Background and aim: Studies have shown that bending the trunk to lift or place a load on the ground is a risk factor for developing back pain. The aim of this study was to evaluate the co-contraction ratio of selected trunk muscles during forward and backward bending with different loads in people with and without chronic low back pain. Materials and Methods: Thirty-one subjects in two groups of 15 patients with chronic low back pain and 16 healthy subjects participated in this study voluntarily. Surface electromyography signals were recorded from selected trunk muscles (Rectus abdominis, External oblique, Internal oblique, Thoracic erector spine, Lumbar erector spine, Multifidus) during forward and backward bending with different loads (0, 2, 5, and 10% of body mass) in 6 seconds (2 seconds standing, 2 seconds forward bending and 2 seconds backward bending). Level of activity and co-contraction ratio was employed to assess muscle coordination and compare it between the chronic low back pain and healthy people. Results: Findings showed the amount of load had a significant effect on the co-contraction ratio in different groups (p=0.01) and different muscles (p=0.02), but there is no significant difference between the muscles co-contraction ratio of the two groups (p=0.46). Also, the effect of load on muscle co-contraction ratio has increased in the chronic low back pain group (p<0.05). Conclusions: Increasing the co-contraction in the chronic low back pain group causes more muscle fibers to be called; as a result, the muscles that are called more often have a lack of strength. These results could facilitate the design of efficient rehabilitation methods for the treatment of trunk muscle dysfunctions.
