The trapezius muscle, often called the “traps,” spans the neck and upper back. Its size and location, extending toward the shoulder, frequently cause confusion about whether it belongs to the back or the shoulder girdle. Although it is a primary mover and stabilizer of the shoulder blade (scapula), its anatomical classification places it firmly within the musculature of the back. Understanding its structure clarifies its official designation and its significant role in upper body movement and posture.
Anatomical Classification: Why the Traps are Back Muscles
The trapezius is classified as a superficial muscle of the back, based on its origin points along the skull and spine. It begins high up at the external occipital protuberance at the base of the skull and extends down the spine through the nuchal ligament and the spinous processes of all twelve thoracic vertebrae (T1-T12). This broad attachment across the cervical and thoracic spine establishes its identity as a back muscle. The muscle fibers fan out from these spinal origins to insert onto the bones of the shoulder girdle: the lateral third of the clavicle (collarbone), the acromion, and the spine of the scapula (shoulder blade).
The Three Functional Divisions
The trapezius is composed of three distinct fiber groups. Each group performs a specialized action on the scapula, allowing for complex arm and shoulder function.
Upper Trapezius
The upper trapezius fibers run downward and laterally from the neck to the clavicle and acromion. Their primary role is to elevate the scapula, which is the action used when shrugging the shoulders. These fibers also assist with neck extension, side bending, and rotation of the head.
Middle Trapezius
The middle trapezius fibers travel horizontally from the upper thoracic spine to the spine of the scapula. This section is responsible for scapular retraction, pulling the shoulder blades toward the spine. This action stabilizes the shoulder blade during arm movements.
Lower Trapezius
The lower trapezius fibers run upward and laterally from the mid-to-lower thoracic spine to the medial aspect of the scapular spine. The main action of this lower portion is to depress the scapula, pulling the shoulder blade downward. The upper and lower fibers also work together to upwardly rotate the scapula when raising the arm above shoulder height.
Practical Implications for Training and Posture
Understanding the three functional divisions is important for effective training and maintaining good posture. Simply performing shrugs targets only the upper trapezius, which can lead to muscle imbalances if the middle and lower sections are neglected. For comprehensive development, exercises must be selected to engage all three fiber orientations.
Targeting the middle trapezius requires horizontal pulling movements, such as rows, which emphasize scapular retraction. The lower trapezius is activated through movements that involve depressing and upwardly rotating the scapula, like specific arm raises or “Y” raises. Developing strength across all parts of the trapezius is important for supporting the shoulder and spine.
A strong, balanced trapezius muscle helps maintain proper scapular position, which directly impacts upper body posture. Weakness in the middle and lower fibers, combined with tightness in the upper fibers, can contribute to forward head posture and rounded shoulders. Strengthening the lower trapezius, in particular, helps depress the shoulder blades and stabilize the thoracic spine, contributing to a more upright posture.