The Shoulder muscles ,also called “ball-and-socket” are joint between the humerus bone of the upper arm and scapula bone (shoulder blade).
Six main movements occur at the shoulder: flexion, extension, abduction, adduction, internal rotation, and external rotation.
- Flexion – upper arm is elevated forward toward the face
- Extension – the arms move backward behind the plane of the body
- Abduction – the arm moves up and out to the side of body
- Adduction – the arm is pulled in toward the side of body
Horizontal adduction occur when the arm moves in a horizontal plane at shoulder level, such as during chest flys or rear deltoid flys.
The deltoid muscle of the shoulder consists of three 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 three 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 (at the side) attaches to the acromion and lifts the arm outward to the side (abduction). The posterior deltoid (behind) attaches to the scapula and moves the arm backward (shoulder extension).
The rotator cuff is a group of four muscles that form a protective sleeve around the shoulder joint. Despite being a barely visible muscle group, the rotator cuff is essential for shoulder stability and strength. All four muscles originate from the scapula (shoulder blade) and pass across the shoulder joint to attach onto 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. Infraspinatus and teres minor are located behind and act to rotate the arm out-as when hitchhiking. Subscapularis is situated in front and rotates the arm inward-as when folding your arms across the chest.
Source: wikipedia.org | book – Bodybuilding Anatomy by Nick Evans