Purpose: The flexion relaxation phenomenon (FRP) is characterized by the reduction of paraspinal muscle activity at maximum trunk flexion. The FRP is reported to be altered (persistence of spinal muscle activity) in nonspecific chronic low back pain (NSCLBP) and is considered a promising biomarker. The aim of this systematic review was to synthetize current knowledge on FRP in the NSCLBP population regarding prevalence, the reliability of FRP measurement using surface electromyography (sEMG), the average value, and variation of the relaxation ratios (RR). Methods: Five databases were searched (PubMed, EMBASE, Web of Sciences, Cochrane Library, Pedro). A qualitative analysis was done for all included studies and meta-analysis studying prevalence, mean value of flexion relaxation ratio (FRR) and extension relaxation ratio (ERR), and difference between asymptomatic and NSCLBP FRR.