OBJECTIVE: To develop a reliable and repeatable way to monitor changes in the flexion relaxation phenomenon of the lumbar paraspinal muscles during forward flexion by the development of a flexion relaxation ratio and observation of the sEMG activity in standing and during forward flexion in patients with chronic low back pain (CLBP) and healthy controls. DESIGN: Two experiments were conducted, the first to assess the test-retest reliability of the measure in a group of CLBP (n = 11) patients; the second compared the results between a group of normal healthy controls (n = 20) and a group of CLBP patients (n = 70). RESULTS: Repeated measurements over 4 weeks demonstrated between session reliability of between 0.81 and 0.98 for the dynamic activity. The levels of sEMG activity in the fully flexed position was significantly greater in the fully flexed position in the CLBP group than the controls. The flexion relaxation ratio (FRR), a comparison of the maximal sEMG activity during 1 s of forward flexion with activity in full flexion, demonstrated significantly lower values in the CLBP than the control group. The combined discriminant validity for the FRR for all four sites resulted in 93% sensitivity and 75% specificity. CONCLUSION: The FRR clearly discriminated the patients from the healthy controls. These results indicate that dynamic sEMG activity of the paraspinal muscles can be reliably measured and is useful in differentiating CLBP patients from normal controls. RELEVANCE: Analysis of the pattern of different levels of muscle activity during a forward flexion can be used in CLBP where normalization of the sEMG signal to the maximum voluntary contraction may be difficult. The FRR may be used in the assessment of change in the flexion relaxation phenomenon following treatment interventions.