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Background Exercise is recommended for the management of chronic low back pain (CLBP). Trialists have proposed numerous mechanisms to explain why exercise improves pain and function in people with CLBP, but these are yet to be synthesised. Objective To synthesise the proposed mechanisms of benefit for exercise in people with CLBP. Design Review. Methods The Physiotherapy Evidence Database (PEDro) was searched from inception to July 2019. Randomised controlled trials of adults with CLBP, indexed in PEDro as ‘fitness training’, were included. Two reviewers independently screened and extracted data from each study. Data were analysed quantitatively and qualitatively using thematic analysis. Results 186 studies were identified and 110 were included in the analysis. Thirty-six studies (33%) did not provide a mechanism of benefit for exercise in people with CLBP. Of the remaining studies, most provided more than one mechanism, from which 33 unique mechanisms were identified. These were grouped into five themes which, from most to least common, were: neuromuscular (n = 105 (44%)); psychosocial (n = 87 (36%)); neurophysiological (n = 22 (9%)); cardiometabolic (n = 15 (6%)); and tissue healing (n = 12 (5%)). The effects of these proposed mechanisms on outcomes for people with CLBP were seldom examined. Conclusions This review identified a variety of mechanisms proposed in clinical trials to explain why ‘fitness training’ works for people with CLBP, but these mechanisms were seldom tested. Randomised controlled trials investigating the mediating effects of these mechanisms may be warranted to better understand why exercise works for CLBP.