In healthy adults, it is well demonstrated that a single bout of exercise can acutely reduce pain (Naugle et al., 2012), a phenomenon known as exercise-induced hypoalgesia. However, the mechanisms of pain reduction after exercise are not clear. The methods commonly used to study pain responses after exercise in humans provide little insight as to where in the nervous system changes are occurring. For example, exercise could reduce pain through a change in the peripheral nervous system, the central nervous system, or both. It is possible that by better understanding the mechanisms of exercise-induced hypoalgesia, more effective exercise therapies could be developed for patients with chronic pain to help them manage their pain and improve their quality of life. Therefore, understanding how a single bout of exercise acutely reduces pain is of clinical importance.
In our study, pressure pain thresholds were measured before and after five minutes of high intensity cycle exercise. During this time, we blocked blood flow to the participant’s arm using an inflated blood pressure cuff. This meant that any pain relieving substances that might be released into the blood during exercise (e.g. opioids or cannabinoids) would not be able to reach the periphery where pain thresholds were being assessed. Our results showed that blocking blood flow to a limb during exercise resulted in less exercise-induced hypoalgesia. That is, the increase in pain threshold after exercise was smaller in the arm where blood flow was occluded.
SIGNIFICANCE AND IMPLICATIONS
This was the first demonstration in humans that a peripheral mechanism mediates changes in pain after exercise. Our methods did not allow us to say exactly what agents were involved in this effect or exactly where in the peripheral nervous system the changes occurred. One possibility is that agents that were released into the blood during exercise (e.g. opioids, cannabinoids and catecholamines) reduced the sensitivity of nociceptors – specialised receptors that respond to potentially damaging stimuli. Similarly to the way in which some common analgesic medications act to reduce pain. Therefore, it may be that combining exercise with peripherally acting analgesic drugs could have additive effects on relieving pain, which may be particularly useful for patients with chronic pain who avoid exercise for fear of it exacerbating their pain and symptoms.
Jones MD, Taylor JL, Barry BK (2017). Occlusion of blood flow attenuates exercise-induced hypoalgesia in the occluded limb of healthy adults. J Appl Physiol. Article in Press.
Naugle KM, Fillingim RB, Riley JL 3rd (2012). A meta-analytic review of the hypoalgesic effects of exercise. J Pain, 13(12): 1139-50.