365) Is it Exercise or Pain Associated with Exercise that Leads to Hypoalgesia

Abstract

The opioid epidemic has brought attention to the pain relieving effect of exercise, but exercise prescription for pain relief may be quite different from that typically recommended for cardiovascular and other health benefits. Exercise induced hypoalgesia (EIH) is complex and not entirely understood but possible mechanisms include the opioid and non-opioid systems. Hypoalgesia is also known to occur in response to pain itself. Conditioned pain modulation (CPM) is the inhibition of pain from a test stimulus by a painful conditioning stimulus. Studies of EIH often involve exercise paradigms in which long duration muscle contractions are used.\xa0 Long duration muscle contractions are often painful so it raises the question, “Is it exercise or the pain associated with exercise that leads to EIH?” Nine individuals underwent pain testing on the right and left knees and middle finger before and after three conditions: 10 voluntary isometric contractions, 10 contractions elicited by neuromuscular electrical stimulation (NMES) and 10 bursts of noxious electrical stimulation without muscle contraction all on the right quadriceps muscles. The contractions or stimuli lasted 10 s separated by 50 s off. The average pain rating (0-100 VAS) during the noxious stimulation (84.4 +/- 12.1) was higher than during the NMES elicited contractions (61.4 +/- 18.9; p=0.001). \xa0Subjects reported no pain during the volitional contractions. The force of the contractions is reported in % maximum voluntary contraction (%MVIC) and the volitional contraction force averaged 59.6 (+/- 18.0) % MVIC while the NMES elicited contraction force averaged 52.0 (+/-14.5) % MVIC. Hypoalgesia was observed in the right and left knees response to the noxious stimulation and NMES elicited contractions but not the volitional contractions. The results suggest that the hypoalgesia associated with the NMES elicited contractions was due to the noxious nature of the contractions and that the mechanism involves spinal mechanisms.