Dysfunctional Endogenous Analgesia During Exercise in Patients with Chronic Pain: To Exercise or Not to Exercise?

Background: Exercise is an effective treatment for various chronic pain disorders, includingfibromyalgia, chronic neck pain, osteoarthritis, rheumatoid arthritis, and chronic low back pain.Although the clinical benefits of exercise therapy in these populations are well established (i.e.evidence based), it is currently unclear whether exercise has positive effects on the processesinvolved in chronic pain (e.g. central pain modulation).Objectives: Reviewing the available evidence addressing the effects of exercise on central painmodulation in patients with chronic pain.Methods: Narrative review.Results: Exercise activates endogenous analgesia in healthy individuals. The increased painthreshold following exercise is due to the release of endogenous opioids and activation of(supra)spinal nociceptive inhibitory mechanisms orchestrated by the brain. Exercise triggers therelease of β-endorphins from the pituitary (peripherally) and the hypothalamus (centrally), whichin turn enables analgesic effects by activating µ-opioid receptors peripherally and centrally,respectively. The hypothalamus, through its projections on the periaqueductal grey, has thecapacity to activate descending nociceptive inhibitory mechanisms.However, several groups have shown dysfunctioning of endogenous analgesia in response toexercise in patients with chronic pain. Muscle contractions activate generalized endogenousanalgesia in healthy, pain-free humans and patients with either osteoarthritis or rheumatoidarthritis, but result in increased generalised pain sensitivity in fibromyalgia patients. In patientshaving local muscular pain (e.g. shoulder myalgia), exercising non-painful muscles activatesgeneralized endogenous analgesia. However, exercising painful muscles does not change painsensitivity either in the exercising muscle or at distant locations.Limitations: The reviewed studies examined acute effects of exercise rather than long-termeffects of exercise therapy.Conclusions: A dysfunctional response of patients with chronic pain and aberrations incentral pain modulation to exercise has been shown, indicating that exercise therapy should beindividually tailored with emphasis on prevention of symptom flares. The paper discusses thetranslation of these findings to rehabilitation practice together with future research avenues.Key words: Whiplash, fibromyalgia, chronic pain, low back pain, exercise, rehabilitation,chronic fatigue syndrome, osteoarthritis, rheumatoid arthritis, sensitization, shoulder