Trunk kinematics and muscle activities during arm elevation

Bilateral ES activity may be required for back-extension torques, especially for the early phase of rapid elevating motion. The anterior muscles' activity may contribute to the production of anterior force against the backward movement of the center of mass of the upper limb in the late phase. Trunk rotation, controlled by the trunk muscles in harmony, may assist the scapular movement to align the scapular plane in the arm elevating plane.