Shoulder flexion is the action of lifting one’s arm forward in front of the body. Initiated by the pectoralis major and anterior deltoid muscles with assistance from the biceps, flexion occurs at the glenohumeral, or shoulder, joint, which is a ball-in-socket joint. From a starting position with the arm down at the side, approximately 180 degrees of flexion may be attained until the arm is directly overhead. Flexion may happen with the palm facing in, palm facing backward, or palm facing forward.
The glenohumeral joint is an articulation between the head or ball of the humerus bone in the upper arm and the socket formed by the glenoid cavity of the scapula. A type of synovial, or movable, joint called a ball-and-socket joint, the shoulder allows movement of the arm in nearly every direction as the head of the humerus rolls around in the glenoid cavity like a ball in a cup. One such movement is shoulder flexion, in which the arm is lifted vertically in a forward direction.
Shoulder flexion is made possible mostly by two major muscles of the upper body: the pectoralis major and the anterior deltoid. In the pectoralis major muscle of the chest, an upper section known as the clavicular head that arises from the collarbone and attaches to the front of the humerus is responsible for flexing the arm forward. Adjacent to the clavicular head of the pectoralis major is the anterior section of the deltoid, the muscle covering the shoulder. Originating on the collarbone just lateral to the pectoralis and inserting on the front outer aspect of the humerus, the anterior deltoid works with the clavicular head of the pectoralis to produce shoulder flexion. It is particularly active when the elbow is angled outward.
Other muscles become involved in shoulder flexion depending on the rotation of the shoulder during flexion. When the arm is rotated so that the palm of the hand faces forward, the biceps brachii and coracobrachialis muscles in the anterior upper arm assist in lifting the arm forward. Their contribution is weaker when the arm is neutral. In a medially, or internally, rotated position, however, the pectoralis minor in the chest as well as the subscapularis muscle of the rotator cuff assist in shoulder flexion, especially by stabilizing the head of the humerus in the glenoid cavity as the arm is raised.