Teres Major Innervation: Lower Subscapular Nerve Path

The teres major is innervated by the lower subscapular nerve, a branch of the posterior cord of the brachial plexus. This holds true in the large majority of people, but not all of them. A cadaveric study found that roughly one in seven shoulders received its teres major nerve supply from the thoracodorsal nerve instead, a variation that matters for surgeons, rehabilitation specialists, and anyone trying to understand why shoulder procedures sometimes produce unexpected outcomes.

The Lower Subscapular Nerve and Its Path

The lower subscapular nerve originates from the posterior cord of the brachial plexus, which itself forms from the merging of nerve fibers that exit the spinal cord at the level of the lower neck. After branching off the posterior cord, the lower subscapular nerve travels inferiorly to reach two muscles: the subscapularis, which sits on the front surface of the shoulder blade, and the teres major, which wraps around from the back. The nerve supplies motor signals that allow the teres major to contract, pulling the arm inward (adduction), rotating it toward the body (internal rotation), and extending the shoulder when the arm is raised.

The lower subscapular nerve also supplies the subscapularis muscle, which means these two muscles share a common neural pathway from the brachial plexus. This shared innervation reflects a deep anatomical kinship between the two muscles and becomes relevant during nerve blocks, surgical dissection, and rehabilitation after shoulder injuries.

How Often the Standard Pattern Varies

Textbooks typically describe the lower subscapular nerve as the sole nerve to the teres major, but cadaveric dissections reveal that anatomy does not always follow the textbook. In a study of 30 shoulders, the lower subscapular nerve innervated the teres major in about 87% of cases. In the remaining roughly 13%, the thoracodorsal nerve provided the supply instead.¹ The thoracodorsal nerve normally serves the latissimus dorsi, the large fan-shaped muscle of the back. When it also takes over innervation of the teres major, it means both muscles are effectively wired together, driven by the same nerve trunk.

This variation is not just anatomical trivia. Surgeons who perform tendon transfer procedures, where the teres major or latissimus dorsi tendons are rerouted to replace a torn rotator cuff, need to know exactly where the nerve enters the muscle. Cutting or stretching the nerve during mobilization would leave the transferred muscle paralyzed and the surgery pointless. In patients whose teres major happens to be innervated by the thoracodorsal nerve, the neurovascular anatomy around the muscle is arranged differently than expected, and a surgeon operating on autopilot could inadvertently damage the nerve supply.

Where the Nerve Enters the Muscle

Knowing which nerve supplies the teres major is only half the picture. The other half is knowing where along the muscle belly the nerve actually dives in. One anatomic study measured the entry point and found that the lower subscapular nerve entered the teres major at an average of 4.1 centimeters from the scapula.² A separate surgical anatomy study, approaching from the humeral (arm bone) side, placed the neurovascular pedicle to the teres major at an average of 7.4 centimeters from the muscle’s insertion on the humerus.³ These two measurements approach the same landmark from opposite directions and together give surgeons a working map of where the nerve lives relative to the bony anchors they can see and feel during an operation.

Within the muscle itself, nerve endings are not spread evenly. A cadaveric study mapping the intramuscular nerve distribution found that the greatest density of nerve endings concentrates in the middle fifth of the muscle belly, while the tendinous portions at either end contain relatively few.⁴ This finding has direct clinical applications. When clinicians inject botulinum toxin to treat shoulder spasticity, aiming for the middle of the muscle belly gives the toxin the best chance of reaching the motor end plates where it needs to act. Injecting too close to the tendinous ends wastes medication in tissue that has little nerve density.

Why Intramuscular Nerve Mapping Matters for Spasticity Treatment

Spasticity of the teres major is a common problem after stroke and brain injuries. The muscle can become chronically overactive, pulling the arm tightly against the body and internally rotating it into what clinicians describe as an adducted, internally rotated posture. This posture makes it hard for patients to reach, dress, or perform basic hygiene. Botulinum toxin injections into the teres major can relax the muscle enough to improve range of motion and reduce pain, but the injections work best when they hit the zones where motor nerves branch into the muscle fibers.

