The trapezius muscle is a large, triangular, paired muscle located on the posterior aspect of the neck and thorax. When viewed together, this pair forms a diamond or trapezoid shape, hence its name. The trapezius has many attachment points, extending from the skull and vertebral column to the shoulder girdle.
The trapezius belongs to the superficial layer of the extrinsic muscles of the back, along with latissimus dorsi, rhomboid major and minor, and levator scapulae muscles. The trapezius is largely involved in movements of the shoulder girdle, and is therefore functionally considered as a muscle of the upper limb rather than of the back.
In this article, we will discuss the detailed anatomy and function of the trapezius muscle.
Descending part (superior fibers): medial third of the superior nuchal line, external occipital protuberance, nuchal ligament
Transverse part (middle fibers): spinous processes and supraspinous ligaments of vertebrae T1-T4 (or C7-T3)
Ascending part (inferior fibers): spinous processes and supraspinous ligaments of vertebrae T4-T12
Descending part (superior fibers): lateral third of clavicle
Transverse part (middle fibers): medial acromial margin, superior crest of spine of scapula
Ascending part (inferior fibers): lateral apex of the medial end of scapular spine
Motor: accessory nerve (CN XI)
Motor/Sensory: anterior rami of spinal nerves C3-C4 (via cervical plexus)
Descending part (superior fibers)
- Scapulothoracic joint: draws scapula superomedially
- Atlantooccipital joint: extension of head and neck, lateral flexion of head and neck (ipsilateral)
- Atlantoaxial joint: rotation of head (contralateral)
Transverse part (middle fibers)
- Scapulothoracic joint: draws scapula medially
Ascending part (inferior fibers)
- Scapulothoracic joint: Draws scapula inferomedially
|Occipital artery (descending part), superficial or transverse cervical artery (transverse part), dorsal scapular artery (ascending part)
- Origin and insertion
- Structure and relations
- Blood supply
- Clinical notes
Origin and insertion
The trapezius muscle has several origin points along the midline of the posterior neck and back.
- The superior fibers attach on the medial third of the superior nuchal line, the external occipital protuberance of the occipital bone and the nuchal ligament, that is attached to the spinous processes of the C1-C6 vertebrae. These fibers have a descending course towards their insertion point, hence why this part of the trapezius is referred to as the descending part.
- The middle fibers originate from the spinous processes of T1-T4 (some sources state C7-T3) vertebrae and their intervening supraspinous ligaments. These fibers are directed horizontally, running laterally towards the shoulder. Thus, these fibers represent the transverse part of the trapezius.
- The inferior fibers originate from the spinous processes of the T4-T12 vertebrae and their respective supraspinous ligaments. These fibers course superiorly and laterally towards their insertion point and hence, represent the ascending part of the trapezius.
Along their course, all fibers of the trapezius converge laterally on the superior angle of the scapula to attach on their respective insertion points.
- The superior fibers insert on the posterior border of the lateral third of the clavicle.
- The middle fibers insert onto the medial margin of the acromion of the scapula, as well as the superior crest of the scapular spine.
- The inferior fibers insert via an aponeurosis on a tubercule at the lateral apex of the medial end of the scapular spine.
Structure and relations
According to their attachments, course and location discussed earlier, we can divide the trapezius into three functional parts. Each part is implicated in different movements, as we’ll explore in more detail later in this article. The parts of the trapezius include:
- The descending (upper) part, composed of superior fibres
- The transverse (middle) part, composed of middle fibres
- The ascending (lower) part, composed of inferior fibres
Along with latissimus dorsi, the trapezius is the most superficial of the superficial extrinsic back muscles. It overlies the remaining muscles of this group, namely the rhomboids and levator scapulae. Deep to the trapezius muscle we can also find the serratus posterior superior, which belongs to the intermediate layer of extrinsic muscles of the back.
In addition, the trapezius covers several muscles of the superficial layer of the intrinsic muscle group, such as the splenius capitis, splenius cervicis, spinalis, longissimus and iliocostalis muscles. The upper part of the trapezius also overlies the suboccipital region.
The anterior margin of the trapezius muscle forms the posterior border of the posterior triangle of the neck. Moreover, its free inferior margin forms the medial boundary of the triangle of auscultation, an area of the chest wall not obscured by the scapula and only covered by a thin layer of muscle.
The trapezius muscle is the only muscle of the upper limb that does not receive its innervation from the brachial plexus. Instead, motor innervation to the trapezius is conveyed by the accessory nerve (CN XI) as well as the anterior rami of the C3 and C4 spinal nerves, which also contain proprioceptive/sensory fibers from the muscle.
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The arterial supply to trapezius varies depending on the level.
- The descending (upper) part of the muscle is supplied by transverse muscular branches arising from the occipital artery (branch of the external carotid), which passes along the deep surface of the muscle.
- The transverse (middle) part of the muscle is supplied by the superficial cervical artery, or by a branch from the transverse cervical artery.
- The ascending (lower) part is supplied by muscular branches of the dorsal scapular artery, which arises from the subclavian artery.
The main function of the trapezius is stabilizing the scapula in its anatomical place, as well as controlling it during movements of the shoulder and upper limb. The throwing action is a common manoeuvre in which the trapezius muscle is active, along with the deltoid muscle and the rotator cuff muscles. In addition, the trapezius is also involved in the movements of the head and neck.
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The actions of the trapezius muscle is largely dependent on the direction of the fibres that are contracting.
Descending (upper) fibers
The descending (upper) fibers act with the levator scapulae muscle to produce an elevation of the scapula at the scapulothoracic joint. In the same manner, they also maintain the level of the shoulders against gravity e.g. when a weight is being carried in the hand.
When the muscle is acting unilaterally, the descending fibres produce an ipsilateral lateral flexion of the head and neck by acting on the atlanto-occipital joint and upper cervical vertebrae, respectively. Unilateral contraction may also result in contralateral rotation of the head at the atlantoaxial joint. Bilateral contraction of the descending part of trapezius (i.e. when both left and right muscles contract) causes an extension of the head and neck.
Ascending (lower) fibers
The ascending (lower) fibers are responsible for depression of the medial part of the scapula, and thus lowering the shoulder. This action is especially important for activities in which the shoulders are lowered against resistance, for example when using the hands to help oneself up from a seated position.
Together with the descending part, the ascending fibers also produce a rotation of the scapula around an axis which runs anteroposteriorly through the base of the scapular spine.
Transverse (middle) fibers
The transverse (middle) fibers act together with the rhomboids to produce a retraction of the scapula, by pulling it towards the midline. The trapezius muscle is also responsible for upward rotation of the scapula, along with the serratus anterior muscle. This allows us to raise our arm above our heads beyond the level of the shoulder.
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The function of the trapezius muscle can be tested by placing a hand on the patient’s shoulder and assessing their ability to elevate or ‘shrug’ the shoulder against resistance. This test, coupled with the functionality test for sternocleidomastoid muscle, can be used to assess damage of the accessory nerve.
Weakness of the trapezius muscle with intact functioning of the sternocleidomastoid muscle would indicate damage to the accessory nerve at a more distal point, such as in the posterior triangle of the neck. Weakness of both the trapezius and sternocleidomastoid muscles would allude to damage closer to where the accessory nerve exits the base of the skull.
As the trapezius muscle has an extensive vascular supply, it can be used as a site for musculocutaneous tissue flap harvesting for reconstructive purposes in other areas of the body, such as for breast reconstruction.
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