Final answer:
The pectoralis major is the primary muscle involved in shoulder horizontal adduction, supported by other muscles that stabilize and position the pectoral girdle such as the subclavius, pectoralis minor, and serratus anterior, as well as muscles like the latissimus dorsi and various scapular muscles that facilitate movement of the humerus in different directions.
Step-by-step explanation:
The pectoralis major is a key muscle that performs the movement known as shoulder horizontal adduction, which involves bringing the elbows together in front of the body. This muscle covers much of the superior portion of the anterior thorax and is assisted by other muscles to stabilize the shoulder joint during movement. Muscles located on the anterior thorax, such as the subclavius, pectoralis minor, and serratus anterior, as well as posterior thorax muscles including the trapezius, rhomboid major, and rhomboid minor help to stabilize and position the pectoral girdle. They provide a stable base for movement and contribute to the inferior rotation movement of the scapula during upper limb adduction, where the glenoid cavity of the scapula moves in a downward direction. This ensures the efficient function of the shoulder joint and the girdle, comprising the clavicle and scapula, which supports the proximal end of the humerus.
Muscles such as the latissimus dorsi, and scapular muscles like the deltoid, subscapularis, supraspinatus, infraspinatus, teres major, teres minor, and coracobrachialis also play roles in moving the humerus. These muscles enable a range of movements for the arm, including abduction, medial rotation, lateral rotation, flexion, extension, and adduction.