Which muscle is primarily responsible for shoulder flexion?

The anterior deltoid is primarily responsible for shoulder flexion due to its anatomical positioning and function. This muscle is located at the front of the shoulder, and its fibers are oriented in a way that enables them to contract effectively during the upward movement of the arm in front of the body. When the anterior deltoid contracts, it pulls the humerus forward and upward, which is essential for actions such as lifting objects in front of you or performing overhead movements.

While the biceps brachii and pectoralis major can assist in some shoulder flexion activities, their primary functions differ. The biceps brachii is mainly involved in elbow flexion and supination of the forearm, though it does play a minor role in shoulder flexion due to its attachment at the shoulder joint. The pectoralis major, meanwhile, facilitates movements such as horizontal adduction and internal rotation of the shoulder but is not the primary mover in shoulder flexion.

The triceps brachii, on the other hand, is primarily involved in elbow extension and does not play a significant role in shoulder flexion, as its fibers are oriented to extend the elbow rather than lift the arm forward. Therefore, the anterior deltoid is the clear primary muscle