Shoulder external rotation (or lateral rotation) is a rotary movement at the shoulder (glenohumeral) joint around the longitudinal axis of the humerus (upper arm bone) away from the centre of the body – turning the upper arm outwards – see Figures 1 and 2.
Figure 1. Shoulder External Rotation.
Figure 2. Shoulder external rotation with cable.
The key muscles involved in this movement are the posterior deltoid, the infraspinatus, and the teres minor. The supraspinatus also plays a weak role when the shoulder is abducted.
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Golf Anatomy and Kinesiology, a collection of articles describing the roles of the muscles involved in the golf swing.
The Role of the Deltoids in the Golf Swing
The Role of the Teres Minor in the Golf Swing
Introduction to the Swing like a Champion System.
Overview of a Great Golf Swing which summarises the correct movements in a great golf swing.
» Golf swing instruction home page.