Overview of back muscles
The back plays a major role in how the entire body functions. By virtue of its attachments to the vertebral column, the back integrates the activity of the lower limbs, upper limbs, spine and pelvis. The muscles of this region can be easily divided into two major groups: the extrinsic back muscles, which functionally belong to the upper limbs but are situated on the posterior aspect of the trunk also known as “immigrant” muscles, and the intrinsic back muscles, which act specifically on the vertebral column.
In this article, we will present a brief overview of the back muscles, listing their origin and insertion points and giving their general action. You may find it helpful to review the skeleton of the thorax and the bones of the shoulder girdle alongside this article.
Extrinsic Muscles of the Back
Let’s begin with the superficial layer of the extrinsic muscles of the back. They are the trapezius, latissimus dorsi, rhomboid major, rhomboid minor, and levator scapulae. These muscles for the most part, receive their nerve supply from the ventral rami of cervical nerves, with the exception being the trapezius muscle. The trapezius muscle receives its motor innervation from the eleventh cranial nerve, also known as the spinal accessory nerve.
The trapezius muscle (G. trapezium, a four sided flat shape) is a broad triangular shaped muscle that extends over the dorsum of the neck and shoulders. It originates on the external occipital protuberance and superior nuchal line of the occipital bone, nuchal ligament, and spinous processes of the seventh cervical to the twelfth thoracic vertebrae (C7 – T12). Distally, the trapezius inserts on the clavicle and scapula, thus connecting the axial skeleton (i.e.: trunk) to the appendicular skeleton (i.e.: upper limb).
The trapezius is divided into three functionally distinct parts. Each part produces a different movement at the scapulothoracic joint.
Superior part: The fibers of the superior (or descending) part of the trapezius muscle run from their medial attachments on the external occipital protuberance of the occipital bone, nuchal ligament and spine of the C7 vertebra, down and towards the clavicle on the anterior aspect of the shoulder. The muscle inserts on the trapezoid line, a lateral roughening on the inferior surface of the lateral one third of the clavicle, and acts to elevate the scapula.
Middle part: The horizontal fibers of the middle part of the trapezius muscle also originate on the spine of the C7 vertebra, as well as those of the first to fifth thoracic vertebrae (C7 – T5). They traverse the back and insert distally on the acromion of the scapula and the scapular spine. This part of the trapezius muscle mainly acts to retract the scapula.
Inferior part: Proximally, the inferior (or ascending) part originates on the spines of the T6 to T12 vertebrae. The muscle fibers course superolaterally, and insert distally near the medial end of the scapular spine. This part of the trapezius muscle mainly acts to depress the scapula. The superior and inferior parts of the trapezius muscle act together to rotate the scapula.
The latissimus dorsi muscle (L. latissime, meaning wide, dorsi, meaning back) is a very large muscle of the back. Medially, it is attached to the spines of the seventh to twelfth thoracic vertebrae (T7 – T12), thoracolumbar fascia, posterior part of the iliac crest, and lower three to four ribs just lateral to their angles. The fibers course superolaterally and end in a tendon that inserts distally on the floor of intertubercular sulcus on the anterior side of the humerus. The muscle may also attach to the inferior angle of the scapula. The latissimus dorsi mainly acts to adduct and internally rotate the arm.
The rhomboids (Gr. Rhombus, meaning kite) comprise both the rhomboid minor and the rhomboid major muscles.
The rhomboid minor muscle has its medial attachment on the lower part of the nuchal ligament and spinous process of the seventh cervical vertebra and the first thoracic vertebra (C7 – T1). It inserts distally on the medial border of scapula at the level of scapular spine.
The rhomboid major muscle exists just inferior to the rhomboid minor muscle, extending from the second to fifth thoracic vertebrae (T2 – T5). From the spines and supraspinous ligaments of these four vertebrae the muscle inserts distally on the medial border of the scapula inferior to the scapular spine.
The rhomboids act to retract the scapula and depress the glenoid cavity, such as when lowering the arm from an elevated position. The rhomboids also hold the scapula close to the dorsum of the trunk.
The levator scapulae muscle (L. levare, meaning ‘to elevate’) is the last of the superficial extrinsic muscles. Superiorly, it attaches to the transverse process of the upper four cervical vertebrae (C1 – C4) and inferiorly, from the superior angle of the scapula down to the root of the scapular spine. As its name may suggest, the levator scapulae elevates and rotates the scapula to depress the glenoid cavity.
Serratus posterior muscles
The intermediate extrinsic back muscles are the serratus posterior muscles. These are very thin muscles commonly designated superficial respiratory muscles.
