Musculus trapezius (xlas)

Preview:

simple/clean drawings; arm movements included; lots of additional info (+latin names)

Synonyms: Trapezius muscle; Trapezius; Neck muscle

Summary

The Musculus trapezius is the biggest neck muscle (surface area) we have and very relevant in several head and arm movements. Its name giving shape, the trapezoid, significantly shapes the upper back, especially around the neck and the shoulder blades (scapulae). There are 3 parts to the trapezius (top part, middle part, bottom part) and each one pulls in a slightly different direction.

Visual Introduction

left: general view (white parts = tendons); right: the 3 parts of the muscle + pull direction

two ways of interpreting the name: 1. muscle shape, viewed from the back; 2. front view: trapezius muscle and clavicles form a trapezoid

Origin

Occipital bone (Os occipitale; lower back of the skull)
Cervical vertebrae (spine, neck part)
Down to the 12th thoracic vertebra (spine, chest part)

Originating from a vertebra means the tendons connect to the spinous process (little bumps you can feel on your spine) of each vertebra.

Insertion

Spine of the shoulder blade (Spina scapula)
Acromion (Pars transversa) = highest part of the shoulder blade
Clavicle (clavicula)

origin and insertion; back and front view

Functions

Top part:
- Lifting the shoulder
- Twisting the shoulder blade (scapula)
- Tilting and turning the head, also leaning the head backwards
Middle part:
- Pulling the shoulder back
- Twisting the shoulder blade (scapula)
Bottom part:
- Pulling the shoulder/shoulder blade (scapula) down

Different Viewpoints

muscle is marked in red; notice how the muscle varies with arm movement

a more realistic view; only a few parts of the muscle are visible on average

Details

Everything beyond the basics

There is one vertebra, that sticks out at the center of the trapezius. This is the seventh cervical vertebrae, the last neck vertebra before the thoracic vertebrae start.

Drawing the trapezius for a woman you should always consider that the subcutaneous fat layer is thicker. This means that bumps and depressions caused by muscles, bones, etc. are less visible on women. Concentrate on the main depression, the corner where the trapezius meets the shoulder blade, and don't go for too many details to make it look convincing. Of course there are exceptions. The same goes for the average man (especially if the body fat is high), focus on the shoulder blade depression.

On many people the significant areas of the trapezius aren't visible at all.

The trapezius is also used when the shoulder blade needs to be pressed against the body, for example in a handstand. Chest muscles (+ serratus anterior) and back muscles, including the trapezius (mostly the middle part) contract at the same time to achieve that.

The rhomboids often contract simultaneously with the trapezius. They pull in a similar direction with the top and middle part of the trapezius and have similar origin and insertion. The bottom part of the trapezius on the other hand pulls down, so its kinda opposite to the rhomboids. The rhomboids are located below the trapezius, so you cannot see them on a person usually. However with very particular shoulder movement both muscles can be visible at the same time - at the very bottom of the trapezius.

The rhomboids are partly causing the middle part of the trapezius to look relatively big.

On first sight one could think that the trapezius has a large diameter, but thats not true. The trapezius is large in surface area but its cross-section shows that its relatively thin. In the following drawing I compare the cross section of the trapezius and the Musculus erector spinae (lower back muscle for straightening your upper body) to give you a comparison.