Anatomists refer to the arm, or brachium, as the region between the shoulder and the elbow joint. This upper limb segment is not a single, undivided space but is organized into defined functional and structural areas. To understand the musculature and function of the backside of this area requires using the specific terms that describe these distinct anatomical regions.
Identifying the Posterior Compartment
The entire upper arm is structurally divided into two main sections by sheets of tough connective tissue known as intermuscular septa. These partitions, which connect the bone of the upper arm (the humerus) to the surrounding deep fascia, create distinct anatomical spaces called compartments. The segment located on the rear side is formally known as the Posterior Compartment of the Arm.
This posterior section is also frequently referred to as the Extensor Compartment. This alternate name is based on the primary action performed by the muscles housed within this region. The compartment is dominated by a single, large muscle, which is responsible for the characteristic shape and function of the back of the arm.
The Triceps Brachii Muscle
The most prominent structure defining the posterior compartment is the Triceps Brachii muscle, whose name literally translates from Latin as “three-headed muscle of the arm.” The three heads—the long head, the lateral head, and the medial head—each originate from different points to span the length of the arm. This multi-headed structure allows the muscle to exert force across the elbow joint from various angles and positions.
The long head is unique among the three because it originates on the scapula, or shoulder blade, specifically from the infraglenoid tubercle. This origin means the long head crosses both the shoulder and the elbow joints, giving it a role in both movements. In contrast, the lateral and medial heads arise directly from the back surface of the humerus. The lateral head originates superior to a feature known as the radial groove, while the medial head originates inferior to it.
Despite having three separate points of origin, all three heads converge to form a single, powerful tendon. This common tendon crosses the elbow joint and inserts onto the olecranon process of the ulna, which is the prominent, bony point of the elbow. This extensive attachment across two joints and two bones illustrates the Triceps Brachii’s dominance in the structure of the posterior arm.
Primary Role in Movement
The Triceps Brachii is recognized as the sole extensor of the forearm at the elbow joint. Its main job is to straighten the arm, moving the forearm away from the upper arm, a movement known as elbow extension. When the muscle contracts, it pulls on the olecranon process, causing the elbow to open and the arm to unbend.
This function is the reason the entire area is anatomically labeled the Extensor Compartment. While the primary action is at the elbow, the long head of the Triceps Brachii also assists with extension and adduction of the arm at the shoulder joint because of its attachment to the scapula. This secondary role helps to stabilize the shoulder and bring the arm back toward the body’s midline.