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Muscles of the back

Transcript

Here at Osmosis, we support each other - we’ve got each other’s backs. And it turns out, our backs support all of us! Let’s look at the muscles of the back that help maintain our posture and also provide movements of the trunk and upper limb.

The muscles of the back are divided into two major groups: the extrinsic back muscles and the intrinsic back muscles. The extrinsic back muscles include the superficial back muscles, which produce and control upper limb movements, and the intermediate back muscles, which are thought to be involved in respiratory movements.

The superficial extrinsic back muscles include the trapezius, latissimus dorsi, levator scapulae, and the two rhomboids - rhomboid major and rhomboid minor.

All these muscles are posterior axioappendicular muscles, which connect the axial skeleton, primarily the vertebral column, to the superior appendicular skeleton, specifically the pectoral girdle and the humerus.

The posterior axioappendicular group is divided into two subgroups. The first subgroup consists of two superficial muscles: the trapezius and latissimus dorsi.

The trapezius is a large triangular muscle that covers the posterior aspect of the neck and the superior half of the back. There are two trapezius muscles in the back, which when seen together, look like a trapezium.

Proximally, the trapezius originates on the medial third of the superior nuchal line, the external occipital protuberance, the nuchal ligament, and the spinous processes of the C7 to T12 vertebrae.

Distally, the trapezius inserts on the lateral third of the clavicle, the acromion, and the spine of the scapula. Based on the orientation of its fibers, the trapezius is divided into the descending - or superior - part, the middle part, and the ascending - or inferior - part.

Contraction of the descending part results in elevation of the scapula, like when shrugging the shoulders. Contraction of the middle part retracts the scapula, and contraction of the ascending part depresses the scapula, lowering the shoulder.

And finally, when the ascending and descending fibers work together, it results in upward rotation of the scapula which causes the glenoid cavity to tilt superiorly.

The trapezius is innervated by a plexus of nerves formed by the spinal accessory nerve - or cranial nerve 11 - which provides motor innervation to the muscle, as well as by branches from the anterior rami of the C3 and C4 spinal nerves which carry sensory information - specifically pain and proprioception - from the muscle.

The arterial supply for the trapezius is primarily from the superficial branch of the transverse cervical artery, which arises from the thyrocervical trunk. These neurovascular structures travel through the posterior triangle and then along the deep surface of the trapezius muscle.

The second superficial muscle is latissimus dorsi, which extends from the trunk to the humerus, acting directly on the glenohumeral joint and indirectly on the pectoral girdle. The latissimus dorsi originates on the iliac crest, thoracolumbar fascia, spinous processes of the inferior six thoracic vertebrae, and the inferior three or four ribs.

Distally, the latissimus dorsi inserts on the floor of the intertubercular groove of the humerus. The latissimus dorsi is innervated by the thoracodorsal nerve, which consists of fibers from the anterior rami of C6, C7, and C8.

The contraction of latissimus dorsi extends, adducts and medially rotates the humerus, like when trying to scratch that annoying itch in the middle of your back. In fact, because it is such a powerful extensor and adductor, this muscle is used when pulling yourself up during rock climbing, chin ups, or movements such as swimming or chopping wood.

Okay, next up, we have the deep subgroup of the posterior axioappendicular muscles, consisting of the levator scapulae and the two rhomboids.

The levator scapulae is found in the neck, deep to the sternocleidomastoid and trapezius muscles. Proximally, it originates on the posterior tubercles of the transverse processes of the C1 to C4 vertebrae and then descends to insert on the medial border of the scapula, superior to the spine of the scapula.

“Levator” roughly translates to “lifter”, so when this muscle contracts, it elevates - or ‘lifts’ - the scapula. It also fixes the scapula and holds it in place to resist downward forces like when carrying a heavy piece of luggage up the stairs. It also produces downward rotation of the scapula, causing the glenoid cavity to tilt inferiorly.

Now, let’s have a look at the rhomboids, which are located immediately deep to trapezius. There are two muscles: rhomboid minor and rhomboid major, where the rhomboid major muscle is found immediately below the rhomboid minor.

Rhomboid minor originates on the nuchal ligament and spinous processes of the C7 and T1 vertebrae and inserts on the medial border of the scapula at the level of the spine of the scapula. The much larger rhomboid major originates on the spinous processes of the T2 to T5 vertebrae and inserts on the medial border of the scapula inferior to the spine of the scapula.

Rhomboid minor and major always contract together to produce retraction and downward rotation of the scapula. They also assist the serratus anterior in holding the scapula against the thoracic wall. These movements enable us to forcibly lower the arm, like when driving a stake... but not, like, literally driving a steak.

The rhomboids and the levator scapulae are all innervated by the dorsal scapular nerve - which arises from the anterior rami of C4 and C5, with the levator scapulae also receiving branches directly from the anterior rami of the C3 and C4 spinal nerves.

Now, the dorsal scapular nerve is accompanied by the dorsal scapular artery, which may arise directly from the subclavian artery, or as a branch from the transverse cervical artery, in which case it is also referred to as the deep branch of the transverse cervical artery.

Regardless of its origin, the artery buddies up with the dorsal scapular nerve, where they travel deep to the levator scapulae, and continue deep to the rhomboids, descending parallel to the medial border of the scapula, supplying all three muscles throughout their course.

Now, deep to the posterior axioappendicular muscles, are the intermediate extrinsic back muscles which are the two thin serratus posterior muscles. On each side, there’s a serratus posterior superior muscle and serratus posterior inferior muscle.

Serratus posterior superior originates from the nuchal ligament and the spinous processes of the C7 through T3 vertebrae and inserts on the superior borders of 2nd through 5th ribs. Based on these attachments, it is thought that this muscle acts to elevate these superior ribs during inspiration, and it is innervated by the anterior rami of the T2 to T5 spinal nerves.

The serratus posterior inferior originates from the spinous processes of the T11 through L2 vertebrae and inserts at the inferior borders of the 9th through 12th ribs near their angles. Based on these attachments, it is thought to depress the inferior ribs during inspiration , and it is innervated by the anterior rami of the T9 to T12 spinal nerves.

Although these muscles are often designated as respiratory muscles that cause movement of the ribs, these actions are controversial with many sources suggesting they may function primarily in proprioception by providing sensory information about the position of the vertebral column and ribs.

Let’s take a quick break and try to identify all the extrinsic back muscles.

Now, the intrinsic back muscles - also sometimes called the deep back muscles - are actually innervated by the posterior rami of spinal nerves, and function to maintain posture and control movements of the vertebral column.

They extend from the pelvis to the cranium and are enclosed by deep fascia which attaches medially to the nuchal ligament, the tips of the spinous processes of the vertebrae, the supraspinous ligament, and the median crest of the sacrum. Laterally, this fascia attaches to the cervical and lumbar transverse processes and the angles of the ribs.

The portions of the deep fascia in the thoracic and lumbar regions are collectively referred to as the thoracolumbar fascia, with the fascia forming a thin covering over the intrinsic back muscles in the thoracic region and a tough, thick covering in the lumbar region.