Shoulders

The shoulders, more formally called the deltoids, are the triangular muscles that cap the shoulder joint and give it its rounded contour. They are one of the most visible muscles in the upper body and play a critical role in arm movement.

Anatomy

The deltoid muscle is a large triangular muscle that envelopes the shoulder joint from the front, side, and back. It is named for its resemblance to the Greek letter delta. The muscle originates from three separate points: the lateral third of the clavicle, the acromion process of the scapula, and the spine of the scapula. These fibers converge to insert on the deltoid tuberosity of the humerus¹².

The deltoid is traditionally divided into three parts based on their fiber orientation. The anterior (clavicular) fibers originate from the clavicle and flex and medially rotate the arm. The lateral (acromial) fibers originate from the acromion and are the primary abductors of the arm. The posterior (spinal) fibers originate from the spine of the scapula and extend and laterally rotate the arm.

The deltoid is innervated by the axillary nerve, which arises from the fifth and sixth cervical spinal roots (C5, C6)³.

Functions

The primary function of the deltoid is arm abduction. The lateral fibers are responsible for raising the arm from 15 to 100 degrees. The deltoid cannot initiate abduction below 15 degrees because its fibers run parallel to the humeral axis; the supraspinatus handles the first 15 degrees.

The anterior deltoid assists the pectoralis major in flexing the arm and works with the subscapularis to internally rotate the humerus. The posterior deltoid assists the latissimus dorsi in extending the arm during ambulation and other activities.

The deltoid also plays a key role in shoulder stability, preventing inferior displacement of the humeral head. This becomes especially important in cases of rotator cuff injury, where the deltoid must compensate for reduced function in other shoulder muscles¹².

Training

The deltoid is targeted through various pressing and raising movements. The overhead press targets the entire deltoid but places greatest emphasis on the anterior and lateral fibers. Lateral raises directly target the lateral deltoid, while front raises target the anterior deltoid. Rear deltoid flyes or face pulls target the posterior deltoid.

The lateral deltoid is often considered lagging in many lifters because it is difficult to train with compound movements. Isolation exercises like lateral raises are particularly important for developing shoulder width.

Clinical Significance

The deltoid is a common site for intramuscular injections due to its large size and superficial location. Proper injection technique is important to avoid damaging the axillary nerve, which runs through the muscle.

Rotator cuff injuries may force the deltoid to compensate for lost function in the supraspinatus, infraspinatus, teres minor, and subscapularis. This can lead to deltoid overuse and fatigue if the rotator cuff injury is not properly rehabilitated.

References

1. Anatomy, Shoulder and Upper Limb, Deltoid Muscle(accessed Apr 18, 2026)
2. Deltoid muscle(accessed Apr 18, 2026)
3. Deltoid Muscle Anatomy(accessed Apr 18, 2026)