The strengthening component of exercise is usually the focus when including them into a rehabilitation program. However, exercise also has a pain relieving aspect known as exercise-induced hypoalgesia. This article will discuss the research on which exercises have been shown to stimulate exercise-induced hypoalgesia and the potential mechanisms that are involved.

Which exercises produce hypoalgesia?

Aerobic exercise and isometric muscle contractions have been studied for their effects on hypoalgesia after exercise.A study in the Pain Journal by H.B Vaegter compared exercise-induced hypoalgesia (EIH) produced with aerobic exercise to isometric muscle contractions.¹ The aerobic exercise group cycled for two ten minute sessions at either 50 percent Max VO₂ or 75 percent Max VO₂. The study found that aerobic exercise increased the pain threshold during exercise, however, the pain threshold returned to baseline within 15 minutes after exercise. Low intensity cycling (50 percent VO₂ Max) did not significantly activate exercise-induced hypoalgesia.In the isometric muscle contraction group, contraction of the biceps and quadriceps were tested at 30 percent and 60 percent maximal voluntary contraction. Isometric contractions were found to stimulate exercise-induced hypoalgesia during exercise as well as 15 minutes following exercise. The higher intensity muscle contraction (60 percent MVC) produced significantly greater exercise-induced hypoalgesia compared with the lower intensity muscle contraction (30 percent MVC).Aerobic exercise and isometric muscle contractions both produced local increases in pain thresholds as well as global increases, although increases were higher locally than globally. The global hypoalgesic effects suggest that there is a central mechanism also involved with exercise-induced hypoalgesia.In another study from the European Journal of Pain, the effects of aerobic exercise and isometric muscle contractions on pain tolerance and temporal summation of pain were studied.² Temporal summation of pain (TSP) is a term used to describe how repetitive painful stimuli can become more painful (ie. repetitive bending forward becomes progressively more painful for someone with chronic low back pain).The study found that aerobic exercise (stationary cycling) slightly increased pain threshold but did not reduce temporal summation of pain. Isometric contraction of the quadriceps and biceps increased pain thresholds as well as reduced temporal summation of pain. Low and high intensity isometric contraction (30 and 60 percent MVC, respectively) of the quadriceps for two sets of 90 seconds reduced temporal summation of pain, however, only high intensity isometric contraction of the biceps reduced temporal summation.In a third study in the British Journal of Sports Medicine, isotonic exercises were compared to isometric muscle contractions for patellar tendon pain.³ Heavy isometric exercises (70 percent of MVC) reduced patellar tendon pain for 45 minutes following exercises. The isometric exercises were held for 45 seconds for 5 repetitions. Isotonic exercises had a smaller reduction in patellar tendon pain compared to isometric exercises and the reduction in pain was not sustained following the exercise.

How does exercise-induced hypoalgesia work? 

There are several mechanisms believed to be involved with exercise-induced hypoalgesia: the endogenous opioid system, endogenous cannabinoid system, and cardiovascular system.Several studies have used naloxone injections to determine if the opioid system is involved with exercise-induced hypoalgesia.^1,4,5 Naloxone is an opioid antagonist. The studies have demonstrated a reduction in exercise-induced hypoalgesia after naloxone injections which suggests that the endogenous opioid system is involved.While naloxone reduced the effects of exercise-induced hypoalgesia, the opioid system may not be the only mechanism involved. Another mechanism potentially involved with exercise-induced hypoalgessia is the cardiovascular system.Since exercise elevates the systolic blood pressure, it has also been suggested that the activation baroreceptors may be involved in exercise-induced hypoalgesia.⁶ The activation of baroreceptors has been shown to stimulate areas of the brain that are involved with pain modulation. Experimentally, the stimulation of baroreceptors has also been associated with a decrease in pain perception.The endogenous cannabinoid system has also been suggested as a potential mechanism for exercise-induced hypoalgesia. A study in the Journal of Pain measured the circulating endocannabinoid plasma concentrations (N-arachidonylethanolamine and 2-arachidonoylglycerol) following isometric exercises.⁵ The study included a placebo group as well as a group injected with naltrexone (opioid antagonist). The study found no difference in pain responses following isometric exercises between the placebo and naltrexone groups, suggesting a non-opioid mechanism for exercise-induced hypoalgesia. There was also a significant increase in circulating endocannabinoids following isometric exercise, suggesting that the endogenous cannabinoid system is a potential mechanism for EIH.

What is the clinical significance? 

Opioids and other pharmacologic options are commonly prescribed for pain management with the potential for addiction and abuse becoming a public health concern. Exercise-induced hypoalgesia stimulates both the endogenous opioid and non-opioid systems (ie. endocannabinoids) that are same targets for pharmacologic pain medications.The activation of these systems may vary depending on the stimulus (exercise-induced compared to pharmacologically-induced), but explaining to patients that their bodies are capable of stimulating the same pain modulating systems through exercise may empower them to feel less dependent on pharmacologic treatment options.Exercise is usually prescribed as a way to stabilize the joints in the body (ie. core exercises to stabilize the lumbar spine) but the anti-nociceptive effects of exercise are frequently ignored. The anti-nociceptive effects of exercise, specifically aerobic and isometric exercises, suggests that they can be included in the pain management stages of treatment.Rehabilitation programs tend to shy away from heavier loads with pain because of the fear of causing more pain or more tissue damage. However, the research mentioned above suggests that heavier loading (60 percent of MVC) produced greater local hypoaglesic effects. The exact amount of loading to include in a rehabilitation program remains a clinical decision and should be based on the patient’s tolerance to loading.

Conclusion:

Traditional rehabilitation exercises (ie. core exercises, rotator cuff strengthening, etc…) are usually prescribed to strengthen the stabilizing muscles around the joint. While these rehabilitation exercises can improve the strength and stabilization of a joint, aerobic and isometric exercises can also be an important part of a pain management strategy.

Resources:

1. Vaegter, Henrik Bjarke, Gitte Handberg, and Thomas Graven-Nielsen. "Similarities between Exercise-induced Hypoalgesia and Conditioned Pain Modulation in Humans." Pain 155.1 (2014): 158-67.
2. Vaegter, H.b., G. Handberg, and T. Graven-Nielsen. "Isometric Exercises Reduce Temporal Summation of Pressure Pain in Humans." European Journal of Pain 19.7 (2014): 973-83.
3. Rio, Ebonie, Dawson Kidgell, Craig Purdam, Jamie Gaida, G. Lorimer Moseley, Alan J. Pearce, and Jill Cook. "Isometric Exercise Induces Analgesia and Reduces Inhibition in Patellar Tendinopathy." British Journal of Sports Medicine 49.19 (2015): 1277-283.
4. Janal, Malvin N., Edward W.d. Colt, Crawford W. Clark, and Murray Glusman. "Pain Sensitivity, Mood and Plasma Endocrine Levels in Man following Long-distance Running: Effects of Naloxone." Pain 19.1 (1984): 13-25.
5. Koltyn, Kelli F., Angelique G. Brellenthin, Dane B. Cook, Nalini Sehgal, and Cecilia Hillard. "Mechanisms of Exercise-Induced Hypoalgesia." The Journal of Pain 15.12 (2014): 1294-304.
6. Koltyn, Kelli F., and Masataka Umeda. "Exercise, Hypoalgesia and Blood Pressure." Sports Medicine 36.3 (2006): 207-14.