Muscles of the Shoulder

The muscles of the shoulder are responsible for several important movements. They are called extrinsic and intrinsic, and are located in two parts of the shoulder joint. These muscles are responsible for three primary movements: shoulder adduction and abduction. The muscles involved in shoulder adduction are the pectoralis major, latissimus dorsi, teres major, triceps, coracobrachialis, deltoid, and supraspinatus.

Extrinsic

The clavicle, or collar bone, connects the arm and the chest. It also attaches to several muscles. The scapula, or shoulder blade, is a triangular bone that “floats” off the back of the chest. The scapula and clavicle are connected by a muscle, the acromion. The scapula and clavicle are separated by a shallow socket, called the glenoid.

Intrinsic

The intrinsic muscles of the shoulder are the muscles that originate from the torso and attach to the humerus. These muscles are connected to the shoulder joint through joints called acromioclavicular joints. The scapula and clavicle are flat bones in the center of the chest. These joints also allow movement of the humerus. Intrinsic muscles of the shoulder are deltoid, teres major, and four rotator cuff muscles.

Coracoacromial ligament

The muscles of the shoulder are part of the rotator cuff. They consist of four musculotendinous attachments. The muscles include the supraspinatus muscle, which arises from the supraspinous fossa of the scapula and inserts on the superior aspect of the shoulder capsule. These muscles are supplied by the suprascapular nerve. The subacromial bursa is located between the coracoacromial ligament and the tendon of the superspinatus muscle.

Origins

Shoulder muscles come from two different places. Intrinsic muscles originate from the torso, and external muscles arise from the upper arm and attach to the humerus. The scapulohumeral group consists of four intrinsic muscles: the deltoid, teres major, supraspinatus, and biceps brachii. All four of these muscles are connected to the humerus by tendons. They act on the humerus and shoulder joint.

Insertion

The scapula and the humerus form a joint and the muscles that stabilize it are called intrinsic shoulder muscles. These muscles originate from the acromion process of the scapula and the spine and attach to the humerus. The rotator cuff is made up of four muscles that attach to the scapula and the humerus, forming the scapulohumeral joint.

Innervation

The rotator cuff muscles are the stabilizers of the shoulder joint and are located in the pectoral region of the upper arm. These muscles have short lever arms and a small cross-sectional area, which allows them to generate small forces. They are integrated into the scapular-thoracic complex, which controls the shoulder joint’s rhythm. In addition, these muscles are responsible for shoulder abduction, as well as prevention of subluxation of the humerus head.

Tendons

Injuries to the tendons of the shoulder muscles can result in a condition known as impingement syndrome. Symptoms include pain and weakness in the shoulder. Treatment for this condition varies, but a positive diagnosis is typically obtained with an x-ray or ultrasound imaging. In some cases, invasive treatment may be necessary, such as corticosteroid injections. In other cases, dry needling may be necessary.

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