The back is made up of a complex series of bones, discs, nerves, joints and muscles. The back muscles support the spine, link the pelvis and shoulders to the trunk, and provide mobility and stability to the trunk and spine.
The anatomy of the back muscles can be complex. There are several different layers of muscles that tend to pull in different and varied directions. Understanding the anatomy and function of the back muscles can help us determine if we need professional medical attention when a back problem develops.
extrinsic muscles
The back consists of three distinct layers of muscles. These layers help mobilize and stabilize the trunk during daily activities. They also join the shoulders and pelvis to the trunk, creating a bridge between the upper and lower body.
Surface layer
The superficial layer of the back muscles lies just below the skin and fascia. Many of these muscles are easily identified and located simply by looking and feeling the body. The superficial muscles of the back include:
Trapeze
The trapezius is divided into three distinct parts: the upper, middle, and lower trapezius. This muscle originates from the cervical spine and skull and from spinous processes from cervical level seven to the thoracic. The flat, triangular-shaped muscle runs down the back of the shoulder blade to attach to the clavicle, acromion, and spine of the scapula.
Its main functions are to stabilize, elevate, lower, retract, and rotate the scapula at the scapulothoracic joint. It also contributes to ipsilateral lateral flexion and contralateral rotation of the head during unilateral contraction, as well as extension of the head during bilateral contraction.
rhomboid major and minor
The rhomboid muscles are the two small quadrangular muscles located between the medial margins of the scapulae. They originate from the spinous process of the cervical seven to the five thoracic. They then attach to the medial border of the scapula. The innervation of the rhomboids is through the dorsal scapular nerve.
The function of the rhomboid muscles is to stabilize and retract the scapula at the scapulothoracic joint.
dorsal
Lactus Dorsi is the widest muscle in the body and occupies almost the entire surface of the back. It is an important muscle of the back and a motor of the shoulder joint. They originate from the thoracolumbar fascia, the spinous processes of the sixth to twelfth thoracic, the iliac crest, and the lower three ribs. The muscle goes up to the shoulder and attaches to the upper arm bone. The innervation of the latissimus dorsi is through the thoracodorsal nerve.
The functions of the latissimus dorsi muscle are to extend the flexed arm, adduct and internally rotate the arm, and pull the pectoral girdle posteriorly as a whole when the humerus is fixed against the scapula.
levator scapulae
The levator scapulae is an elongated muscle that extends from the upper cervical vertebrae to the scapula. It originates from the transverse processes of the C1-C4 vertebrae and inserts on the medial margin of the scapula.
The main function of this muscle is to elevate the scapula, as its name suggests. It also contributes to ipsilateral lateral flexion of the neck when contracted unilaterally, as well as extension of the neck when contracted bilaterally.
intermediate layer
The middle layer of back muscles includes the serratus posterosuperior and inferior . These muscles run from the spine to the ribs and help the diaphragm and intercostal muscles to move the ribs during breathing.
The serratus posterosuperior originates from cervical seven through thoracic three and courses to ribs two through five. The serratus posterior inferior originates from thoracic 11 to lumbar level 3 and inserts on ribs 9 through 12. Both serratus posterior muscles are innervated by intercostal nerves.
The function of the serratus posterior muscles is to assist breathing by elevating the ribs (serratus posterior superior) during inspiration and depressing them (serratus posterior inferior) during exhalation.
intrinsic muscles
The true intrinsic muscles of the back are the deepest layer of muscles attached to the spine. The muscles of the thoracic area lie deep within the thoracolumbar fascia, while the muscles of the lumbar area lie between the superficial and middle layers of the fascia.
Almost all of them receive their innervation from the posterior (dorsal) rami of the spinal nerves and are called the intrinsic group because they act specifically on the spinal column.
deep layer
The deep layer of the back muscles is also known as the intrinsic layer and resides closest to the spine.
interspinal muscles
The interspinalis are the short muscles that extend between the adjacent spinous processes of the vertebrae.
They are divided into cervical, thoracic and lumbar interspinal. However, only the cervical and lumbar parts are well developed, while the thoracic part may often be absent or rudimentary. The function of the interspinal muscles is to contribute to the extension of the cervical and lumbar spine.
intertransverse muscles
The intertransversarii interval between the adjacent transverse processes of the vertebrae. They develop best in the cervical and lumbar spine, while in the thoracic region they are usually absent. The colli intertransversarii are divided into anterior and posterior groups, their joint function being to contribute to lateral flexion and stabilization of the cervical spine.
The intertransversarii lumborum consist of medial and lateral glides and their function is to assist in lateral flexion of the spine.
lumbar square
This is the deepest muscle in the back. It originates from the iliac crest and inserts on the transverse process of lumbar one through five and the bottom of its twelfth rib. The innervation of the quadratus lumborum is via the twelfth thoracic nerve and the spinal branches of the lumbar nerves.
Each muscle in the deep, middle, and superficial layers comes in pairs; there is one on the left side and one on the right side of the body.
transversospinalis muscles
The transversospinalis muscles bring together three groups of back muscles:
- Semispinalis , which is topographically divided into semispinalis capitis, semispinalis cervicis, and semispinalis thoracis. They extend between the transverse and spinous processes of the regional vertebrae.
- Multifidus , which consists of the multifidus cervicis, multifidus thoracis, and multifidus lumborum. They originate from the articular, transverse, and mammillary processes of the regional vertebra and insert on the spinous processes of the vertebrae 2-5 levels above.
- Rotators are divided into short and long rotators. The short rotators connect the transverse processes of the thoracic vertebrae with the spinous processes of the vertebra one level above, while the long rotators span two levels above the point of origin.
Surface layer
The back muscles also have superficial intrinsics.
splenius muscles
The splenius muscle group is made up of two muscles:
- Splenius capitis muscle, which originates from the spinous processes of the C7-T3 vertebrae and the ligamentum nuchae, and inserts on the superior lateral nuchal line of the occipital bone and the mastoid process of the temporal bone.
- Splenius muscle of the cervix, which originates from the spinous processes of vertebrae T3-T6 and inserts on the transverse processes of vertebrae C1-C3.
The function of the splenius muscles is to rotate, laterally flex, and extend the neck.
spinal erector
The spinal erectors are long-strap muscles that are made up of the iliocostal, spinal, and longissimus muscles. These muscles originate from the common tendinous origin of the iliac crest, sacrum, lumbar vertebrae, and sacroiliac and supraspinatus ligaments.
- Spinal , which is regionally divided into spinal cord, cervical spinal, and thoracic spinal. They join between the spinous processes of the vertebrae of their corresponding regions.
- Longissimus is also divided into longissimus muscle, longissimus cervicis (neck), and longissimus thorax. They join between the transverse processes of the vertebrae of their corresponding regions.
- Iliocostal divides into the iliocostal muscle of the cervix (the neck), the iliocostal nerve of the thorax, and the iliocostal lumbar. They extend between the angles of the ribs and the transverse processes of their corresponding regional vertebrae.
They travel along the spinal column and insert on the ribs, the transverse processes of the vertebrae, and the skull. The innervation of the erector spinae is through the posterior rami of the spinal nerves. The function of each part of the erector spinae, and thus of the muscle as a whole, is to extend (bilateral contraction) and laterally flex the spine (unilateral contraction).
