BACKGROUND: The World Health Organization states that low back pain is the leading cause for disability worldwide. Patients with chronic low back pain (CLBP) show important decreases in lumbar strength and can now be assessed by using the new isokinetic dynamometer BioniX Sim3 Pro which offers very detailed measurements. OBJECTIVE: To compare lumbar flexion and extension strength values, as well as extension-flexion ratio (EFR) based on isokinetic velocity and gender, for patients with CLBP on the BioniX Sim3 Pro. METHODS: A retrospective analysis was performed on data from 20 men and 22 women with CLBP. Maximum torque in flexion and extension was measured isometrically and isokinetically. Statistical analysis was performed on these parameters. RESULTS: EFR shows a downward trend with increasing speed of measurement (isometric to isokinetic 30∘/s and isokinetic 30∘/s to isokinetic 90∘/s, respectively): in men from 1.21 to 1.04 (p< 0.001) and from 1.04 to 0.93 (p= 0.207). In women, EFR decreased from 1.41 to 1.13 (p< 0.001) and from 1.13 to 1.00 (p= 0.144). CONCLUSIONS: Patients with CLBP have a decreased strength in lumbar extension and flexion compared to the age-corrected normal values of Bionix Sim3 Pro. With increasing speed of testing protocol, a decrease in EFR is observed in this population.