Which muscle produces lateral rotation of the shoulder?

• A. Supraspinatus
• B. Infraspinatus
• C. Teres minor
• D. Subscapularis

Answer: (B)

External rotation of the shoulder is produced mainly by the posterior rotator cuff muscles, with the infraspinatus doing the bulk of the work. This muscle sits on the infraspinous fossa of the scapula and attaches to the humerus, so when it contracts it twists the arm outward and helps keep the head of the humerus centered in the socket. The teres minor also contributes to external rotation, but it is smaller, so the infraspinatus is the primary mover. The supraspinatus is primarily involved in initiating arm abduction, not external rotation, and the subscapularis is responsible for internal rotation. So the muscle most responsible for turning the arm laterally is the infraspinatus.