Which action is primarily performed by the infraspinatus muscle at the glenohumeral (shoulder) joint?

• A. Scapular elevation
• B. Medial (internal) rotation of the humerus
• C. Lateral (external) rotation of the humerus
• D. Elbow flexion

Answer: (C)

Verified step by step guidance

1. Identify the anatomical location and function of the infraspinatus muscle. It is one of the rotator cuff muscles located on the posterior aspect of the scapula, specifically in the infraspinous fossa.
2. Recall the primary actions of the rotator cuff muscles at the glenohumeral joint. These muscles stabilize the shoulder and assist in various movements of the humerus.
3. Understand that the infraspinatus muscle's main role is to produce lateral (external) rotation of the humerus. This means it rotates the arm outward, away from the midline of the body.
4. Eliminate other options by matching their actions to different muscles: scapular elevation is mainly performed by the trapezius and levator scapulae; medial (internal) rotation of the humerus is performed by muscles like the subscapularis and pectoralis major; elbow flexion is performed by muscles such as the biceps brachii and brachialis.
5. Conclude that the correct action primarily performed by the infraspinatus muscle at the glenohumeral joint is lateral (external) rotation of the humerus.