Deltoid muscle

The deltoid muscle is a large, triangular, and thick muscle that envelops the shoulder joint from the front, side, and back. It has three distinct parts—anterior, middle, and posterior—that work synergistically to produce various shoulder movements.

Origin

• Clavicular part: Anterior border of the lateral one-third of the clavicle.

• Acromial part: Lateral margin of the acromion, attached to four septa of origin.

• Scapular spinal part: Lower lip of the crest of the spine of the scapula.

Insertion

From its broad origin, the muscle fibers converge toward a thick tendon:

• Middle fibers: Pass vertically

• Anterior fibers: Pass obliquely backward and laterally

• Posterior fibers: Pass obliquely forward and laterally.

All fibers unite to form a tendon that inserts into the deltoid tuberosity on the middle of the lateral side of the humerus.

Structure

The deltoid muscle features distinct fiber arrangements:

• Acromial (central) part: Shows a multipennate arrangement, where fibers originate from four septa connected to the acromion. These fibers converge onto three septa of insertion attached to the deltoid tuberosity. The multipennate arrangement provides significant strength by increasing the density of fibers relative to its volume.

• Clavicular and scapular spinal parts: Arranged as unipennate muscles, inserting along the margins of the inferior tendon.

Nerves

The deltoid muscle is innervated by the axillary nerve, which is derived from the fifth and sixth cervical spinal roots (C5, C6).

Actions

• Middle (acromial) fibers: Primary abductors of the arm, responsible for lifting the arm from 15 to 90 degrees (taking over from the supraspinatus).

• Anterior fibers: Flex and medially rotate the arm.

• Posterior fibers: Extend and laterally rotate the arm.