Trunk kinematics and muscle activities during arm elevation.

BACKGROUND

While trunk movement accompanies arm elevation, trunk muscle activities during arm elevation at different speeds are unclear. The purpose of this study was to examine the trunk muscle activities at various speeds of arm elevation and then to evaluate their roles in trunk kinematics.

METHODS

Twenty-two healthy subjects participated. The participants performed right shoulder flexion at three different speeds. Surface electromyography was used to measure the activities of bilateral external oblique muscles (EO), internal oblique muscles (IO), rectus abdominis muscles (RA) and lumber erector spinae muscles (ES). A three-dimensional motion analyzer was used to measure arm and trunk movements.

RESULTS

In natural and slow movements, the muscle activities of left ES, right EO and left IO were significantly augmented compared with those of the contralateral muscles, in the relatively late phase. In fast movement, the muscle activities of both ES were significantly augmented during the early phase compared with later in the action. The muscle activities of the left ES and the right EO were significantly augmented compared with those of the contralateral muscles. There was a consistent pattern of trunk extension, lateral flexion and rotation during arm elevation, irrespective of the speed.

CONCLUSIONS

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.