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Attachments, innervation and functions of the superficial muscles of the back.
Hands up if you go to the gym or even like to climb or, well, at least have heard someone else talking about working out their traps or lats, but what does that mean? Where are those muscles, and what do they do? Well, if you’re wondering, you’ve come to the right place. We’ll discover those muscles and more in this tutorial on the superficial muscles of the back.
Before we get started, left’s have a quick overview of what we’ll cover today. We’ll start by looking at the bones these muscles attach to including some of their specific bony features, then we’ll look at the muscles of the back starting superficial and then working our way deeper to the intermediate layer of the muscles of the back. We’ll then finish up with some clinical correlates before we let you run away. Are you ready? Here we go!
So, the muscles of the back can be largely divided into two major groups. The superficial or extrinsic back muscles which functionally belong to the upper limbs but are situated on the posterior aspect of the trunk and these are interestingly sometimes referred to as immigrant muscles due to the fact that these muscles migrate from the embryological upper limb during development. The other group of back muscles are known as the deep or intrinsic back muscles, which act specifically on the vertebral column. These are known as the true back muscles.
We’ll be focusing solely on the superficial muscles today, but, of course, if you’re interested in learning about the deep muscles of the back, you’ll find great articles, videos, and quizzes about all of these at kenhub.com.
Alright, let’s begin by taking a look at the bones and some of their bony features that these superficial back muscles attach to. We’ll start with the most central set of bones – the vertebral column.
The vertebral column runs the length of the neck and the back, so I’m sure you can imagine some muscles attaching onto these bones. There are three main regions of the vertebral column that we’ll look at today – the cervical region consisting of seven vertebrae, the thoracic region where there are twelve vertebrae, and finally, the lumbar region which is made up of five vertebrae.
Vertebrae have many bony features, but there are two specific ones I’d like to remind you of today. The first is the spinous process highlighted in green in this image of a vertebra from a superior view. There is one spinous process per vertebra which projects posteriorly. If you palpate the back of your neck in the midline, the bony feature you feel is a spinous process.
The second bony feature we’ll look at today are the paired transverse processes. There is one on either side of the vertebral canal projecting a bit posteriorly, but mostly laterally. Superiorly, there’s one bone of the skull where we’ll see a muscle attachment, and that’s the one highlighted now – the occipital bone. Specifically, one of the muscles will originate from this bony feature just here – the external occipital protuberance. At the inferior end of the vertebral column is where we’ll find our pelvis. We’ll see that one of the superficial back muscles attaches to the iliac crest which is this structure here.
Attached to the thoracic vertebrae are the twelve ribs. These, of course, wrap around anteriorly and form much of the thoracic cage. Most of the muscles we look at today will have an attachment point somewhere on this bone here – the scapula. We’ll particularly see insertions along the medial border of the scapula and the spine of the scapula.
The final bone we’ll be working with today is the bone of the arm which is called the humerus. On the anterior surface of the humerus at the proximal end is a sulcus or groove called the intertubercular sulcus. It is also called the bicipital groove because the long head of the biceps brachii runs along it. One of our superficial back muscles attaches within this groove which we’ll see soon.
Alright, enough teases, right? Ready to get to it? Here it comes – the superficial muscles of the back.
First up is the most superficial muscle of the back – the trapezius. The trapezius is a triangular, flat muscle that can be divided into three parts – the superior part, the middle part, and the inferior part. Each part has a second name based on the direction its fibers travel in. The superior part is also called the descending part. Its fibers travel from superior to inferior descending along the neck. The middle part is also called the transverse part as its fibers travel horizontally across the upper back, and finally, the inferior part is also called the ascending part because its fibers ascend back up towards the scapula.
Each part of the trapezius has its own origin and insertion points which we’ll run through now. The descending part originates from the medial third of the superior nuchal line, the external occipital protuberance, and the nuchal ligament, which runs along the spinous processes of the cervical neck. These muscle fibers travel inferiorly and laterally to insert on the lateral third of the clavicle. The transverse part originates from the spinous processes of T1 to T4. They travel transversely to insert on the medial aspect of the acromion of the scapula as well as the superior crest of the spine of the scapula. The ascending part originates from the spinous processes of T5 to T12 and ascends superiorly to insert on the medial end of the spine of the scapula.
