The deltoid muscle is the triangular complex that gives the shoulder its rounded contour. Located at the top of the shoulder, it has three heads: anterior, lateral, and posterior. Its primary responsibility is to lift and rotate the arm, executing movements like forward flexion, abduction, and extension. As a primary mover and stabilizer during nearly all arm movements, the deltoid is frequently subjected to stresses that can lead to pain originating from various sources.
Deltoid Muscle Overload and Acute Injury
Pain can arise directly from the muscle fibers, most commonly due to a muscle strain (a pulled muscle). A strain occurs when muscle fibers are overstretched or torn, typically from a sudden, forceful contraction or lifting a heavy load with improper form. This type of injury presents as a sharp, immediate pain felt directly within the muscle belly, localized to the specific head of the deltoid that was overloaded.
Acute injuries are graded by severity, ranging from a Grade I strain (minor tearing and mild soreness) to a Grade III tear (a complete rupture causing severe pain, significant swelling, and a palpable defect or gap in the muscle). Movement assessment often reproduces the pain precisely when the injured muscle head is contracted against resistance. This is distinct from Delayed Onset Muscle Soreness (DOMS), a dull ache appearing 12 to 24 hours after intense exercise, which represents normal microscopic muscle damage.
Associated Tendon and Bursa Inflammation
Pain felt over the deltoid is frequently referred from inflamed soft tissues located beneath the muscle. The subacromial-subdeltoid bursa, a fluid-filled sac cushioning the space between the deltoid and the rotator cuff tendons, is susceptible to inflammation (subacromial bursitis). The inflamed bursa causes a diffuse, aching pain often felt over the lateral deltoid. This discomfort worsens when lifting the arm, especially overhead, and can be painful at night when lying on the affected shoulder.
Irritation or degeneration of the rotator cuff tendons (tendinopathy) also refers pain to the deltoid region, particularly from the supraspinatus tendon. This pain is described as a deep ache that radiates down the side of the upper arm. Tendinopathy pain is exacerbated by specific movements like raising or lowering the arm, as the compromised tendon is loaded or momentarily pinched. While historically called tendinitis, the condition is now understood to be chronic degeneration (tendinosis) rather than purely an inflammatory process.
Mechanical Compression and Joint Instability
Structural issues within the shoulder joint can also mechanically irritate tissues, causing pain felt in the deltoid area. Shoulder Impingement Syndrome occurs when the space beneath the acromion narrows, pinching the rotator cuff tendons and the subacromial bursa during arm movement. This compression results in pain, often described as a painful arc, occurring when the arm is elevated between 60 and 120 degrees. The pain is typically felt on the outer side of the shoulder, directly over the lateral deltoid.
Shoulder instability (subluxation or dislocation of the humeral head) causes deltoid pain through a different mechanism. The deltoid, along with the rotator cuff, contracts intensely to stabilize the joint and prevent further movement. This protective muscle guarding and subsequent strain leads to pain and weakness, especially during activities that challenge stability, such as throwing or reaching behind the body. Acromioclavicular (AC) Joint Arthritis, degeneration of the joint where the collarbone meets the shoulder blade, can cause pain that radiates downward over the anterior deltoid. This localized pain is usually worsened by movements that bring the arm across the body or by lifting objects overhead.
Pain Originating from Nerves and Spine
Pain felt in the deltoid can originate outside the shoulder complex, specifically from compressed nerves in the neck. Cervical radiculopathy (a pinched nerve in the neck) can refer pain down the shoulder and into the deltoid area. Compression of the C5 nerve root, which exits between the C4 and C5 vertebrae, is associated with pain and numbness in the superior shoulder and lateral upper arm. This nerve issue can also lead to weakness in the deltoid muscle, making it difficult to raise the arm against resistance.
The axillary nerve, which directly supplies the deltoid muscle, is vulnerable to injury, most often following a shoulder dislocation or a fracture of the upper arm bone. Axillary nerve injury causes pain in the shoulder region, but its defining feature is severe weakness in the deltoid muscle when attempting to lift the arm away from the body. Chronic lesions can also result in muscle wasting (atrophy) of the deltoid and numbness over the lateral part of the shoulder.