The trapezius muscle is a large, flat, triangular-shaped muscle located in the upper back and neck region. It is one of the superficial muscles of the back and is primarily responsible for connecting the shoulder girdle to the axial skeleton.[1] The trapezius is divided into three functional parts: upper, middle, and lower fibers, based on their orientation and attachment points.
Anatomy
The trapezius muscle is a large, flat, and triangular muscle covering the posterior neck, shoulders, and upper back.[4] It plays a vital role in connecting the scapula and clavicle to the axial skeleton and has a unique structure that allows for diverse functions.
Location and Orientation
- The trapezius muscle spans the posterior aspect of the neck and upper back, forming a diamond-shaped structure when combined with the opposite trapezius muscle.
- It is positioned superficially, covering the deeper muscles of the neck and back, such as the rhomboids, levator scapulae, and the erector spinae group.[6]
Attachments
Origin
The trapezius originates from the following structures:
- External occipital protuberance (a bony prominence on the occipital bone at the base of the skull).
- Medial third of the superior nuchal line of the occipital bone.
- Nuchal ligament, which spans the cervical spine.
- Spinous processes of C7 to T12 vertebrae.
- Supraspinous ligament, which connects the spinous processes of these vertebrae.[8]
Insertion
The trapezius muscle inserts onto the following structures:
- The lateral third of the clavicle (upper fibers).
- The acromion process of the scapula (middle fibers).
- The spine of the scapula (lower fibers).
Divisions and Fiber Orientation
The trapezius muscle is divided into three distinct parts based on the direction of the muscle fibers and their respective functions:
- Upper Fibers:
- Fibers run downward and laterally from the occipital bone and cervical vertebrae to the clavicle and acromion.
- These fibers are thicker and more robust compared to the other parts.
- Middle Fibers:
- Fibers run horizontally from the upper thoracic vertebrae to the acromion and spine of the scapula.
- Lower Fibers:
- Fibers run upward and laterally from the lower thoracic vertebrae to the medial end of the scapular spine.
Relations
- Superficial: The trapezius lies just beneath the skin and superficial fascia, making it one of the most visible muscles of the upper back.
- Deep: Covers several deeper muscles, including the rhomboid major, rhomboid minor, levator scapulae, splenius, and semispinalis muscles.
- Medial: Adjacent to the spinous processes of the cervical and thoracic vertebrae.
- Lateral: Inserts onto the clavicle, acromion, and scapular spine, connecting the axial skeleton to the shoulder girdle.
Innervation
The trapezius muscle is innervated by:
- Spinal accessory nerve (cranial nerve XI): Provides motor innervation.[7]
- Cervical plexus (C3 and C4): Contributes sensory innervation (proprioception).
Vascular Supply
The blood supply to the trapezius muscle includes:
- Transverse cervical artery (a branch of the thyrocervical trunk).
- Dorsal scapular artery (in some cases).
- Additional contributions from the occipital artery and posterior intercostal arteries.
Fascial Connections
The trapezius is enclosed in the superficial layer of the thoracolumbar fascia, which provides support and separates it from deeper structures.
Function
The trapezius muscle is a multifunctional muscle that plays a critical role in the movement, stabilization, and positioning of the scapula and neck.[5] Its broad attachments and distinct fiber orientation allow it to perform various actions, often in coordination with other muscles of the shoulder girdle and neck. Below is a detailed explanation of its functions:
Primary Functions
Scapular Movements
The trapezius muscle is vital for controlling the scapula’s position and movement, facilitating arm and shoulder mechanics.
- Elevation of the Scapula (Upper Fibers): The upper fibers contract to lift the scapula, as seen when shrugging the shoulders or raising the arm overhead.
- Depression of the Scapula (Lower Fibers): The lower fibers pull the scapula downward, assisting in lowering the shoulders from an elevated position.
- Retraction of the Scapula (Middle Fibers): The middle fibers draw the scapula medially toward the spine, bringing the shoulder blades closer together (e.g., during rowing or pulling motions).
- Upward Rotation of the Scapula (Upper and Lower Fibers): The upper and lower fibers work together to rotate the scapula upward, a movement critical for fully raising the arm above the head. This movement positions the glenoid cavity of the scapula to face upward, allowing for greater shoulder mobility.
Neck Movements
- Extension of the Neck (Upper Fibers): When the upper fibers contract bilaterally, they extend the neck, enabling the head to tilt backward.
- Lateral Flexion of the Neck (Upper Fibers): When one side of the upper fibers contracts, it causes ipsilateral lateral flexion, bending the neck toward the same side.
- Contralateral Rotation of the Neck (Upper Fibers): Unilateral contraction also aids in rotating the head to the opposite side.
Scapular Stabilization
- The trapezius muscle stabilizes the scapula during dynamic upper limb movements, providing a stable base for the rotator cuff and deltoid muscles to act upon.[3]
- This stabilization is essential during pushing, pulling, and overhead activities.
Secondary Functions
Postural Support
The trapezius helps maintain an upright posture by counteracting the forward pull of gravity on the shoulders and neck, especially during prolonged sitting or standing.
Support of Arm Movements
By controlling scapular movements, the trapezius facilitates efficient shoulder mechanics, allowing for a full range of arm movements, including flexion, abduction, and rotation.
Accessory Role in Breathing
During labored breathing, the upper fibers can assist in elevating the ribcage when the scapula is fixed, functioning as an accessory respiratory muscle.
Synergistic Actions
The trapezius works synergistically with other muscles for coordinated shoulder and neck movements:
- Levator Scapulae and Rhomboids: Assist in scapular elevation and retraction.
- Serratus Anterior: Works with the trapezius for upward rotation of the scapula during arm elevation.
- Deltoid and Rotator Cuff Muscles: Depend on scapular stability provided by the trapezius for efficient arm movements.
- Sternocleidomastoid: Complements the trapezius in neck rotation and lateral flexion.
Specific Roles in Activities
- Overhead Movements: The upper and lower fibers are critical for upward rotation of the scapula, enabling activities such as throwing, lifting, or reaching.
- Carrying Loads: The upper fibers help sustain elevated scapular positioning during carrying or holding heavy loads.
- Pulling Motions: The middle fibers retract the scapula during rowing or pulling exercises.
Clinical Significance
The trapezius muscle is clinically significant due to its role in scapular and neck movements, posture maintenance, and susceptibility to strain and dysfunction:
Muscle Strain and Pain
- The trapezius is commonly affected by muscle strain, leading to pain in the neck, shoulders, and upper back. This is often caused by overuse, poor posture, or carrying heavy loads.[2]
- Upper trapezius myofascial pain syndrome is a frequent source of tension headaches and referred pain.
Postural Disorders
Weakness or dysfunction of the trapezius contributes to forward shoulder posture and kyphosis, causing discomfort and impairing shoulder mechanics.
Nerve Injury
Damage to the spinal accessory nerve (cranial nerve XI), often due to trauma or surgical procedures, can result in trapezius paralysis, leading to scapular winging and difficulty in shoulder movements.
Rehabilitation Target
The trapezius muscle is frequently targeted in physical therapy for improving posture, shoulder mobility, and neck alignment, particularly in cases of cervical spine disorders or scapular dyskinesis.
