Muscles of the Back
Back muscles are divided into two specific groups: the extrinsic muscles that are associated with upper extremity and shoulder movement, and the intrinsic muscles that deal with movements of the vertebral column. Several small muscles in the cervical area of the vertebral column are also important.
The extrinsic muscles
Superficial extrinsic muscles connect your upper extremities to the trunk, and they form the V-shaped musculature associated with the middle and upper back. They include the trapezius, latissimus dorsi, levator scapulae, and the rhomboids. Intermediate extrinsic muscles include the serratus posterior superior and inferior. Most of their function is involved with respiration.
The intrinsic muscles
Intrinsic muscles, which stretch all the way from the pelvis to the cranium, help to maintain your posture and move the vertebral column. They’re divided into three groups: the superficial layer, the intermediate layer, and the deep layer. The muscles in all of the layers are innervated by the posterior rami of spinal nerves:
Injuries of the intrinsic back muscles often occur while using improper lifting technique. You can protect the back muscles by bending from the hip and knee when you lift objects from the ground.
The superficial layer
Thick splenius muscles form the superficial layer of muscles and are located on the lateral and posterior portions of the neck. They laterally flex, rotate, and extend your head and neck. Bodies have two kinds of splenius muscles:
Splenius capitis muscles: These muscles originate from the nuchal ligament and spinous processes of the 7th cervical vertebra and the upper thoracic vertebrae. They run superiorly to the mastoid processes of the temporal bone.
Splenius cervicis muscles: These muscles originate with the splenius capitis but insert onto the transverse processes of the upper cervical vertebrae.
The intermediate layer
The erector spinae muscles lay on either side of the vertebral column, running from the lumbosacral area superiorly to various places along the ribs and up to the base of the skull. Their job is to extend the vertebral column and maintain the normal curvature (posture) of the vertebral column. The erector spinae muscles, detailed in the following list, all originate from the posterior sacrum, sacroiliac ligaments, sacral and lumbar spinous processes, and iliac crest:
Iliocostalis muscles: These muscles run superiorly where they insert onto the angles of the ribs and the transverse processes of the lower cervical vertebrae.
Longissimus muscles: These muscles travel superiorly to their insertions on the ribs, the transverse processes of the thoracic and cervical vertebrae, and the mastoid process of the temporal bone.
Spinalis muscles: These muscles run superiorly to insert on the spinous processes of the upper thoracic vertebrae and to the cranium.
The deep layer
Underneath the intermediate intrinsic back muscles is another layer of muscles that help to support posture and assist the intermediate muscles in moving the spine. The deep intrinsic muscles are smaller than the erector spinae muscles, and none of them traverse more than six vertebral segments.
Semispinalis muscles: This group is the most superficial of the deep intrinsic muscles. These muscles run from the midthoracic spine superiorly through the cervical spine. They have three divisions (thoracis, cervicis, and capitis) that originate from the transverse processes of the 4th cervical vertebra through the 10th, 11th, or 12th thoracic vertebra. The fibers travel superiorly for about four to six segments each and attach on spinous processes and the occipital bone.
Multifidus muscles: These short, triangular muscles originate in various places but always travel superiorly and medially for two to four segments and attach on the spinous processes.
Rotatores muscles: The rotatores lie underneath the multifidus muscles. They originate from the transverse processes of a single vertebra and travel superiorly to insert into the spinous process of the vertebra one or two segments superior to it. The rotatores help with rotation and proprioception.
The suboccipital muscles
The suboccipital region includes the posterior part of the 2nd cervical vertebra to the area inferior to the occipital region of the head. Four small muscles located on each side of the suboccipital region help with posture and assist with extension and rotation of the head:
Rectus capitis posterior muscles: These two muscles insert onto the occipital bone; the rectus capitis posterior major originates at the spinous process of the 2nd cervical vertebra (the axis) and the rectus capitis posterior minor originates from the posterior arch of the 1st cervical vertebra (the atlas).
Obliquus muscles: These two muscles complete the suboccipital quartet. The obliquus capitis inferior travels from the spinous process of the 2nd cervical vertebra to the transverse process of the 1st cervical vertebra, and the obliquus capitis superior has its origin at the transverse process of the 1st cervical vertebra and inserts onto the occipital bone.