Flexion Relaxation Ratio Not Responsive to Acutely Induced Low Back Pain from a Delayed Onset Muscle Soreness Protocol

Background. The flexion relaxation ratio (FRR) has been suggested as a measure of muscular performance in patients with low back pain (LBP). The purpose of this study was to investigate whether the FRR was responsive to acute LBP produced from a delayed onset muscle soreness (DOMS) protocol. Methods. Fifty-one pain-free volunteers performed DOMS to induce LBP. Current pain intensity, trunk flexion range of motion (ROM), and passive straight leg raise (SLR) were measured at baseline, 24 and 48 hours after DOMS. Participants were categorized into pain groups based on reported current pain intensity. Changes in FRR, trunk flexion ROM, and SLR ROM were examined using two-way repeated measures analysis of variance. Results. Pain group was not found to have a significant effect on FRR (F (1,29) = 0.054, P = 0.818), nor were there any two-way interactions for changes in FRR. The pain group had decreased trunk flexion ROM compared to the minimal pain group (F (1,38) = 7.21, P = 0.011), but no decreases in SLR ROM (F (1,38) = 3.51, P = 0.057) over time. Interpretation. There were no differences in FRR based on reported pain intensity of LBP from a DOMS protocol. The responsiveness of FRR might be limited in patients with acute onset LBP of muscular origin.