Abstract Background: Low Back Pain (LBP) is the mosthighly prevalent musculoskeletal disorder. Now, it can be considered the most common cause of disability in the world. Strongcon-nections have been reported between low back pain prevalence and disturbance of back muscle flexion relaxation phenomenon (FRP). Aim of Study: To determine the effect of lumbar posture on back muscles flexion relaxation phenomenon during sitting in chronic non-specific low back pain (CNSLBP) among faculty of physical therapy students. Material and Methods: Thirty male and female university students aged between 18-25 years old were assigned into2 groups, 15 students with chronic non-specific low back pain-were assigned into the experimental group and 15 age-matched healthy students were assigned into the control group among students of physical therapy, Misr University for Science and Technology. Surface electromyography (sEMG) was used to assess erector spinae(ES) muscle activation in specific lumbar postures (lordoticand kyphotic) from sitting with specific lumbar flexion angles (0º, 30, 60º, and maximum trunk flexion), the static EMG (lumbar erector spinae data) data for each participant were collected. Results: There was a significant difference between the study and control group regarding mean EMG values, asback muscles' electrical activity significantly decreased in subjects with low back pain at several lumbar flexion positions in lordotic and kyphotic postures.A kyphotic posture produced very low and unchanged ES EMG values over the full range of trunk flexion, whereas a lordotic posture inducedcompar-atively significant lumbar ES activations. Conclusion: Lumbar posture has asignificant effect on erector spinae activation and consequently the flexion-relaxation phenomenon among students withchronic non-specific low back pain.