Lumbar Extension during Stoop Lifting is Delayed by the Load and Hamstring Tightness

[Purpose] This study investigated the relationship between lumbar pelvic rhythm and the physical characteristics of stoop lifting. [Subjects and Methods] Participants performed a stoop lifting task under two conditions: with and without load. We assessed the lumbar kyphosis and sacral inclination angles using the SpinalMouse(®) system, as well as hamstring flexibility. During stoop lifting, surface electromyograms and the lumbar and sacral motions were recorded using a multi-channel telemetry system and flexible electrogoniometers. [Results] In the initial phase of lifting, lumbar extension was delayed by load; the delay showed a negative correlation with sacral inclination angle at trunk flexion, whereas a positive correlation was observed with electromyogram activity of the lumbar multifidus. Additionally, a positive correlation was observed between sacral inclination angle and hip flexion range of motion during the straight leg raise test. [Conclusion] We found that a disorder of the lumbar pelvic rhythm can be caused by both load and hamstring tightness. In the initial phase of stoop lifting, delayed lumbar extension is likely to lead to an increase in spinal instability and stress on the posterior ligamentous system. This mechanism shows that stoop lifting of a load may be harmful to the lower back of people with hamstring tightness.