The deltoid muscle consists of 3 fiber regions.  The anterior portion flexes and medially rotates the humerus.  The posterior portion extends and laterally rotates the humerus. The middle portion abducts the humerus.  From the rest position, the deltoid generates a superiorly directed vector that pulls the head of the humerus toward the acromion.  The humeral head should not glide superiorly.   If the posterior head of the deltoid has become the dominant lateral rotator, the result is anterior glide of the humeral head during motion.  The humeral head should not glide anteriorly.

If the anterior head if the deltoid has become a dominant medial rotator and/or the subscapularis, a medial rotator, is dominating the lateral rotators, the resting glenohumeral position is one of medial rotation.  The combination of abduction and medial rotation predisposes the subject to an impingement syndrome.

It is essential that the depressors of the humeral head primarily the supraspinatus, infraspinatus, teres minor, and subscapularis adequately offset the proximal pull of the deltoid muscle.

Actions

All fibers:

• Abduct the shoulder (glenohumeral joint)

Anterior fibers:

• Flex the shoulder (g/h joint)
• Medially rotate the shoulder (g/h joint)
• Horizontally abduct the shoulder (g/h joint)

Posterior fibers:

• Extend the shoulder (g/h joint)
• Laterally rotate the shoulder (g/h joint)
• Horizontally abduct the shoulder (g/h joint)

Origin

• Lateral 1/3 of the clavicle, acromion, and spine of scapula

Insertion

• Deltoid tuberosity

Nerve Innervation

• Axillary from brachial plexus