The shoulder is a ball-and-socket type of joint that is surrounded by a group of muscles, tendons, and ligaments. The muscles that make up the shoulder include the deltoid, rotator cuff, and trapezius. The tendons attach the shoulder muscles to the bone, and the ligaments attach the shoulder bones to each other.
The deltoid muscle is the largest muscle in the shoulder. It attaches to the clavicle, scapula, and humerus. The rotator cuff consists of four muscles that attach to the scapula and humerus. Finally, the trapezius muscle attaches to the neck and spine.
The Deltoids
The deltoids are a shoulder muscle that is responsible for movement at the shoulder joint. The muscle has three heads: the anterior, middle, and posterior. The deltoid muscle is used in various movements, such as raising the arm, rotating the arm, and moving the arm away from the body. The muscle attaches to the clavicle, scapula, and humerus. The deltoid muscle is innervated by the radial nerve.
The anterior head of the deltoid muscle is located at the front of the shoulder. It is a large, triangular muscle that covers the shoulder joint. The anterior head of the deltoid muscle attaches to the clavicle and the acromion process of the scapula. It inserts onto the humerus. The anterior head of the deltoid muscle aids in shoulder flexion, external rotation, and abduction.
The middle head of the deltoid muscle is the largest of the three heads and is located in the middle of the shoulder. It attaches to the clavicle and acromion and inserts into the deltoid tuberosity of the humerus. The middle head of the deltoid muscle is responsible for abducting (lifting) the arm away from the body.
The posterior head is the deltoid muscle located at the back of your shoulders. It originates from the upper posterior surface of the humerus and inserts into the outer lip of the bicipital groove. The function of the posterior head is to extend, adduct, and medially rotate the arm at the shoulder joint.
The Rotator Cuff
The rotator cuff is a group of muscles and tendons that attach the shoulder blade to the upper arm bone. The rotator cuff muscles and tendons help lift the arm up and out to the side. The rotator cuff also stabilizes the shoulder joint and keeps the upper arm bone from slipping out of the socket.
The rotator cuff has four main muscles: supraspinatus, infraspinatus, teres minor, and subscapularis. The supraspinatus muscle is responsible for lifting the arm up and out to the side. The infraspinatus muscle helps stabilize the shoulder joint. The teres minor muscle helps rotate the arm outward. The subscapularis muscle stabilizes the shoulder joint and helps keep the upper arm bone in place.
The Trapezius Muscle
The trapezius muscle is a large, triangular muscle that extends from the base of the skull to the lower thoracic spine. It is one of several muscles that make up the posterior part of the neck and shoulder girdle. The trapezius muscle has three parts: the upper, middle, and lower fibers. The upper muscle-fibers are responsible for elevating the scapula, while the middle and lower fibers depress and retract it. The trapezius muscle is innervated by the accessory nerve (CN XI).