[Purpose] Although lifting tasks has been recognized as a primary risk factor in low back pain, the concept of lifting asymmetry is relatively new subject. To address trunk function, biomechanical studies generally measure trunk muscle activity using surface electromyography (EMG). But so far, magnitude and similarity index (SI) obtained from EMG have not been studied as indicators of the motor control during lifting task. So, the purpose of this study is to compare the trunk muscles magnitude and SI during symmetric and asymmetric lifting. [Subjects and Methods] A total of 20 healthy male with no history of lumbar spine disorders participated. Surface electromyography data were recorded from the 7 trunk muscles while the participants performed symmetric and asymmetric lifting and lowering different loads. [Results] According to Multivariate ANOVAs the phase of motion (lifting, lowering) and condition (symmetry, asymmetry) have a significant effect on SI and magnitude (p≤0.05). Load changes have no effect on SI (p=0.969) but have a significant effect on magnitude (p≤0.05). The magnitude and SI value is higher in asymmetrical lifting and lowering compare to symmetrical condition. [Conclusion] The findings reveal the SI value is higher in asymmetric conditions. This means that the amount of muscles co-contracture increased during asymmetrical conditions. Increased muscles co-contracture reinforces the hypothesis of exerting more compression on the spine in asymmetrical movement.

Keywords: Asymmetrical lifting, Motor control, Electromyography