Influence of thoracic posture on scapulothoracic and glenohumeral motions during eccentric shoulder external rotation

•Thoracic extension increased scapular posterior tilting and external rotation.•Thoracic extension reduced glenohumeral horizontal extension.•Thoracic posture had no effect on upward rotation of the scapula.•Thoracic extension increased the maximum shoulder external rotation. Throwing injuries to the shoulder joint often occur during shoulder external rotation. An appropriate combination of thoracic, scapular, and humeral motion during throwing is important to prevent such injuries, but it is unclear how thoracic posture contributes to shoulder motion during throwing. The purpose of this study, therefore, was to clarify the influence of thoracic posture on scapulothoracic and glenohumeral motion during shoulder external rotation. Eccentric external rotation at 90° of shoulder abduction in thoracic flexion and extension postures was performed by 15 asymptomatic participants. Three-dimensional scapulothoracic and glenohumeral movements were measured with an electromagnetic tracking device at 75°, 80°, 85°, and maximum shoulder external rotation. The thoracic angle and maximum shoulder external rotation in absolute coordination were measured with a three-dimensional motion capture system. The results showed that scapular posterior tilting and external rotation in the thoracic extension posture were significantly greater than those in the flexion posture (p