Trapezius
A huge diamond across the upper back whose three parts lift, pull back and pull down the shoulder blade — often at the same time.
- Type
- Muscle
- Region
- Shoulder
- Standard name
- m. trapezius
- Origin
- External occipital protuberance, nuchal ligament, and the spinous processes of the seventh cervical and all thoracic vertebrae
- Insertion
- Lateral third of the clavicle, the acromion, and the spine of the scapula
- Actions
- scapular elevationscapular retractionscapular depressionscapular upward rotation
- Verify
- TA2 2226 TA98 A04.3.01.001
What it is
A large flat muscle covering the upper back and the back of the neck. The pair of them together form a diamond — a trapezium — from the base of the skull to the middle of the back, and out to each shoulder.
Three parts, with genuinely different directions of pull:
- Upper (descending) — from skull and neck, running downward and outward to the clavicle
- Middle (transverse) — running horizontally to the acromion and scapular spine
- Lower (ascending) — from the lower thoracic spine, running upward and outward
It is the most superficial muscle of the upper back, and unusually it is supplied by a cranial nerve — the accessory nerve — rather than by ordinary spinal nerves.
What it does
Moves the scapula, in a different direction per part. Upper elevates it, middle retracts it (pulls it towards the spine), lower depresses it.
The important behaviour is the parts working together. When you raise your arm overhead, the upper and lower parts pull in opposite vertical directions on different corners of the shoulder blade, and the result is not a tug of war but a rotation — the blade turns so the socket faces upward. Serratus anterior does the same job from the front. This is a force couple, and it is one of the neater pieces of mechanics in the body.
The upper part also extends and side-bends the neck.
What loads it
Shrugging, rowing, carrying loads at the side, and any overhead pressing where the shoulder blade must rotate.
What is commonly got wrong about it
"Overactive upper traps". An extremely popular diagnosis: the upper fibres are said to be tight and dominant, the lower fibres weak, and the imbalance the cause of neck and shoulder trouble. The pattern is described far more confidently than the evidence supports, and the idea of a specific ratio to be corrected is not well established.
That shrugging is bad for you. Elevating the shoulder blade is what the upper fibres are for.
"Tension in the traps" as a muscle problem. The sensation of tightness across the top of the shoulders is extremely common and its relationship to actual muscle state is loose. Sensation is produced by the nervous system, not read off a strain gauge.
That it is one muscle doing one thing. The upper and lower parts are direct opponents in the vertical plane and cooperate in rotation.