The muscles of the shoulder account for several arm movements. According to Exrx.net, the shoulder can flex, extend, abduct, adduct and internally and externally rotate. Flexion is the movement of the arm up in front of the body. Extension is the movement of the arm up and in back of the body. Abduction and adduction are opposing movements -- adduction is movement of the arm toward the midline of the body and abduction is away from the midline of the body. Internal and external rotations are rotary movements around the longitudinal axis of the humerus toward and away from the midline of the body, according to exerciseprescription.net. Several shoulder muscles work in coordination to make these movements possible.
The Rotator Cuff
The supraspinatus, infraspinatus, teres minor and subscapularis form the rotator cuff, according to "Anatomy and Physiology" by Kenneth Saladin. The tendons of these muscles converge on the capsule of the shoulder joint and hold the head of the humerus in the glenoid cavity. This stabilizes the shoulder joint. These muscles allow the shoulder to extend, abduct, adduct and medially rotate.
Pectoralis Major
This muscle is located on the front of the body. It originates at the clavicle, sternum and costal cartilage and inserts at the humerus. Saladin says this muscle is primarily responsible for shoulder flexion, adduction and medial rotation. The pectoralis major helps with pushing and throwing motions.
Latissumus Dorsi
This muscle is a large muscle located on the back. It originates at vetebrae T7 through L5, the lower three ribs, iliac crest and the inferior angle of the scapula, according to Saladin. It inserts into the intertubercular groove of the humerus. This muscle helps with adduction, medial rotation, and extension -- and it is responsible for downward movements of the arms.
Deltoids
These muscle are composed of anterior, middle and posterior sections. They are located directly over the top of the shoulder joint. Origination occurs at the clavicle, scapular spine and acromion, according to Saladin. Insertion is at the deltoid tuberosity of the humerus. These muscles help with adbuction, flexion and medial and lateral rotation.
Teres Major
This muscle is located on the back above the latissimus dorsi. "Anatomy and Physiology" states that teres major originates at the inferior angle to the lateral border of the scapula and inserts at the medial part of the humerus. It helps with adduction, medial rotation and extension.
Coracobrachialis
This muscle is located on the medial side of the humerus. It orignates at the coracoid process and inserts at the medial part of the shaft of the humerus, according to "Anatomy and Physiology." And it aids in adduction and flexion.
References
- ExercisePrescription.net
- "Anatomy and Physiology"; Saladin, Kenneth S.; 2004
