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Introduction

There are three major groups of back muscles:

  • Superficial: attached to the shoulder girdle
  • Intermediate: attached to the posterior thorax
  • Deep: attached to the vertebral column

The former two groups, superficial and intermediate, are referred to as the extrinsic back muscles. The latter group is the intrinsic muscle group. This article will focus on the superficial group.

The superficial back muscles are covered by skin, subcutaneous connective tissue and a layer of fat. The muscles of this group include:

  • Trapezius
  • Latissimus dorsi
  • Levator scapulae
  • Rhomboid major and rhomboid minor

Trapezius

The trapezius is a primary mover of the scapula that assists in superior and inferior rotation, retraction, and elevation and depression of the scapula and shoulder complex.

It is a triangular muscle unilaterally and appears as a trapezoid-shaped muscle when the left and right trapezius are viewed on the back.

It is divided into three distinct segments, named the superior (s), middle (m) and inferior (i) trapezius, that have slightly varied attachment points.

Origin and insertion

Originating from the superior nuchal line (s), nuchal ligament (s; m) and the spinous processes of C7 to T12 (m; i), the trapezius muscle converges onto the lateral 1/3 of the clavicle (s), acromion (m) and spine of the scapula (i).

Fibre orientation

The trapezius fibres originate broadly along the vertical midline of the vertebral column before converging on their insertion points around the acromion, spine of the scapula and lateral 1/3 of the clavicle.

Function

We will consider the different heads of the trapezius:

  • Superior head – elevation, rotation during arm abduction
  • Middle head – retraction
  • Inferior head – depression, rotation during arm adduction
  • All three heads together – retraction and fixation of the scapula

Innervation

The motorinnervation is derived from the spinal accessory nerve (cranial nerve XI). The sensory innervation is derived from the dorsal ramus of spinal nerves C3-C4.

Arterial supply

The trapezius is supplied by either of the superficial cervical arteries, or the superficial branch of the transverse cervical artery.

Clinical relevance

Lower brainstem and upper cervical cord lesions can interfere with the function of cranial nerve XI, leading to paresis or paralysis of the trapezius (and sternocleidomastoid) muscles. Patients may not be able to shrug their shoulders, or will have significant weakness against inferior shoulder resistance.1,2

The trapezius, with sensory innervation from C3 and C4 may also be the site of origin of tension headaches. Chronic tension or contraction of these muscles can cause localised elevations of prostaglandins and lactate, irritating nociceptive receptors. This irritation can cause a referred pain inn the distribution of C2-C4, manifesting as a headache or neck pain.3

Trapezius
Trapezius 1

Latissimus dorsi

The latissimus dorsi is a broad, flat muscle that covers the entire lower back.

Origin and insertion

The latissimus dorsi originates from the iliac crest, thoracodorsal fascia and T7-L5 spinous processes, converging to insert via a single, flat tendon in the floor of the intertubercular (bicipital) groove of the humerus.

Fibre orientation

The fibres of latissimus dorsi begin broadly and converge onto a single common tendon.

Function

A powerful extensor and adductor of the shoulder, latissimus dorsi also assists in internal rotation of the humerus.

Innervation

The motor and sensory innervation is supplied by the thoracodorsal nerve, which derives fibres from spinal roots C6-C8.

Blood supply

Latissimus dorsi is supplied by the thoracodorsal branch of the subscapular artery.

Clinical relevance

Latissimus dorsi is a large muscle that, when atrophied, can cause significance asymmetry in the back. While it is a strong muscle for adduction, internal rotation and extension of the humerus, we can do without it. The tendon for latissimus dorsi can be harvested for tendon repairs or tendon transfers. An asymmetric or non-functional latissimus dorsi is not always due to a nerve lesion, and can result from prior transfer surgery.2

Latissimus dorsi
Latissimus dorsi 2

Levator scapulae

Levator scapulae is a relatively short and thin strap muscle of the superficial back compartment.

Origin and insertion

Leavator scapulae originates from the transverse processes of T1-T4, and inserts onto the medial border of the scapula at the superior angle.

Fibre orientation

The fibres of levator scapulae have a straight orientation, consistent with a strap.

Function

Levator scapulae is a synergist (assistor) in elevation of the scapula.

Innervation

The motor and sensory innervation of levator scapulae is derived from the dorsal scapula nerve – the first branch of the brachial plexus arising from the C5 nerve root.

Blood supply

Levator scapulae is supplied by the similarly named dorsal scapular artery.

Levator scapulae
Levator scapulae 3

Rhomboid major

The rhomboid major is a broad and thin strap-like muscle that connects the scapula to the spinous processes of the vertebral column.

Origin and insertion

Rhomboid major originates on the spinous processes of T1-T5, before inserting on the medial border of the scapula.

Fibre orientation

The fibres of rhomboid major resemble a strap muscle.

Function

Rhomboid major is a scapula retractor and inferior rotator (when adducting the humerus).

Innervation

The motor and sensory innervation of the rhomboid major muscle is derived from the dorsal scapular nerve of the C5 nerve root.

Blood supply

Rhomboid major is supplied by the dorsal scapular artery.

Rhomboid major
Rhomboid major 3

Rhomboid minor

The rhomboid minor is a narrow and thin strap-like muscle that connects the scapula to the spinous processes of the vertebral column. It is smaller and more superiorly located than the adjacent rhomboid major.

Origin and insertion

Rhomboid minor originates on the spinous processes of C7-T1, before inserting on the medial border of the scapula (superior to rhomboid major).

Fibre orientation

The fibres of rhomboid minor resemble a narrow strap muscle.

Function

Rhomboid minor is a scapula retractor and inferior rotator (when adducting the humerus).

Innervation

The motor and sensory innervation of the rhomboid minor muscle is derived from the dorsal scapular nerve of the C5 nerve root.

Blood supply

Rhomboid minor is supplied by a deep branch of the transverse cervical artery.

The next article will focus on the intermediate muscles of the back.

Clinical relevance

Weakness in the rhomboid major and minor muscles can contribute to scapula winging. This refers to weakness in the muscles that assist the scapula in adhering closely to the ribs and thoracic cage. Serratus anterior is the primary muscle contributing to this, and scapula winging is classically described as a lesion to the long thoracic nerve of the brachial plexus. Next time you see a patient with a winged scapula, be sure to consider their rhomboids.

Rhomboid minor
Rhomboid minor 5

References

Reference Images

  1. Trapezius: By Mikael Häggström, used with permission. [Public domain]
  2. Latissimus dorsi: By Mikael Häggström, used with permission. [Public domain]
  3. Levator scapulae: modified by Uwe Gille [Public domain]
  4. Rhomboid major: modified by Uwe Gille [Public domain]
  5. Rhomboid minor: modified by Uwe Gille [Public domain]

Reference Texts

  1. Sinnatamby, C. S. (2011). Last’s Anatomy, International Edition: Regional and Applied. Elsevier Health Sciences.
  2. Moore, K. L., Dalley, A. F., & Agur, A. M. (2013). Clinically oriented anatomy. Lippincott Williams & Wilkins.
  3. Nardone, R., & Tezzon, F. (2003). The trigemino‐cervical reflex in tension‐type headache. European journal of neurology10(3), 307-312.

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