processes of all of these possible processes, it is important to evaluate specific exercise regimens that may provide most pain relief within the context of clinical practice. Although there is good evidence that long-duration aerobic, and high-intensity exercises modulate pain, these strategies may be difficult to apply in a clinical setting due to the disabling nature of painful conditions. Patients with such conditions may be more apt to comply with shorter duration exercises at lesser intensity. No studies to date have specifically addressed differences in pain response when comparing the EIH following anaerobic exercises using concentric and isometric training bouts as well as to a non-exercising control condition. Investigation of EIH in this study will improve understanding of nociceptive changes following therapeutic exercises in a clinical setting.

Aim 1: To examine changes in pain following two different types of exercise (isometric versus concentric contractions).

Hypothesis 1a: Pain will be significantly lower following both types of exercise in comparison to the non-exercising control condition. Currently, it is unclear whether pain responses will differ between the two exercise conditions since no previous research has been conducted in this area.