Mechanisms hypothesized for pain-relieving effects of exercise in fibromyalgia: a scoping review

BACKGROUND: Exercise is one of the most recommended management strategies by treatment guidelines for fibromyalgia (FM); however, the mechanism through which exercise improves pain in FM is still unknown. OBJECTIVE: We aimed to summarize the hypothesized theoretical mechanisms for the pain-relieving effects of exercise in people with FM. ELIGIBILITY CRITERIA: Randomized controlled trials (RCTs) in English reporting mechanisms for pain-relieving effects of exercise in the ‘Introduction’ and ‘Discussion’ sections and significant within- group or between-group effects of exercise interventions were included. SOURCES OF EVIDENCE: We searched the databases Ovid MEDLINE(R), EMBASE, CINAHL, COCHRANE, Sports Discuss, and AMED with the keywords: exercise and fibromyalgia until December 2021. CHARTING METHODS: Two authors independently performed title/abstract, full-text review, and data abstraction using a data abstraction form. The hypothesized mechanisms from individual studies were grouped into three categories. RESULTS: The literature search resulted in 2147 studies, out of which 220 studies were considered for full-text review. A total of 50 RCTs proposing 29 unique mechanisms for the pain-relieving effects of exercise were included. These mechanisms were divided into three categories: physical, neuro-physiological, and psychological. The neuro-physiological category was further subdivided into exercise-induced hypoalgesia (EIH), pain sensitization, the autonomic system, the immune system, the endocrine system, and miscellaneous categories. The most frequently hypothesized mechanisms were EIH (n = 15), autonomic modulation (n = 7), improved sleep (n = 6), muscle oxygenation (n = 6), self-efficacy (n = 5), mental health (n = 4), and benefits of the aquatic environment (n = 12). While all exercise interventions involved FM patients, most of the supporting evidence for these mechanisms was cited from previous studies conducted on healthy samples. No studies performed analyses to demonstrate causal associations between the mechanisms and outcomes. CONCLUSION: Multiple mechanisms were hypothesized for the positive influence of exercise in people with FM. Future studies using causal analyses, such as mediation analysis, are recommended to validate these mechanisms.