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A large vertebrate skeletal muscle divided into an ascending, descending, and transverse portion, attaching the neck and central spine to the outer extremity of the scapula; it functions in scapular elevation, adduction, and depression.
Muscles of the shoulder include those that attach to the scapula, humerus, and clavicle and act to move the joint, or those muscles of the rotator cuff, that act to stabilize the shoulder.
The serratus anterior is a segmented muscle that consists of three parts: superior, intermediate, and inferior, which refers to the angle of insertion (superior, medial, and inferior, respectively). The action of the muscle is to pull the scapula forward, which is needed for anteversion (movement forwards) of the arm. The subclavius is a muscle of the axilla located between the clavicle and the first rib. It is small, and triangular in shape. Upon innervation and contraction, the subclavius depresses the shoulder, through action on the clavicle, to bring both forward.
The pectoralis minor is a paired muscle of the upper chest that lies beneath the pectoralis major. The muscle acts on the point of the shoulder to depress the joint and draw the scapula downwards and anteriorly. The pectoralis major, superficial to the pectoralis minor, is a broad and thick paired muscle of the chest. As a large and major muscle of the chest, the pectoralis major is innervated by two nerves, the medial pectoral nerve and the lateral pectoral nerve. Innervation of the pectoralis major can have several actions, including flexion of the humerus, adduction of the humerus, rotation of the humerus medially, and aiding deep respiration. The clavicular part of the muscle, located near the deltoid, controls the flexion, horizontal adduction, and rotation of the humerus. The sternocostal portion act antagonistic to the clavicular portion, and moves the arm forward and downwards, as well as rotation inwards.
The sternocleidomastoid is a superficial, paired muscle of the anterior neck. This muscle acts to flex and rotate the head, and have additional actions, in concert with the scalene muscles, in assisting inspiration through raising the sternum, while the levator scapulae, which functions to elevate the scapula, is a paired muscle which extends across the side of the neck to insertion at the medial border of the scapula. The muscle usually lies deep to the sterncleidomastoideus at its origin. The levator scapulae most often works with other muscles to coordinate movements, including the rhomboids and pectoralis minor. When the shoulder is fixed, the levator scapulae rotates and flexes the cervical spine. However, when rotating the shoulders, the cervical spine is fixed.
The rhomboid major, a paired muscle that acts with the rhomboid minor, acts to press the scapula against the thoracic wall, retracting the scapula towards the spine. The rhomboid minor is small relative to the major, and is located below the levator scapulae. The rhomboids act with the levator scapulae to elevate the scapula. The serratus anterior and trapezius act antagonist to this group of muscles.
The trapezius acts on both the scapula and spine, moving one when the other is fixed . The muscle consists of three parts: the superior region, which acts to support the arm, the intermediate region, which acts on the scapula, and the inferior region, when rotates and depresses the scapula.
The deltoid is a muscle that forms the rounded edge of the shoulder . The action of the muscle is complex, with the components acting in opposing and separate ways during the course of a contraction. Each portion of the deltoid also acts singularly to produce movement distinct in origination from the other deltoid portions. The anterior deltoid assists the pectoralis major during transverse flexion of the shoulder, and acts weakly in strict transverse flexion. The lateral deltoid assists in shoulder flexion when the shoulder is rotating, although it also assists the transverse abduction of the shoulder. The posterior deltoid is involved with extension, being the hyperextensor of the shoulder, contributing to transverse extension.
Major muscles of the shoulder are those responsible for its movement. Anatomically, they form the shoulder cap and underarm. They produce movement of the shoulder via insertion at the scapula, humerus and clavicle. A second group of muscles, referred to as the rotator cuff, are those muscles that stabilize movement of the shoulder . The associated tendons of the rotator cuff hold the ball of the humerus in the socket of the shoulder joint. These muscles originate from the scapula and insert at the head of the humerus. During abduction of the arm, the rotator cuff compresses the glenohumeral joint, allowing the deltoid to elevate the arm. Four muscles comprise the rotator cuff: the supraspinatus, infraspinatus, teres minor, and subscapularis.
levator scapulae: elevates scapula; when shoulder fixed, rotates and flexes cervical spine, rotator cuff: during abduction, cuff compresses glenohumeral joint, allowing deltoid to elevate arm, pectoralis major: flexion and abduction of humerus along with lateral rotation, or serratus anterior: pulls scapula forward for forward movement of arm
Source: Boundless. “Muscles of the Shoulder.” Boundless Anatomy and Physiology. Boundless, 21 Jul. 2015. Retrieved 14 Feb. 2016 from https://www.boundless.com/physiology/textbooks/boundless-anatomy-and-physiology-textbook/the-muscular-system-10/muscles-of-the-upper-limb-106/muscles-of-the-shoulder-577-10094/