Final answer:
The deltoid muscle is key for shoulder movement, originating from the lateral clavicle and scapula, inserting into the humerus, and receiving innervation from the axillary nerve. It facilitates arm abduction, flexion, and internal rotation, while also contributing to horizontal adduction. The stability of the glenohumeral joint is bolstered by muscles making up the rotator cuff.
Step-by-step explanation:
The deltoid is a critical shoulder muscle involved in various arm movements at the glenohumeral joint, which is the meeting point of the glenoid cavity of the scapula and the head of the humerus. The anterior deltoid allows for the abduction, flexion, and internal (medial) rotation of the arm, with the deltoid muscle also contributing to horizontal adduction. The deltoid originates from the lateral third of the clavicle, the acromion, and the spine of the scapula; it inserts into the deltoid tuberosity on the humerus and is innervated by the axillary nerve, which consists of nerve fibers from the C5 and C6 spinal nerves.
Ligaments and the labrum offer some support to the joint, but primary stability is provided by the muscles crossing the joint, with tendons forming the rotator cuff. These tendons are shielded by the subacromial and subscapular bursae, which reduce friction during shoulder movements. Overall, the shoulder muscles play a vital role in enabling a wide range of movements for the humerus, contributing to the versatility and mobility of the upper limb.