Left trapezius is a broad skeletal muscle, a muscle that moves and stabilises the skeleton, lying superficially—close to the skin—over the back of the neck and upper trunk. It extends from the skull and vertebral column to the left shoulder girdle, positioning the shoulder blade and helping move the neck.
Where is the left trapezius located?
The muscle covers the back of the left side of the neck and extends across the upper back. Its broad attachment near the midline narrows towards the shoulder. The shoulder girdle consists of the clavicle, or collarbone, and the scapula, or shoulder blade.
The left trapezius lies beneath the skin, subcutaneous tissue—the tissue immediately below the skin—and a covering of connective tissue called fascia. It overlies muscles including the levator scapulae, which raises the scapula, and the rhomboid muscles, which draw it towards the spine. Its lower region also overlaps part of the latissimus dorsi, a broad back muscle that acts on the upper arm.
At the shoulder, trapezius attaches beside the deltoid, the muscle covering the shoulder. In the neck, its front edge forms the back boundary of the posterior triangle, a region between trapezius, sternocleidomastoid and the clavicle. The sternocleidomastoid is the prominent muscle running between the chest and skull on the side of the neck.
What are the parts of the trapezius?
Within the muscular system, trapezius is described in upper, middle and lower parts according to the direction of its muscle fibres, the elongated cells that contract. These are continuous regions of one muscle, not separate muscles.
- Upper fibres run downwards and outwards from the skull and neck towards the clavicle.
- Middle fibres run approximately horizontally from the upper back towards the scapula.
- Lower fibres run upwards and outwards from the lower thoracic region towards the scapula.
These different directions explain why the parts can produce different movements. They also allow the muscle to hold the scapula steady when the arm moves.
What are the origin and insertion of the left trapezius?
An origin is conventionally the more fixed attachment of a muscle; an insertion is the attachment that usually moves towards it during contraction. These roles can reverse when the shoulder girdle is held still. The attachments of trapezius link the central axis of the skeletal system to the shoulder girdle.
Its origins are:
- The medial part of the superior nuchal line, a bony ridge near the back of the skull; medial means towards the midline.
- The external occipital protuberance, a prominence on the back of the skull.
- The nuchal ligament, a strong band of connective tissue along the back of the neck.
- The spinous processes from the seventh cervical vertebra to the twelfth thoracic vertebra, and the supraspinous ligaments connecting them. Spinous processes are the rearward bony projections of vertebrae; cervical and thoracic refer to the neck and chest regions.
The upper fibres insert onto the lateral part of the clavicle, towards the shoulder. The middle fibres insert onto the acromion, the scapular projection forming the top of the shoulder, and the spine of the scapula, the ridge across its back. The lower fibres insert near the medial end of that ridge.
What movements does the left trapezius produce?
The main actions depend on which fibres contract and which attachments remain fixed. Scapular movement is coordinated with other muscles rather than produced by trapezius alone.
| Part | Main action on the scapula | Plain-language description |
|---|---|---|
| Upper fibres | Elevation and contribution to upward rotation | Raise the shoulder blade and help turn its socket upwards |
| Middle fibres | Retraction | Draw the shoulder blade towards the spine |
| Lower fibres | Depression and contribution to upward rotation | Lower the shoulder blade and help turn its socket upwards |
Upward rotation turns the glenoid cavity—the scapular socket for the upper arm—more upwards. The upper and lower fibres cooperate with serratus anterior, a muscle running from the ribs to the scapula, during this movement as the arm is raised.
Within the articular system, or system of joints, the sternoclavicular and acromioclavicular joints accommodate shoulder girdle movement. These joints connect the clavicle to the sternum, or breastbone, and to the acromion respectively.
When the shoulder girdle is fixed, the upper fibres can help bend the neck towards the left and turn the head towards the right. Acting together, the left and right trapezius muscles can help extend the neck, moving the head backwards.
What nerve supplies the left trapezius?
Motor innervation—the nerve supply that causes muscle contraction—comes from the spinal accessory nerve, part of the nervous system. This nerve reaches the deep surface of trapezius after crossing the posterior triangle of the neck.
Branches from the third and fourth cervical spinal nerves provide sensory fibres, including those for proprioception. Proprioception is the sense of muscle position and movement.
What is the blood supply of the left trapezius?
Arteries of the cardiovascular system supply the muscle, principally through the superficial branch of the transverse cervical artery. This vessel approaches the deep surface of trapezius and distributes branches within it.
How can I find the left trapezius in the 3D model?
In the 3D model beside this article, rotate to a view of the back, then select the left trapezius and isolate it. Zoom in for a closer view, or zoom out to return to a wider view.
Fade the selected muscle to reduce its visibility, or hide it before selecting another part. Switch layers on and off, rotate the view again, or pull selected parts apart to adjust the display as you explore.