Now because this large muscle has so many origin and insertion points, it can perform several actions. The three parts can either work together or separately and this will change the resulting action. We’ll look firstly at the scapulothoracic joint where the scapula meets the thoracic cage and what actions the trapezius muscle can perform there. When the descending part of the trapezius contracts, it pulls the scapula superomedially elevating it. These muscles are your shrugging muscles. However, if the ascending part contracts, then we have muscle fibers pulling down on the scapula from below drawing the scapula inferomedially and suppressing it. The middle part then by pulling medially towards the vertebral column retracts it.
Contraction of multiple parts of the trapezius can also result in rotation of the scapula. Since the trapezius muscle is attached to the occipital bone, it can also move the head. This is mainly done by the descending or the superior part of the trapezius. If one side of the trapezius contracts which is called unilateral contraction, it will cause lateral flexion of the head on the same or the ipsilateral side. If both sides of the trapezius muscle work together – that is, bilateral contraction – this will lead to the extension of the head and the upper cervical spine.
The last bit of information we need relating to the trapezius muscle is its innervation. Although this muscle spans along most of the length of the neck and back, its innervation actually comes from a very superior nerve called the spinal accessory nerve. The spinal accessory nerve is cranial nerve eleven and actually arises from the brainstem rather than the spinal cord. Sensory fibers for the trapezius are derived from the anterior rami of the C3 and C4 spinal nerves by branches of the cervical plexus.
The second muscle that makes up the superficial layer of back muscles is another one we hear a lot about locally as the lats. The proper name for this large muscle is the latissimus dorsi, and you can see this massive muscle highlighted in green now. This is the widest muscle of the body and covers many of the posterior back muscles in this area except for the trapezius.
The broadness of this muscle means that it has a lot of origin sites with four defined parts. Firstly, a vertebral part of the muscle originates from the spinous processes of T7 to S1. Inferior to that, the main origin site is found which is the thoracolumbar fascia. Laterally, at its inferior end, we find the iliac part of the origin. The latissimus dorsi also originates from the posterior third of the iliac crest. The costal part originates from ribs nine to twelve, and finally, the scapular part has a small origin on the inferior angle of the scapula.
All of these fibers come together to insert onto a single insertion point which is found at the floor of the intertubercular sulcus. Remember, this sulcus is on the anterior aspect of the humerus. So the latissimus dorsi comes from the back, runs between the rib cage and the humerus, and inserts onto the anterior part of the humerus in the floor of the intertubercular sulcus.
As the muscle travels through the armpit region, it makes up the posterior axillary fold within the teres major muscle. At this point, it also begins spiraling so the fibers end up flipping one hundred and eighty degrees. This means that the superior part of the latissimus dorsi inserts more distally than the inferior part which inserts proximally.
Since the latissimus dorsi is such a large muscle, it can pull on the humerus in a few different ways to cause various movements. Firstly, muscle contraction leads to medial or internal rotation of the humerus at the shoulder joint. Also, the muscle will pull the humerus closer to the trunk, therefore, causing adduction of the humerus also occurring at the shoulder joint, and finally, the latissimus dorsi pulls the humerus posteriorly causing extension of the arm.
The sheer size and strength of the latissimus dorsi combined with its ability to adduct and extend the arm at the shoulder joint makes it one very useful muscle if you’re into climbing or doing pull-ups.
In terms of innervation, this large muscle is supplied by the thoracodorsal nerve, which arises from the brachial plexus. The thoracodorsal nerve is also called the middle subscapular nerve. It arises from the posterior cord of the brachial plexus and contains fibers from the anterior rami of C6 to C8 spinal nerves.
We’ll now start diving a little deeper to muscles that underlie the trapezius and the latissimus dorsi. They’re still very superficial and are considered extrinsic muscles of the back. Firstly, is the muscle that we can see highlighted now called the levator scapulae. In order to see this, the only muscle that would be removed is the trapezius.
The levator scapulae muscle originates from the transverse processes of C1 to C4. You can see part of the muscle coming off each of those four cervical vertebrae and joining together to form the main muscle. Inferiorly, this muscle inserts on the superior part of the medial border of the scapula from the superior angle to the root of the spine of the scapula.
