Which muscle is primarily involved in the movement of the shoulder?

The deltoid muscle is primarily responsible for the movement of the shoulder. This muscle covers the shoulder joint and plays a crucial role in several key movements, such as arm abduction, flexion, and extension. Its unique triangular shape allows it to stabilize the shoulder and function effectively in lifting the arm away from the body, as well as in performing overhead activities.

In contrast, while the pectoralis major also aids in shoulder movement, particularly in actions like flexing and rotating the arm, it does not serve as the primary mover in shoulder abduction. The serratus anterior assists with the movement of the scapula and helps in the upward rotation necessary for certain shoulder movements but does not directly move the shoulder joint itself. Lastly, the trapezius muscle is important for stabilizing and moving the shoulder blade rather than the shoulder joint, playing a supportive role in shoulder movement rather than being the main muscle responsible for it.