Abstract Roussel NA, Truijen S, De Kerf I, Lambeets D, Nijs J, Stassijns G. Reliability of the assessment of lumbar range of motion and maximal isometric strength in patients with chronic low back pain. Objective To examine the interobserver reliability of the assessment of lumbar range of motion (ROM) and maximal isometric strength in patients with chronic low back pain (CLBP) using commercially available equipment. Design A prospective repeated-measures design. Setting Ambulatory care in a university hospital. Participants Twelve patients (5 men, 7 women; age range, 20−52y) with CLBP, with a mean visual analog scale score of 31.5±25.8mm, volunteered for the study. The duration of their symptoms was 63±115 months and the mean Oswestry Disability Index score was 31%. Interventions Not applicable. Main Outcome Measures Movements of the lumbar spine were assessed with commercially available equipment. Both the range of motion (ROM) and the maximal isometric strength for flexion, extension, lateroflexion, and rotation of the lumbar spine were evaluated twice to analyze the interobserver reliability. The same test procedure was performed on 2 separate days by 2 investigators who were blinded to the outcome of the assessment of their colleague. The order of investigator was balanced, so that each investigator tested the same number of patients as first investigator. Results The intraclass correlation coefficient varied between .91 and .98 for the measurements of the lumbar ROM and was between .93 and .97 for all the strength measurements. Post hoc power analysis confirmed previous power analysis, that is, despite the small sample size, an excellent power was found for the observed interobserver reliability coefficients (power range, 0.93−1.00). No learning effect was found when comparing the results of the second measurement with the first measurement ( P>.05). Conclusions The interobserver reliability is excellent for the measurement of the ROM of the lumbar spine and for the maximal isometric strength using specific devices in patients with CLBP.