The flexion relaxation ratio (FRR) of the lumbar extensor muscles is often assessed in experimental and clinical studies. This study evaluated within- and between-session test--retest reliability and measurement error for different FRR formulations. Participants completed two identical data collection sessions 1-week apart. Spine flexion and erector spinae electromyographic data were recorded during two blocks of full forward spine flexion. Participants who numerically self-reported low back pain ([greater than or equal to]3/10) during either session were excluded from analysis. Two FRR formulations and their reciprocals were calculated. Generalizability coefficients (GC) was calculated to assess reliability. Standard error of measurement was also determined. Fifty participants were recruited, with six excluded from the analysis. Within-session reliability was moderate-to-good (GC = 0.519-0.791). Between-session reliability was poor-to-moderate (GC = 0.376-0.538). Within-session and between-session measurement errors, respectively, were 38% and 49% of the grand mean FRR. These data suggest limited FRR utility in longitudinal studies.