Which muscle is a primary mover in flexion of the arm at the shoulder and also contributes to medial rotation and adduction?

• A. Pectoralis Major
• B. Deltoid
• C. Latissimus Dorsi
• D. Biceps Brachii

Answer: (A)

The arm at the shoulder is brought forward most effectively by the pectoralis major, especially its clavicular portion. This part of the muscle originates on the clavicle and pulls the humerus forward, making it the primary mover for shoulder flexion. It also crosses the shoulder in a way that allows it to contribute to medial rotation and adduction of the arm.

Other muscles either oppose or only assist in these actions. The deltoid, while its anterior fibers can assist in flexion and medial rotation, is not the main flexor and is more renowned for initiating abduction after the initial movement. The latissimus dorsi primarily extends, adducts, and medially rotates the arm, which would oppose flexion rather than drive it. The biceps brachii can aid shoulder flexion but is mainly a forearm/elbow flexor, not the principal shoulder flexor or primary contributor to adduction.

So, the pectoralis major best fits all parts of the action described.