Definition of shoulder abduction :
Arm abduction occurs when the arms are held at the sides, parallel to the length of the torso, and are then raised in the plane of the torso. This movement may be broken down into two parts: True abduction of the arm, which takes the humerus from parallel to the spine to perpendicular; and upward rotation of the scapula, which raises the humerus above the shoulders until it points straight upwards.
True abduction: supraspinatus (first 15 degrees), deltoid;
Upward rotation: trapezius, serratus anterior
Deltoid Muscle :
This thick muscle covers the shoulder protecting the joint. It assists in many movements of the arm, moving it forward, backward, and outward abduction.
It arises from two bones and three places: from the lateral third of the clavicle, from the acromion, and from the spine of the scapula. The fibers cover the shoulder joint to merge as a small tendon which inserts onto the deltoid tuberosity, a small projection on the lateral aspect of the shaft of the humerus, about midway down.
The supraspinatus muscle assists in lifting the arm outward abduction. Another of the rotator cuff muscles it arises from the top of the scapula, the area above the spine. Its fibers merge into a tendon that crosses the joint to insert on the greater tubercle of the humerus. This tubercle is just behind the head of the humerus.
This large, flat muscle, which looks like a pointed cape over the top half of the back, assists in moving the head backward.
It helps in rotating the scapula as well as in drawing it toward the spine. The lower part of the muscle assists in drawing the scapula downward and in drawing the arm
inward. It arises from the back of the skull along a part of the superior nuchal line on the occipital bone, from the spinous process of the 7th cervical vertebra, and from the spinous processes of all the thoracic vertebrae. It drapes over the shoulder to insert on the clavicle and along the superior border of the spine of the scapula.
The serratus anterior is one of the most powerful muscles of the pectoral girdle. It is a strong protractor of the scapula and is used when punching or reaching anteriorly (sometimes called the “boxer’s muscle”).
Proximal Attachment: External surfaces of lateral parts of 1st–8th ribs
Distal Attachment :Anterior surface of medial border of scapula
Main Action: Protracts scapula and holds it against thoracic wall; rotates scapula