The muscles of the back categorize into three groups. The intrinsic or deep muscles are those muscles that fuse with the vertebral column. The second group is the superficial muscles, which help with shoulder and neck movements. The final group is the intermediate muscles, which help with the movement of the thoracic cage. Only the intrinsic muscles are considered true back muscles.
The two muscles in the superficial layer include the splenius cervicis and splenius capitis. They help with movements of the shoulder and neck.
The intermediate muscles are the erector spinae. They include the longissimus, iliocostalis, and spinalis muscles. Their attachments subdivide these muscles, and they all have a common tendinous origin. They play a role in the movement of the thoracic cage and flexion of the upper vertebral column and head.
The intrinsic/deep muscles are well developed and extend from the skull base to the sacrum. These deep muscles are enclosed by fascia. The deep back muscles are posterior to the erector spinae. They are short muscles associated with the spinous and transverse processes of the vertebrae. The three deep muscles of the back include the semispinalis, multifidus, and rotatores. These muscles stabilize the vertebral column and also have a role in proprioception and balance. Moreover, these muscles help with the movements of the vertebral column and maintain posture.
As the muscles of the back develop, they extend caudally. The origins and insertions are described as though the fibers run caudocranially. In this manner, origins are inferior to their insertions.
Skeletal muscle develops by epitheliomesenchymal transformation and originates from the somatic mesoderm. The epaxial myotomes develop the extensor muscles of the vertebral column. The embryological development of the back muscles has been a challenging field to study because current preparation methods make it challenging to identify muscle bundle direction.
The deep cervical, posterior intercostal, subcostal, or lumbar arteries provide the blood supply for all the muscle groups of the back. Arterial supply will vary person-to-person.
The posterior, or dorsal, primary rami of the spinal nerves innervates only the intrinsic or true back muscles.
Ventral rami of the spinal nerves innervate the extrinsic muscles (trapezius, latissimus dorsi, levator scapulae, and rhomboid muscles).
The segmental innervation of the deep muscle groups makes these muscles vulnerable during surgical procedures. Only one branch of the dorsal ramus innervates the medial multifidus with no intersegmental supply. This lack of intersegmental supply can cause injury and paralysis of these small muscle groups. As the paraspinal muscles depend on the antagonistic relationship between bilateral groups, a weakness in the small muscles can cause an imbalance and impairment for the spine.
The chief pathology associated with the back muscles is pain. These muscles can develop spasms that can be debilitating. The lower back muscles are a common cause of low back pain. This entity is often mistaken for spinal stenosis and involves millions of people of all ages and gender. Patients often undergo exhaustive workup, including an MRI.
All individuals with low back pain should perform a straight leg test to differentiate neurologic abnormalities from purely muscular conditions. The treatment of muscular complaints is conservative with a few days of rest, pain control, and physical therapy.
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