Shoulders

The shoulder is a ball or socket joint between the humerus bone of the upper arm and the scapula bone. Six main movements occur at the shoulder:-  flexion, extension, abduction, adduction, internal rotation, and external rotation. During shoulder flexion the upper arm is elevated forward toward the face. During shoulder extension, the arm moves back behind the plane of the body. During the abduction, the arm moves up and out to the side of the body. During adduction, the arm is pulled in toward the side of the body. Horizontal abduction and adduction occur when the arm moves in a horizontal plane at shoulder levels, such as during chest flys or rear deltoid flys. 


The deltoid muscle of the shoulder consists of 3 separate sections, or heads, each capable of moving the arm in different directions. From a broad tendon attachment above the shoulder joint, the deltoid’s 3 heads merge into a single tendon that attaches to the humerus bone of the upper arm. The anterior deltoid (in front) attaches to the clavicle and raises the arm forward (shoulder flexion). The lateral deltoid attaches to the acromion and lifts the arm out to the side (shoulder abduction). The posterior deltoid attaches to the scapula and moves the arm backward (shoulder extension).

A rotator cuff is a group of four muscles that form a protective sleeve around the shoulder joint. Despite being barely visible, the rotator cuff is essential for shoulder stability and strength. All 4 muscles originate from the scapula (shoulder blade) and pass across the shoulder joint to attach to the humerus bone of the upper arm. The supraspinatus lies above the joint and raises (abducts) the arm up and outward—as when hailing a taxi. The infraspinatus and teres minor are located behind the joint and act to rotate the arm out—as when hitchhiking. The subscapularis is situated in front of the joint and rotates the arm inward—as when folding your arms across your chest.