MUSCLE:
Deltoid (Is made up of three heads: anterior, lateral, and posterior)
Muscle: Anterior Deltoid
Origin: At the lateral third of the clavicle (collar bone), on the anterior border.
Insertion: At the deltoid tubercle of the lateral surface of the humerus
Function: Humeral flexion; works with the pectoralis major in drawing the arm forward. It is a strong abductor of the arm. The anterior deltoid’s lower fibers weakly adduct and rotate the arm inward.
Muscle: Lateral Deltoid
Origin: At the acromion process on the scapula.
Insertion: At the deltoid tubercle on the lateral surface of the humerus
Function: Abduction of the humerus. It is aided by the supraspinatus, and the upper fibers of the anterior and posterior deltoid in abduction. It is limited by the tightening of the lower shoulder capsule. The lateral deltoid contracts to abduct the humerus, while the anterior and posterior deltoid stretch to steady the humerus.
Muscle: Posterior Deltoid
Origin: At the lateral aspect at the inferior edge (below the top, outside edge of the shoulder blade) of the spine of the scapula
Insertion: At the deltoid tubercle of the humerus
Function: Humeral extension. It assists the latissimus dorsi and teres major in drawing the arm backward into extension. The lower fibers weakly adduct and outwardly rotate the arm.
Indication of Weakness: Anterior Deltoid:
Overhead presses become difficult and painful. Acromioclavicular (AC) joint sprain will cause weakness, since the anterior and lateral deltoid cannot contract properly in an unstable joint.
Shortening of the infraspinatus and pectoralis major will decrease ROM in arm abduction. Fixation of the scapula to the thoracic wall can also decrease ROM in arm abduction.
The posterior deltoid is weakened when the anterior deltoid and pectorals become shortened and hypertonic, causing agonist-antagonist muscle inhibition. If the glenohumeral joint has shifted forward due to shortened muscles, it needs to be treated or stretched to allow the posterior deltoid to contract properly again.