The deltoid muscle is a shoulder muscle, giving the joint its rounded contour. This large, triangular muscle is the primary mover of the arm at the shoulder joint, connecting the arm to the trunk. Confusion about whether it is a shoulder or a back muscle stems from its unique structure, as one of its three sections extends across the back of the shoulder. Understanding the muscle’s anatomy, which is divided into distinct functional heads, clarifies its role as part of the shoulder complex.
Anatomy of the Shoulder: The Three Deltoid Heads
The deltoid is a single muscle composed of three distinct segments, known as heads, which wrap around the shoulder joint. These are the anterior, medial, and posterior heads, each originating from a different part of the shoulder girdle. The anterior head begins at the lateral third of the clavicle, placing it at the front of the shoulder.
The medial head (sometimes called the lateral head) originates from the acromion, the highest point of the scapula. The posterior head begins at the spine of the scapula, a bony ridge on the back of the shoulder blade. All three heads converge into a single tendon that inserts on the deltoid tuberosity, a point located halfway down the side of the humerus. This shared insertion confirms the deltoid’s primary function is to move the arm relative to the shoulder.
Functions of the Deltoid: Movement and Stability
The deltoid muscle is responsible for a wide range of movements, depending on which head contracts. The medial head is the primary driver of arm abduction, lifting the arm straight out to the side. It takes over this action after the initial movement, working up to about 100 degrees of elevation.
The anterior head contributes to arm flexion (lifting the arm forward) and assists in internal rotation. Conversely, the posterior head’s main roles are arm extension (moving the arm backward) and external rotation. The collective action of the deltoid is also important for stabilizing the shoulder joint, helping to hold the head of the humerus securely in its socket.
Addressing the Confusion: Deltoids and the Upper Back
The belief that the deltoid is a back muscle arises because of the posterior head’s location and function. Situated on the back of the shoulder, the posterior deltoid lies next to and partially overlies true upper back muscles, such as the trapezius and rotator cuff muscles. The posterior head’s action of pulling the arm backward involves many surrounding back muscles, creating the appearance of shared function.
True back muscles, like the trapezius and rhomboids, primarily move the shoulder blade itself, retracting or elevating it. The posterior deltoid, however, moves the humerus independently of the shoulder blade. While its function of extending the arm works synergistically with some back muscles, its insertion on the humerus confirms it is structurally and functionally part of the shoulder complex.
Targeting the Three Heads: Practical Application
Because each deltoid head performs a distinct action, targeting all three for balanced development requires a variety of movements.
Anterior Deltoid
The anterior deltoid is heavily engaged in pressing movements that move the arm forward and overhead. Common exercises that target this head include:
- Overhead press variations
- Front raises
Medial Deltoid
The medial head is best isolated by movements that lift the arm directly out to the side, known as abduction. Lateral raises, performed with dumbbells or cables, are the most direct way to focus on this head.
Posterior Deltoid
The posterior deltoid requires exercises that involve pulling the arm backward, often against resistance. Moves effective for isolating this rear section of the shoulder include:
- Reverse flyes
- Face pulls