- Important muscle due to its innervation–spinal accessory nerve and cervical motor roots–and preserved function in most brachial plexus and other peripheral nerve injury patterns.
- With a complete spinal accessory nerve injury, the patient will still demonstrate some preserved upper trapezius function (innervated by cervical motor roots) and will be able to shrug their shoulder (levator scapulae muscle contributes to this as well). However, depending on the amount of glenohumeral joint and other muscle compensation may not be able to abduct the shoulder past 90 degrees.
- With trapezius muscle weakness, the patient can fully abduct the shoulder overhead but with eccentric contraction–as the patient lowers their arm, scapular winging will be noted.
- Note that scapular winging occurs both in long thoracic nerve (to serratus anterior muscle) palsy as well as with spinal accessory nerve (to trapezius muscle) palsy.
- Upper, middle and lower trapezius can be examined separately.
- Bottom line, the trapezius helps move/stabilize the scapula to allow normal shoulder motion (powered by the other muscles) with absent/abnormal trapezius function, the patient will note abnormal shoulder function (which on examination can be picked up on as abnormal scapulothoracic motion):
- Shoulder abduction requires upward rotation of the scapula by the trapezius with the scapula adducted to the midline (to keep it out of the way). With loss of trapezius function, shoulder abduction is limited.
- Forward flexion of the shoulder, on the other hand, requires upward rotation of the scapula by the serratus with the scapula abducted from the midline. Therefore, with abnormal trapezius function, the patient can forward flex the shoulder more.
- Origin Fixed:
- Adduction of the scapula, mainly by the middle fibers with stabilization by the upper and lower fibers.
- Rotates the glenoid cavity cranially, mainly by the upper and lower fibers with stabilization by the middle fibers.
- Elevation of the scapula by the upper fibers.
- Depression of the scapula by the lower fibers.
- Insertion Fixed and Acting Unilaterally:
- Extends, laterally flexes, and rotates the head and joints of the cervical vertebrae towards the opposite side by the upper fibers.
- Insertion Fixed and Acting Bilaterally:
- Extends the neck by the upper fibers.
- Assists in respiration as an accessory muscle.
Palpation: The muscle bulk above the lateral surface of the clavicle following the muscle line superior to the occiput.
Gravity Lessen Test: Position – the subject in supine or prone and the head rotated to the opposite side as the one being tested. Stabilize – the head. Resist – elevation of the shoulder by applying force on the shoulder (in the direction of shoulder depression).
Antigravity Test: Position – the subject in sitting with arms relaxed on the lap of the subject. Stabilize – the trunk and head as needed. Resist – elevation of the shoulder with posterolateral cervical extension and head turned so the face is turned contralateral to the side being tested.
Possible Substitutions: Serratus anterior and levator scapulae. Making sure that the face is not turned towards the tested side will help limit this substitution. Additional possible substitutions are through the pectoralis minor and rhomboids.
Palpation: Medial to the root of the spine of the scapula, at approximately the level of T3 vertebrae.
Strength Testing: Position – the subject in prone with forehead on a small towel roll and head in neutral position. Shoulder in 90º of abduction with lateral rotation. The upper arm supported on the table and in the same plane as the scapula. The subject may require a pillow under the chest if their anterior muscles are short. Stabilize – if the deltoid is weak and support the arm with the scapula pulled laterally to abduct it. This will ensure pure scapular motion. Resist – on the forearm and against adduction of the scapula while the subject is pulling the shoulder blades together and lifting the arm with the humerus laterally rotated.
Possible Substitutions: Rhomboids – downward rotation of the scapula with adduction is noted with rhomboid substitution. Keeping the humerus in lateral rotation or keeping the thumb pointed towards the ceiling with the shoulder in 90º will resolve this. In addition, the levator scapulae, upper trapezius, and lower trapezius may substitute.
Palpation: Lateral to the spinous processes of the lower thoracic vertebrae, with the fibers inserting in the diagonal upward direction towards the spine of the scapula.
Strength Testing: Position – the subject in prone with the forehead on a small towel roll with the head in neutral. The shoulder is abducted to approximately 135º and the arm laterally rotated with thumbs pointed to the ceiling. The intent is to align the arm to the direction of the lower fibers of the trapezius. The arm is supported on the table and on the same plane as the scapula. The subject may require a pillow under the chest if their anterior muscles are short. Stabilize – if the deltoid is weak and support the arm with the scapula pulled laterally to abduct it. Resist – on the forearm and against the arm elevating off the table while the subject is drawing the scapula down and in.
Possible Substitutions: Rhomboids – downward rotation of the scapula is noted with this substitution. Middle trapezius – make sure there is scapular adduction with depression and not pure adduction to avoid this substitution. Upper trapezius – scapular elevation is witnessed rather than scapular depression during this substation. Latissimus dorsi – assists in scapular depression only.
- Roots: XI, C3, C4.
- Nerve: Spinal accessory nerve.
- Innervation Route:
- Brainstem, C2, C3, C4 → spinal accessory nerve → trapezius
- C3, C4 → upper trapezius branches. Note that these branches from the cervical roots go directly to the upper trapezius muscle. Therefore, with a complete transection of the accessory nerve in the posterior triangle of the neck, the patient will still have intact upper trapezius function and shoulder shrug.
Origins of Upper Trapezius: External occipital protuberance, medial ⅓ of superior nuchal line, ligamentum nuchae, and spinous process of C7 vertebra.
Origins of Middle Trapezius: Spinous processes of T1 through T5 thoracic vertebrae.
Origins of Lower Trapezius: Spinous processes of T6 through T12 thoracic vertebrae.
Insertions of Upper Trapezius: Lateral ⅓ of clavicle and acromion process of scapula.
Insertions of Middle Trapezius: Medial margin of acromion and superior lip of spine of scapula.
Insertion of Lower Trapezius: Tubercle at apex of spine of scapula.