The flexion relaxation phenomenon in nonspecific chronic low back pain: prevalence, reproducibility and flexion-extension ratios. A systematic review and meta-analysis

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.

Results: Twenty-seven studies were included for qualitative analysis and 21 studies for meta-analysis. The prevalence of the altered FRP was 55% (95%CI [32-79%]) in the NSCLBP population. The studies on reliability reported good within-session and between-session reproducibility. In the NSLBP population, the mean FRR was 2.96 (95%CI [2.02; 3.90]) and the mean ERR was 4.07 (95%CI [2.08; 6.07]). The difference between asymptomatic and NSCLBP FRR was - 1.19 (95%CI [- 1.92, - 0.45]). In all meta-analysis, the I² index was > 80%.

Conclusion: An altered FRP is frequently found in NSCLBP population using sEMG and the test has a good reproducibility. The difference between asymptomatic and NSCLBP FRR was significant. Nevertheless, considering the high heterogeneity observed, additional research is required to confirm the value of RR.