What organ is primarily responsible for the adduction and internal rotation of the shoulder?

The organ primarily responsible for the adduction and internal rotation of the shoulder is the pectoralis major. This broad muscle spans from the sternum and clavicle to the humerus, and it plays a crucial role in moving the upper arm. When the pectoralis major contracts, it facilitates the movement of the arm toward the body (adduction) as well as rotating the arm inward (internal rotation). This action is essential for various activities, including reaching across the body and performing certain types of lifting.

Each of the other muscles mentioned has different primary functions. The trapezius mainly assists in movements such as elevation, retraction, and rotation of the scapula and upper limb positioning. The latissimus dorsi primarily functions in extending, adducting, and internally rotating the shoulder but is not the primary muscle for adduction in this context. The deltoid muscle primarily functions to abduct the arm and can assist in other movements of the shoulder but is not considered the main muscle responsible for both adduction and internal rotation. Understanding these distinctions clarifies the specific role of the pectoralis major in shoulder mechanics.