The cadaveric study that mapped these zones was explicitly designed to guide clinicians toward the optimal injection site.⁵ Before this kind of mapping data, injection sites were chosen based on general anatomic landmarks or ultrasound-guided guesswork. Understanding that the nerve density peaks in the middle 20% of the muscle gives practitioners a target zone rather than a vague region. The difference can mean faster relief and fewer repeat injections for patients dealing with a stiff, painful shoulder.

The Teres Major in Tendon Transfer Surgery

Massive rotator cuff tears, where two or more of the four rotator cuff tendons are completely torn and cannot be repaired directly, sometimes call for a tendon transfer. The idea is to detach a nearby healthy muscle from its normal insertion and reattach it where the torn rotator cuff used to be, recruiting a new motor to do the old one’s job. Both the latissimus dorsi and the teres major are common donor muscles for this procedure.

Because the nerve to the teres major enters relatively close to the scapula, surgeons face a leash problem: the neurovascular pedicle limits how far the muscle can be moved without tension on the nerve. The pedicle sits roughly 7 centimeters from the humeral insertion, so every centimeter of extra mobilization beyond that distance risks stretching or kinking the nerve supply.⁶ The radial nerve, which controls wrist and finger extension, passes directly in front of the tendons at about 2 to 3 centimeters from the humeral insertion, adding another structure the surgeon has to navigate carefully.⁷ Additionally, when the tendons are tunneled beneath the posterior deltoid to reach their new insertion site, the posterior branch of the axillary nerve crosses superficially over them, creating yet another potential pinch point.

In patients where the thoracodorsal nerve rather than the lower subscapular nerve supplies the teres major, the nerve’s course and length differ. Surgeons who are unaware of this variant may mobilize the muscle to a distance that would be safe under the standard anatomy but stretches the variant nerve supply beyond its tolerance. Preoperative imaging or careful intraoperative identification of the nerve can prevent this.

How the Teres Major Adapts When the Rotator Cuff Fails

The teres major is not just a passive bystander in shoulder disease. When the rotator cuff tears badly enough that the shoulder loses its normal stabilizers, the nervous system reroutes muscle activation patterns in ways that are both clever and problematic. In healthy shoulders, the teres major fires primarily during adduction, pulling the arm toward the body. But in patients with massive rotator cuff tears, the muscle’s activation shifts: it begins firing during forward flexion and abduction, movements it normally has little role in.⁸

This adaptation seems to represent the nervous system’s attempt to compensate for the missing rotator cuff by pressing the teres major into service as a shoulder depressor, holding the humeral head down in the socket during overhead movements. Without this compensation, the humeral head rides upward and jams against the acromion, causing impingement and pain. After pain relief or salvage surgery (such as a reverse total shoulder replacement or a tendon transfer), the teres major’s activation pattern can shift again, sometimes returning closer to its normal adduction role.⁹ This plasticity shows that the nerve supply to the teres major is not just carrying a fixed signal; it responds to the mechanical and sensory environment of the shoulder, recalibrating which movements the muscle participates in.

Age-Related Changes in Teres Major Activation

Even in people with healthy, pain-free shoulders, the way the nervous system drives the teres major changes with age. An electromyography study of 60 asymptomatic adults between the ages of 21 and 60 found that the activation ratio of the teres major during arm abduction decreased with age. The effect was statistically clear, with the teres major showing a stronger age-related decline in activation than the neighboring latissimus dorsi.¹⁰

At first glance, less activation of an adductor during abduction might seem like a good thing: why would you want a muscle pulling the arm down while you are trying to raise it? But the researchers interpreted this as a co-contraction strategy. In younger adults, the deltoid handles abduction with minimal co-contraction from the adductors. In middle-aged adults, the adductors begin co-contracting during abduction, which may help stabilize the humeral head in the socket as the rotator cuff’s capacity gradually declines with age. The decrease in the teres major’s activation ratio, despite the increase in co-contraction overall, suggests that the nervous system is recalibrating how much each adductor contributes as the shoulder ages. These are pain-free individuals, so the shift is not caused by injury; it appears to be a preventive adaptation, a way of maintaining shoulder stability as the tissues slowly change.

