Muscles of the back

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Introduction0:00–0:17

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.

Extrinsic back muscles0:17–1:18

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.

Superficial group - Trapezius1:18–3:03

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.

Superficial group - Latissimus Dorsi3:03–4:02

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.

Superficial group - Levator scapulae4:02–4:58

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.

Superficial group - Rhomboids4:58–6:46

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.

Intermediate group - Serratus posterior superior/inferior6:46–8:09

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.

Quiz8:09–8:28

Intrinsic back muscles8:28–9:39

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.
Now, let’s look at the three subgroups of the intrinsic back muscles: the superficial layer is composed of the splenius muscles, the intermediate layer consists of the erector spinae and the deep layer is composed of the transversospinalis group.The superficial layer consists of the splenius cervicis and splenius capitis.

Superficial group - Splenius muscles9:39–11:04

They are thick, flat muscles that lie on the lateral and posterior aspects of the neck, covering and holding in position the deeper neck muscles kind of like a bandage, which makes sense, given that splenius comes from splenion, which means bandage in Greek.
Splenius capitis originates on the nuchal ligament and the spinous processes of C7 to T4. Its fibers run superolaterally to insert on the mastoid process of the temporal bone and the lateral third of the superior nuchal line of the occipital bone.
Splenius cervicis originates on the spinous processes of T3 to T6. Its fibers run superolaterally to insert on the posterior tubercles of the transverse processes of the C1 to C3 vertebrae.
The splenius capitis is innervated by the posterior rami of the middle cervical spinal nerves where the splenius cervicis is innervated by the posterior rami of the lower cervical spinal nerves.
When the splenius muscles act unilaterally they produce lateral flexion of the neck and ipsilateral rotation - or rotation of the head to the side of the contracting muscles.
When contracting bilaterally, they extend the head and neck.The intermediate layer of the intrinsic back muscles includes the column-like erector spinae muscles, which lie on each side of the vertebral column between the spinous processes of the vertebrae centrally and the angles of the ribs laterally.

Intermediate group - Erector spinae muscles11:04–13:30

Moving from lateral to medial, we have the iliocostalis, the longissimus, which makes up the intermediate column, and finally the spinalis.
You can remember the order of the muscles from lateral to medial by the mnemonic ‘I Love standing’. The erector spinae muscles have a common origin through a broad tendon that attaches inferiorly to the posterior part of the iliac crest, the posterior surface of the sacrum, the sacroiliac ligaments, the sacral and inferior lumbar spinous processes and the supraspinous ligament.
Each erector spinae muscle runs superiorly, and is divided regionally into three parts based on the sites of their superior attachments.
The iliocostalis muscle is divided into iliocostalis lumborum, iliocostalis thoracis, and iliocostalis cervicis with fibers inserting on the transverse processes of the lumbar vertebrae, angles of the lower ribs, and the transverse processes of the cervical vertebrae.
The longissimus muscle has thoracis, cervicis and capitis portions that insert on the ribs between the tubercles and angles, the transverse processes of the thoracic vertebrae, transverse processes of the cervical vertebrae, and the mastoid process of the temporal bone (capitis).
And finally, the spinalis muscle is the most medial erector spinae muscle, running from spinous processes to more superior spinous processes.
It consists of thoracis, cervicis and capitis portions that insert on the spinous processes in the upper thoracic region, the spinous processes in the cervical region and to the occipital bone.
However, spinalis cervicis and capitis are highly variable and may be fused with the other back muscles or be absent entirely.
When acting unilaterally, the erector spinae muscles laterally flex the vertebral column, but when they act bilaterally, they serve to erect the spine; hence their name.
They extend the vertebral column and head, and when the back is flexed, they control movement via eccentric contraction.The deep layer of the intrinsic back muscles consists of the transversospinalis muscle group, which consists of the semispinalis, multifidus, and rotatores.

Deep group - Semispinalis13:30–14:52

The muscles of the transversospinalis group are named this way because they originate from the transverse processes of vertebrae and pass obliquely to insert on the spinous processes of more superior vertebrae, and thus lie in the space between the transverse processes and spinous processes.
The semispinalis, named because it arises from the upper half of the spine, is the most superficial of the transversospinalis muscles and originates on the transverse processes of C4 to T12.
Its fibers run superomedially, spanning 4 to 6 vertebral segments, to insert on the occipital bone and the spinous processes in the cervical and thoracic regions, with each of the three parts of the semispinalis named for its superior attachment - semispinalis capitis, semispinalis cervicis, and semispinalis thoracis.
When acting bilaterally, the semispinalis serves to extend the head and the cervical and thoracic regions of the vertebral column.
When acting unilaterally, it produces contralateral rotation - or rotation to the opposite side. Deep to the semispinalis, and spanning all the way from the sacrum to C2, is the multifidus muscle, which consists of many short muscular bundles, the thickest of which are in the lumbar region.

Deep group - Multifidus14:52–15:38

The multifidus originates from the posterior sacrum, the posterior superior iliac spine of the ilium, the aponeurosis of the erector spinae, the mammillary processes of the lumbar vertebrae, the transverse processes of the thoracic vertebrae, and the articular processes of C4 to C7.
The fibers run obliquely superomedially to insert on the spinous processes located 2 to 4 segments superior to their origin.
The multifidus mainly serves to stabilize vertebrae during local movements of the vertebral column. The deepest muscles of the transversospinalis group are the rotatores, which can be subdivided based on their length into rotatores brevis and rotatores longus.

Deep group - Rotator15:38–16:58

While the rotatores are present in all spinal regions, they are most developed in the thoracic region. The rotatores originate from the transverse processes of vertebrae.
Their fibers run superomedially to insert on the lamina or spinous process of the vertebra either one segment superior to the origin, which is the case for rotatores brevis, or two segments superior to the origin, which is the case for rotatores longus.
Unsurprisingly, the rotatores assist with rotatory movements - specifically they produce contralateral rotation of the vertebral column.
They also stabilize the vertebrae, assist with extension, and may be involved in proprioception.So, all the muscles of the transversospinalis group originate from the transverse processes and pass superomedially to the spinous processes.
To differentiate between them, focus on the length of the muscle fibers! The semispinalis muscle spans 4 to 6 segments.
The deeper multifidus spans 2 to 4 segments. And finally, the deepest muscles - the rotatores - span 1 or 2 segments.Okay.
Before we recap, let’s take a moment and try to recall all of the different intrinsic back muscles.All right, as a quick recap...
There are two major groups of back muscles: the extrinsic back muscles and intrinsic back muscles. The extrinsic back muscles are divided into two subgroups: the superficial extrinsic back muscles and the intermediate extrinsic back muscles.

Quiz16:58–17:24

Review17:24–18:41

The superficial extrinsic back muscles produce and control limb movement and include the trapezius, latissimus dorsi, levator scapulae, and the rhomboids.
The intermediate extrinsic back muscles include the serratus posterior superior and the serratus posterior inferior, which may play a role in respiratory movements and proprioception.
The intrinsic back muscles function to maintain posture and control movement of the vertebral column and are divided into three subgroups: superficial, intermediate, and deep.
The superficial layer of intrinsic back muscles includes the splenius cervicis and the splenius capitis. The intermediate layer is made up of the erector spinae muscles, which are the iliocostalis, longissimus, and spinalis.
And finally, the deep layer of intrinsic back muscles includes the transversospinalis muscle group, which consists of the semispinalis, multifidus, and rotatores muscles.
And finally, the Deep layer of intrinsic back muscles includes. The transition has been Alice muscle Group, which consists of a, semi spinalis,