The levator scapulae is innervated by the anterior rami of C3 and C4 spinal nerves in addition to the dorsal scapular nerve. The dorsal scapular nerve arises directly from the anterior ramus of spinal nerve C5 before the brachial plexus forms. In terms of function, the muscle does exactly as its name suggests – it elevates the scapula. It does this by drawing it superomedially. Because it’s pulling on the superior part of the medial border, as it elevates the scapula, it also rotates it leading to the depression of the glenoid cavity. If only one of the levator scapulae muscles contracts, it can act on the neck causing lateral flexion of the neck on the ipsilateral side.
There are two more muscles that insert onto the medial border of the scapula, and these are our rhomboid muscles. The superior one is called rhomboid minor and the inferior, larger one is called rhomboid major. Both are diamond-shaped muscles. Just like the levator scapulae, the rhomboid muscles are innervated by the dorsal scapular nerve which arises from spinal nerve C5.
We’ll look at the more superior muscle of the pair first – the rhomboid minor. This muscle originates from the spinous processes of vertebrae C7 and T1. It travels inferiorly and laterally to insert on the medial border of the scapula at the level of the scapular spine. And now, we’ve got the larger of the two – the rhomboid major. This muscle originates from the spinous processes of vertebrae T2 to T5, just inferior to the rhomboid minor. It also travels laterally and inferiorly to insert on the medial border of the scapula between the root of the spine and the inferior angle of the scapula. The rhomboid muscles work together to act on the scapula. When they contract, they move the scapula superomedially. Also similar to the levator scapulae, the rhomboid muscles can rotate the scapula leading to the depression of the glenoid cavity.
The final pair of muscles that we’ll look at today are the deepest of the extrinsic back muscles. They are sometimes considered an intermediate layer, just superficial to the deep intrinsic back muscles. These are the serratus posterior muscles. On the top, we can see the serratus posterior superior and towards the bottom of the rib cage is the serratus posterior inferior.
So first up is the serratus posterior superior muscle. This muscle originates from the nuchal ligament and the spinous processes of the vertebrae C7 to T3. It travels laterally and a bit inferiorly to insert on ribs two to five. The serratus posterior superior is innervated by the intercostal nerves that travel along the ribs it inserts into. These are the second to fifth intercostal nerves. This muscle is an accessory muscle of inspiration. It does this by elevating the ribs.
Our final muscle of the tutorial is the serratus posterior inferior. This muscle originates on the spinous processes of T11 to L2 as well as from the thoracolumbar fascia. It travels superiorly and laterally to insert on ribs nine to twelve. Just like the serratus posterior superior, this muscle is also innervated by the intercostal nerves and the same ones that travel along the ribs it inserts onto. So the innervation of the serratus posterior inferior are the ninth to eleventh intercostal nerves and the subcostal nerve, which is the nerve traveling along the twelfth rib. The serratus posterior inferior is also an accessory respiratory muscle, but this time for expiration. It does this by depressing the ribs.
And there we have it! All of the superficial muscles of the back covered and ready to ace in our next exam. Let’s finish up with a quick clinical note.
So, who’s been to the GP before and had the doctor auscultate your lungs on your back? Have you noticed the specific site they try to place the stethoscope in? Doctors will use a space called the triangle of auscultation to listen to breathing sounds through the back. This triangle is a space in the back where breathing sounds can be most easily heard because there’s a thinness of the musculature. The triangle is found medial to the scapula and is bounded above by the trapezius and below by the latissimus dorsi.
And now you’re an expert on the superficial muscles of the back!
Before I let you go, let’s quickly review what we looked at today. We started with the most superficial back muscle – the trapezius. This muscle is made up of three parts, each producing a different action on the scapula. We moved inferior to look at the largest muscle of the back and the widest muscle in the body – the latissimus dorsi. This broad muscle has a large origin site, while all the muscles converge to insert on the humerus and help us with climbing actions.
We then looked at the three muscles that insert along the medial border of the scapula. First, most superiorly, is the levator scapulae, inserting near the superior angle of the scapula. Next were the rhomboids. The rhomboid minor inserts near the level of the spine of the scapula and the rhomboid major inserts inferiorly to that. Finally, we looked at the last pair of muscles that are the deepest of the extrinsic muscles of the back – the serratus posterior muscles. The serratus posterior superior is an accessory muscle of inspiration whereas the serratus posterior inferior is an accessory muscle of expiration.
Finally, we looked at the triangle of auscultation which is bordered by the trapezius, the latissimus dorsi, and the medial border of the scapula.
And that brings us to the end of our tutorial. Thanks for joining me. I hope you enjoyed it. Happy studying!