The Teres Major During Internal Rotation Exercises

For rehabilitation and strength training, the teres major is one of the muscles people are targeting when they perform shoulder internal rotation exercises, even if they do not realize it. An electromyography study of 30 healthy subjects recorded activity from 16 shoulder girdle muscles and muscle sub-regions during three common internal rotation exercises: standing internal rotation with the arm at the side, standing internal rotation with the arm abducted to 90 degrees, and internal rotation in the “zero position” (a plane between pure flexion and pure abduction).¹¹

The teres major was monitored alongside the subscapularis, latissimus dorsi, and pectoralis major, all of which contribute to internal rotation. Understanding which exercises preferentially activate the teres major versus the subscapularis, for example, can help physical therapists design programs that target specific muscles after injury or surgery. If a patient has had a teres major transfer and the surgeon wants to retrain that muscle in its new role, knowing which arm positions drive the most teres major activation is essential for writing a rehab protocol that actually challenges the transferred muscle rather than letting the latissimus dorsi or pectoralis take over.

Evolutionary Origins of the Teres Major’s Nerve Supply

The fact that the teres major receives its nerve from the posterior cord of the brachial plexus, often sharing a trunk with the subscapularis nerve, is not a random arrangement. Comparative anatomy research tracing the muscles of the shoulder across vertebrates, from salamanders through reptiles to mammals, suggests that the teres major evolved from the dorsal-most portion of the latissimus dorsi muscle as seen in reptiles.¹² In reptiles, the latissimus dorsi is a broad muscle with a wide attachment. As mammals evolved a more mobile, rotator-cuff-equipped shoulder, that dorsal portion split off and became the teres major, while the segmentally higher part of the reptilian latissimus dorsi contributed to the subscapularis.

This phylogenetic history explains the innervation puzzle neatly. The lower subscapular nerve and the thoracodorsal nerve both arise from the posterior cord, and both historically supplied portions of what was once a single muscular sheet. When the sheet split into distinct muscles during mammalian evolution, the nerve supply sometimes followed cleanly (lower subscapular to teres major, thoracodorsal to latissimus dorsi) and sometimes retained its ancestral pattern, with the thoracodorsal nerve continuing to supply the teres major. The 13% of shoulders where the thoracodorsal nerve innervates the teres major may represent an echo of this older arrangement, a remnant of the time before the two muscles fully separated. This kind of variation is common throughout the body; evolution does not produce a single fixed wiring diagram but a range of arrangements that all work well enough to survive.

The Brachial Plexus Context

The posterior cord of the brachial plexus gives rise to several nerves beyond the lower subscapular. It also produces the upper subscapular nerve, the thoracodorsal nerve, the axillary nerve, and the radial nerve. The lower subscapular nerve branches off the cord and heads toward the subscapularis and teres major, while the thoracodorsal nerve continues toward the latissimus dorsi.¹³ Understanding this branching pattern matters clinically because injuries to the posterior cord, whether from trauma, compression, or surgical positioning, can knock out the teres major along with the latissimus dorsi, subscapularis, deltoid, and triceps simultaneously. A patient who presents with weakness in all these muscles after a shoulder dislocation or a clavicle fracture likely has a posterior cord lesion rather than individual nerve injuries.

For nerve block procedures targeting shoulder pain, knowing that the teres major is innervated by a posterior cord branch helps anesthesiologists and pain specialists predict which blocks will affect the muscle. A supraclavicular or infraclavicular brachial plexus block that reaches the posterior cord will anesthetize the teres major, which can be useful during shoulder surgery but also means the patient will lose active adduction and internal rotation strength temporarily. Selective nerve blocks that target only the suprascapular nerve, by contrast, will not affect the teres major at all, since that nerve comes from a different trunk of the plexus.