Back and Chest Muscles: Anatomy, Function, and Innervation
Appendicular and dorso-costal muscles relate to the ribs and deeper back muscles. Each has a different innervation: appendicular back muscles are innervated through the brachial plexus, dorsal-rib muscles through the intercostal nerves, and intrinsic back muscles through the posterior branches of spinal nerves.
Appendicular Back Muscles
The most superficial is the trapezius muscle, which is triangular and originates from the external occipital protuberance in the nuchal ligament (supraspinous ligament connecting the spines). In the cervical region, it forms a cervical septum and inserts into the spinous processes of all vertebrae. The fibers are directed outward, with the most superior ascending and the inferior descending. Trapezius fibers terminate in the bones of the shoulder girdle and scapula.
Function: Lateral flexion of the head and neck. The upper fibers elevate the shoulder girdle, the horizontal fibers abduct the scapula, and the lowest fibers pull the scapula downward.
Latissimus dorsi muscle: Partly covers the trapezius. It inserts into the spines of the last thoracic vertebrae in the lumbar spine through a muscle aponeurosis. It is wide, flattened, forming a fascial sheet. The muscle fibers are directed upwards and outwards, attaching to the humerus at the bottom of the bicipital groove.
Function: Brings the arm backward and allows inward rotation. If fixed on the humerus, it can lift the body, liaising between the girdles of the body and the pelvic girdle.
Deep to these two muscles are the rhomboid muscle, which attaches at the first thoracic vertebrae to the spinous process level and extends to the edge of the scapula, and the levator scapulae muscle, which originates in the transverse processes of the first four cervical vertebrae and ends in the upper inner shoulder blade.
Dorsal Rib or Serratus Posterior Muscles
These muscles are visible when raising the rhomboid and levator scapulae. On each side, there is a superior and an inferior muscle.
Serratus posterior superior: Extends from the spine of the last cervical and first three thoracic vertebrae, inserting into the first few ribs.
Serratus posterior inferior: Originates in the last two thoracic and first two lumbar vertebrae, with fibers passing outward and upward, terminating in the ribs.
Intrinsic Back Muscles
These muscles are innervated by the posterior branches of spinal nerves and are also known as the muscles of the quadriceps channels or lumbar spine. Four parts are distinguished, with a common mass that is both fleshy and tendinous, originating behind the sacrum and lumbar vertebrae, and attaching to the iliac crest. From this common mass, four lumbar parts of the quadriceps muscle are distinguished:
- The outer iliocostalis lumborum consists of fibers that extend over the level of the ribs near the rear corner.
- The longissimus dorsi does the same but within the lumbar sacrum, with attachments to the transversus processes.
- The spinalis attaches to the spines of the thoracic vertebrae.
- The transversospinalis is the deepest, consisting of fibers that jump from the spinous to transverse processes, spanning several vertebrae.
Overall, the quadriceps muscle of the back lifts the trunk, so it is also called the erector spinae muscle.
Chest Muscles
These are the muscles of the chest wall.
Intercostal muscles: Occupy the space between two ribs. The external intercostal fibers run forward, outward, and downward, while the internal intercostal muscle fibers run downward and backward, intertwining the fibers of the two layers. Their function is to hold together the ribs. The intercostal vessels and nerves are located between these muscles, on the inner face. Additionally, the sternalis muscle, also known as the transversus thoracis, is found in the anterior chest wall, behind.