The axillary nerve innervates two main muscles of the shoulder: the deltoid and the teres minor. It also supplies sensation to a patch of skin on the outer shoulder and sends a branch to part of the triceps and the shoulder joint itself. Understanding what this nerve controls helps explain why damage to it can make lifting your arm or rotating your shoulder so difficult.
Muscles Controlled by the Axillary Nerve
The deltoid is the large, rounded muscle that caps your shoulder, and it’s the primary muscle the axillary nerve powers. The deltoid has three parts: the front portion lifts your arm forward, the middle portion lifts it out to the side, and the rear portion pulls it backward. All three depend on the axillary nerve to function. When the nerve is damaged, raising your arm away from your body becomes weak or impossible, and the muscle can visibly shrink, especially noticeable in people who are lean or muscular.
The teres minor is a smaller muscle on the back of the shoulder blade. It rotates your arm outward, the motion you’d use to cock your arm back for a throw or reach behind your head. It works alongside another rotator cuff muscle (the infraspinatus) for this job, so some outward rotation may still be possible even if the axillary nerve is injured, though it will be noticeably weaker.
A branch of the axillary nerve also reaches the upper portion of the triceps, the muscle on the back of your upper arm that straightens your elbow. This contribution is relatively minor compared to the radial nerve, which does the heavy lifting for triceps function, so elbow straightening is rarely affected in a meaningful way by axillary nerve problems alone.
Sensory Area on the Outer Shoulder
Beyond muscles, the axillary nerve provides sensation to a small but clinically important patch of skin. This area, called the “regimental badge” region, sits on the outer surface of your upper arm and shoulder, roughly where a military badge would rest on a sleeve. When the nerve is injured, this area often goes numb. Checking for sensation here is one of the quickest ways to assess whether the axillary nerve is intact after a shoulder injury.
The Nerve’s Path Through the Shoulder
The axillary nerve originates from the brachial plexus, the network of nerves that runs from your neck into your arm. It travels through a narrow opening behind the shoulder called the quadrangular space, a gap bordered by the teres minor above, the teres major below, the long head of the triceps on the inner side, and the upper arm bone on the outer side. The posterior circumflex humeral artery, which supplies blood to the back of the shoulder, passes through this space alongside the nerve.
After exiting the quadrangular space, the nerve wraps around the back of the upper arm bone (humerus) and divides into its branches. The anterior branch curves forward to supply the front and middle portions of the deltoid. The posterior branch supplies the rear deltoid, the teres minor, and the sensory patch on the outer shoulder. A third, articular branch heads into the shoulder joint capsule itself, providing some of the joint’s sensory feedback.
The nerve runs an average of about 5 to 6 centimeters below the bony point of the shoulder (the acromion). This proximity to the shoulder joint is one reason the nerve is so vulnerable during dislocations and fractures.
How the Axillary Nerve Gets Injured
Anterior shoulder dislocations, where the upper arm bone pops forward out of the socket, are the most common cause of axillary nerve damage. The axillary nerve is the most frequently injured nerve in this scenario, accounting for roughly 35% of all nerve injuries associated with shoulder dislocations. Fractures of the upper arm bone near the shoulder (proximal humerus fractures) can also stretch or compress the nerve. The overall reported incidence of nerve injury after a shoulder dislocation varies widely, from under 1% to over 60%, depending on how carefully clinicians test for it and how old the patient is. Older adults face a higher risk because their tissues are less elastic.
Blunt trauma to the outer shoulder, repetitive overhead motions in athletes, and compression from improper use of crutches or prolonged pressure on the armpit can also damage the nerve. Surgical procedures around the shoulder carry some risk as well, which is why surgeons pay close attention to the nerve’s location relative to the acromion.
Signs of Axillary Nerve Damage
The hallmark symptom is weakness or inability to lift your arm out to the side. You may also notice difficulty rotating your arm outward. Over weeks to months, the deltoid muscle can visibly flatten or waste away on the affected side, creating an obvious asymmetry between shoulders. Numbness or tingling on the outer shoulder (the regimental badge area) is another telltale sign.
Testing for axillary nerve function is straightforward. A clinician will ask you to raise your arm against resistance while feeling the deltoid contract, and then check your ability to rotate the arm outward. Sensation is tested by lightly touching the outer shoulder. If there’s any question about the extent of injury, nerve conduction studies or imaging can map the damage more precisely.
Many axillary nerve injuries from dislocations or stretching recover on their own over three to six months as the nerve slowly regenerates. Physical therapy during this period helps maintain joint mobility and prevents the shoulder from stiffening. If recovery stalls, surgical options include nerve repair, nerve grafting, or nerve transfer procedures to restore deltoid function.