Background <p>Previous studies have shown that bending the trunk to lift or place a load on the ground is a risk factor for developing back pain.</p> Aim <p>This study aimed to evaluate the effect of external loads on the co-contraction ratio of trunk muscles during forward and backward bending in people with and without chronic low back pain (CLBP).</p> Methods <p>Thirty-one subjects in two groups of 15 patients with CLBP 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 s 2 s standing, 2 s forward bending and 2 s backward bending). The co-contraction ratio was employed to assess and compare muscle coordination between the two groups.</p> Results <p>Findings showed the amount of load had a significant effect on the co-contraction ratios in different groups (<i>p</i> = 0.01) and different muscles (<i>p</i> = 0.02), but there is no significant difference between the muscles co-contraction ratio of the two groups (<i>p</i> = 0.46). Also, the effect of load on muscle co-contraction ratio has increased in the CLBP group (<i>p</i> &lt; 0.05).</p> Conclusions <p>Increasing the co-contraction ratio in the CLBP group causes more muscle fibers to be called; as a result, the muscles that are called more often have a lack of neuromuscular control. These results could facilitate the design of efficient rehabilitation methods for the treatment of trunk muscle dysfunctions.</p>