Background: Non-specific chronic low back pain (NSCLBP) is a complex condition influenced by physical, psychological, and social factors. Conservative treatments, including manual therapy, exercise and other non-invasive approaches (e.g., patient education, lifestyle modifications), are recommended, however, their impact on objective neuromuscular biomarkers remains unclear. The flexion relaxation phenomenon (FRP), commonly assessed using the flexion relaxation ratio (FRR), has been proposed as a potential biomarker for NSCLBP. Objective : This systematic review evaluates the effects of conservative treatments on FRP in individuals with NSCLBP. Methods : A comprehensive systematic search was conducted across four databases (PubMed, EMBASE, Cochrane, and PEDro) up to March 31, 2025, following PRISMA guidelines. Randomized controlled trials (RCTs) examining the effects of conservative treatments on FRP in NSCLBP were included. Risk of bias was assessed using the Cochrane Risk of Bias 2 (RoB 2) tool. The review protocol was registered in PROSPERO (CRD42020162576). Results : Twelve RCTs, including 480 participants, met the inclusion criteria. Most studies were assessed as having a high risk of bias. Several studies reported within-group improvements in FRR following manual therapies; however, the short- and mid-term effects of exercise-based interventions were inconsistent. Only a minority of studies adhered to standardized electromyography (sEMG) electrode placement guidelines, contributing to substantial heterogeneity in FRP assessment. Conclusion : Manual therapies may be associated with immediate changes in FRP; however, these findings should be interpreted with caution due to methodological limitations, including the lack of consistent between-group comparisons. The effects of exercise-based interventions remain unclear. Heterogeneity in FRP recording and analyzis methods further limits the interpretation of results. Further high-quality studies using standardized methodologies are needed to validate FRP as a biomarker for NSCLBP and to clarify its relationship with clinically meaningful outcomes.
