The subscapularis is the largest and strongest muscle in the rotator cuff, a group of four muscles and their tendons that surround the shoulder joint. These muscles provide functional movement while keeping the head of the humerus centered within the shallow socket of the scapula. The subscapularis requires a dedicated nerve supply, or innervation, to receive electrical signals from the brain and spinal cord that trigger contraction. Understanding the specific nerve responsible for this function is important for diagnosing and treating shoulder injuries that involve muscle weakness or paralysis.
The Specific Nerves Supplying the Muscle
The subscapularis muscle is innervated by two distinct nerves: the Upper Subscapular Nerve and the Lower Subscapular Nerve. Both nerves branch directly off the Posterior Cord of the Brachial Plexus, the complex network of nerves that serves the entire arm and shoulder. This dual innervation ensures that the large, triangular muscle receives signals across its broad surface area. The Upper Subscapular Nerve typically supplies the superior portion, or upper half, of the muscle belly. The Lower Subscapular Nerve is responsible for the inferior portion of the subscapularis, and it also sends a branch to the adjacent teres major muscle.
Primary Function of the Subscapularis
The subscapularis functions in both movement and stabilization of the shoulder joint. Its primary action is the powerful internal rotation, or medial rotation, of the humerus, which is the movement used when reaching across the body or tucking a shirt into the back of the pants. It is the only rotator cuff muscle that performs this action, making it a powerful agonist for this type of movement. This rotation works in opposition to the smaller external rotator muscles of the shoulder, helping to control acceleration and deceleration during throwing motions.
The subscapularis is a powerful stabilizer of the glenohumeral joint. Situated on the anterior side of the scapula, it pulls the humeral head inward and forward against the joint socket. This action is particularly important for preventing the head of the arm bone from slipping forward, which can lead to an anterior shoulder dislocation. The muscle contributes significantly to the centering of the arm bone during activity.
Source of the Subscapular Nerves
The Upper and Lower Subscapular Nerves trace their path back to the Brachial Plexus, a structure that begins in the neck. This plexus is formed by the anterior rami, or roots, of the spinal nerves C5 through T1. These five roots combine and split into trunks, divisions, and finally three large cords, which are named for their position relative to the axillary artery.
The subscapular nerves originate specifically from the Posterior Cord, which is one of the three major terminal bundles of the plexus. The Posterior Cord carries nerve fibers from the C5, C6, C7, C8, and T1 spinal levels. However, the fibers that specifically supply the subscapularis muscle primarily arise from the C5 and C6 nerve roots. This pathway provides the electrical connection from the cervical spine down to the subscapularis muscle.
Impact of Nerve Injury
Damage to the subscapular nerves, which can occur due to trauma, surgery, or compression in the shoulder region, results in a loss of muscle function. The immediate clinical consequence is significant weakness or complete paralysis of the subscapularis muscle. This directly impairs the ability to internally rotate the arm and compromises the stability of the shoulder joint.
Patients with this nerve injury often report difficulty with simple daily activities, such as reaching across the body or pushing motions. The lack of dynamic stabilization can make the shoulder feel loose or unstable, especially during overhead movements or when the arm is positioned at certain angles. In cases of chronic denervation, the muscle may begin to show signs of atrophy, or wasting, which is visible as a loss of muscle bulk.
Physicians use specific physical examination maneuvers to test for subscapularis weakness, such as the Lift-off Test or the Belly-Press Test. In the Lift-off Test, the patient is asked to lift their hand off their lower back, a motion that strongly recruits the subscapularis. The Belly-Press Test involves the patient pressing their palm into their abdomen while maintaining a flat wrist. Rehabilitation for nerve injuries often involves physical therapy focused on maximizing the remaining function of the muscle and encouraging surrounding muscles to compensate for the lost strength.