What muscle is primarily responsible for performing *horizontal flexion* of the shoulder?

The muscle primarily responsible for performing horizontal flexion of the shoulder is the pectoralis major, alongside the anterior deltoid. Horizontal flexion, often referred to in the context of actions like bringing the arms forward and across the body, involves these two muscles effectively working together.

The pectoralis major has a significant role in this movement as it originates from the clavicle and sternum, allowing it to pull the upper arm across the front of the chest when contracted. The anterior deltoid also contributes by flexing and internally rotating the arm at the shoulder joint during this action. Together, they enable a range of movements that involve bringing the arm horizontally in front of the body.

The other options do not adequately represent the primary muscles involved in horizontal flexion. For example, while the latissimus dorsi can assist in some shoulder movements, it is not a primary mover for horizontal flexion. The medial and posterior heads of the deltoid are primarily involved in other shoulder actions, such as abduction and extension, not horizontal flexion. The gastrocnemius is related to ankle movement and does not play a role in shoulder actions at all. Thus, the combination of pectoralis major and anterior deltoid stands out