The effect of trunk rotation during shoulder exercises on the activity of the scapular muscle and scapular kinematics

Abstract

Background: In patients with shoulder disease, kinetic chain exercises including hip or trunk movement are recommended. However, the actual muscle activation and scapular kinematics of these exercises are not known. The purpose of this study was to examine the effect of trunk rotation on shoulder exercises that are devised to improve scapular function.

Methods: Thirteen healthy young men participated in this study. Scaption, external rotation in the first and second positions, and prone scapular retraction at 45°, 90°, and 145° of shoulder abduction were performed with and without trunk rotation. Electromyography was used to assess the scapular muscle activity of the upper trapezius (UT), middle trapezius (MT), lower trapezius (LT), and serratus anterior (SA), and electromagnetic motion capture was used to assess scapular motion. The muscle activity ratio, which is the activity of the UT to the MT, LT, and SA, was calculated. These data were compared between 2 conditions (with and without trunk rotation) for each exercise.

Results: Adding trunk rotation to scaption, the first external rotation, and the second external rotation significantly increased scapular external rotation and posterior tilt, and all 3 exercises increased LT activation. In addition, trunk rotation with scapular retraction at 90° and 145° of shoulder abduction significantly decreased the UT/LT ratio.

Conclusions: Our findings suggest that shoulder exercises with trunk rotation in this study may be effective in patients who have difficulty in enhancing LT activity and suppressing excessive activation of the UT or in cases in which a decreased scapular external rotation or posterior tilt is observed.

Keywords: Shoulder exercise; kinetic chain; muscle activation ratio; rehabilitation; scapular kinematics; trunk rotation.