The serratus posterior superior muscle lies deep to the trapezius and rhomboids. It originates on the lower part of the nuchal ligament and the spinous processes of the sixth cervical vertebra down to second thoracic vertebra (C6 – T2; sometimes the lower border of the inferior limit may be the T2/T3 interspinous ligament). The muscle courses inferolaterally to insert distally on the upper borders of the second to fifth, and sometimes sixth ribs. (ribs 2 – 5 or 6).
The serratus posterior inferior muscle lies deep to the latissimus dorsi. It originates on the spines of the eleventh thoracic vertebra to the second or third lumbar vertebra (T11 – L2/L3). Here, its fibers are blended with those of the thoracolumbar fascia. The muscle courses superolaterally to insert distally on the inferior borders of the bottom three to four ribs (ribs 9 or 10 – 12).
Intrinsic Muscles of the Back
The true, intrinsic back muscles are the deepest layer of muscles attached to the vertebral column. They receive their nerve supply from the dorsal rami of spinal nerves and are called the intrinsic group because they act specifically on the vertebral column. The many muscles within this group have been divided into three layers: a superficial layer, deep layer and deepest layer, but act together to help maintain the body’s posture and move the spinal column.
The superficial layer is made up of the splenius and the erector spinae muscles.
Inferiorly, the splenius muscles (Gr. splenion, meaning bandage) attach to the nuchal ligament and the spinous processes of the seventh cervical to sixth thoracic vertebra (C7 – T6). The muscle fibers course diagonally across the dorsum of the neck and depending on their superior attachments, are given different names:
The splenius cervicis muscle (L. cervix, meaning neck) inserts on the transverse processes of the first three cervical vertebrae (C1 – C3).
The erector spinae (L. erector; one who erects) is a confluence three muscles and their tendons. From medial to lateral, the three muscles which make up the erector muscle of the spine are: spinalis, longissimus, and iliocostalis muscles. These columns of muscles are further subdivided into three sub columns. The origin of the erector spinae is on the dorsal surface of the sacrum.
The spinalis muscle, which is the most medial of the erector spinae muscles, inserts on the spines of the thoracic and upper lumbar vertebrae. The three parts of spinalis are Spinalis thoracis, cervicis and capitis.
The longissimus muscle is the longest of the back muscles and inserts on the junction of the transverse and costal elements of the cervical, thoracic and lumbar vertebrae. The three subdivision of this muscle is longissimus thoracis, cervicis and capitis.
The iliocostalis muscle runs superiorly to insert onto the angles of the ribs and the transverse processes of the lower cervical vertebrae.The three parts of this column is iliocostalis lumborum, thoracis and cervicis.
The erector spinae muscles on each side of the vertebral column act to laterally flex the vertebral column. When they act in unison on both sides of the vertebral column, they extend the thoracic and lumbar vertebrae.
Underneath the superficial intrinsic back muscles is another layer of muscles: the deep intrinsic muscles of the back also known as “transversospinalis”. They are the semispinalis, multifidus and rotatores. one of them traverse more than six vertebral segments.
The semispinalis muscles are the most superficial of the deep intrinsic muscles. They run from the mid-thoracic spine superiorly through to the cervical spine. The semispinalis muscles comprise the semispinalis capitis, semispinalis cervicis, and semispinalis thoracis.
The semispinalis capitis muscle originates on the articular processes of C4, C5 and C6 and on the transverse processes of C7 – T6 vertebrae. The muscle inserts between the superior and inferior nuchal lines of the occipital bone. This muscle acts to extend the head.
The semispinalis cervicis muscle originates on the posterior surfaces of the transverse processes of T1 – T5/T6 vertebrae and inserts on the spines of C2 – C5 vertebrae. This muscle acts to extend the vertebral column.
The semispinalis thoracis muscle originates on the transverse processes of T6 – T10 vertebrae and inserts on the spines of C6 – T4 vertebrae.
The multifidus muscle originates between the spinous and transverse crests of the dorsum of the sacrum, mammillary processes in the lumbar region, transverse processes in the thoracic region and articular processes in the cervical region. These short, triangular muscles originate in various places but always travel superiorly and medially for three to five vertebral segments and then insert into the vertebral spines. The multifidus muscle acts to rotate the trunk.
The rotatores lie underneath the multifidus muscle. They originate from the roots of the transverse processes of single vertebrae and travel superiorly to insert into the spinous process of the vertebra above.
The deepest layer of the true, intrinsic back muscles comprises the interspinales and intertransverse muscles.
The interspinales originate on the spine of the vertebra above and insert onto the spine of the vertebra below. These muscles are postural muscles.
The intertransverse muscles originate on the transverse process of the vertebra above and insert onto the transverse process of the vertebra below.