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Posterior compartment of forearm muscles

Origins, insertions, innervation, functions and related clinical anatomy about the the extensors of the forearm.

Show transcript

Hello, everyone! This is Joao from Kenhub, and welcome to another anatomy tutorial where, this time, we’re going to be covering the posterior compartment forearm muscles.

So what we’re going to be essentially do on this tutorial is look at this image that you see right now, which is the posterior side of the arm, as we were looking at the anatomical position of the arm and then looking at the back of your arm and seeing these muscles that you see here, and we’re going to be describing them, not only being able to identify these muscle but also talking about their origins, insertions, innervation, functions, and a little bit of related clinical anatomy.

Now, the posterior compartment of the forearm is divided from the anterior compartment of the forearm muscles, which by the way there is a tutorial here at Kenhub that you can watch and learn about the muscles that you find on the anterior compartment of the forearm. And these two compartments are divided by deep fascia, the lateral deep intermuscular septum, and the interosseous membrane between the radius and the ulna.

First thing I would like to say about the posterior compartment forearm muscles is that they can be divided into two categories or two groups. One of them is known as the superficial muscles or a superficial layer of muscles. You also find a deep layer or a deep… or the deep muscles of the posterior compartment.

Now, we’re going to start off with the very first one here on the list, which I’m going to isolate here. Notice the image. I just erased the deep layer so I can just isolate the superficial muscles of the posterior compartment of the forearm.

And as I mentioned, these will be, then, six muscles which include the brachioradialis, the extensor carpi radialis longus, the extensor carpi radialis brevis. And keep in mind that these three muscles sometimes are divided into a separate group which is known as the radial group or the radial muscles of the forearm, and if you notice here on Kenhub, we have a separate tutorial and a separate quiz which will deal with these muscles separately. But you can also talk about them as part of this superficial layer as well.

The other three muscles will, then, be the extensor digitorum, the extensor digiti minimi, and the extensor carpi ulnaris.

Now, the easiest thing to learn about the superficial muscles of the posterior compartment of the forearm is that they’re all going to be innervated by this nerve that you see here, highlighted in green, on this image of the forearm. And keep in mind that, right now, we’re looking at the arm from an anterior view of the anatomical position, but the nerve is going to course to, then, the posterior side of the forearm where it’s going to be, then, innervating this group of muscles. And this nerve is, then, the radial nerve.

And now that we know that, we’re going to, now, talk about the different origins, insertions, functions, and a little bit of clinical anatomy later on about the different muscles that I listed before. And we’re going to start off with this one that you see here, highlighted in green. And also, we’re looking at an anterior view of the forearm here, but of course, if you were to look at the posterior view, you also see this muscle, which is the brachioradialis.

And this powerful muscle is mainly responsible for the lateral contour of the elbow and forearm.

In terms of origin points, it’s going to be originating from the lateral supracondylar ridge between the brachialis and the lateral head of the triceps muscle. And you notice here that this would be the supra… or the lateral supracondylar ridge of this bone here that you see partially, which is the humerus.

Another point of origin for the brachioradialis is the lateral intermuscular septum of the humerus.

Now, the muscle is going to go all the way distally to, then, insert at the styloid process of the radius, as you can see here on this image.

And notice that the radius should be found in this direction while the ulna is going to be found right next to it, a little bit more medially.

Now, you can see that the radius is covered by this layer of muscles, which are the muscles of the anterior compartment of the forearm. But here, you see a little bit of the styloid process of the radius where the brachioradialis is going to be inserting.

The next thing that we’re going to be covering about the brachioradialis, and you can see here, now, the muscle in action, and this is going to be, then, the functions or actions associated to the brachioradialis.

And if the brachioradialis muscle contracts, it will, then, perform flexion of the elbow joint as well as pronation and supination of the wrist, depending on the position of the arm. And you can see supination and pronation being represented here by these arrows.

The next muscle that we saw on that list is now seen here, highlighted in green, and now, we’re looking at a posterior view of the forearm.

The highlighted muscle is, then, the extensor carpi radialis longus, and the majority of this muscle will be lying underneath the brachioradialis, the muscle that we talked about before.

Now, in terms of origin points, it has an origin slightly below the lateral supracondylar ridge of the humerus, which you can also see here.

The muscle will go, then, distally to insert at the posterior side of the base of the second metacarpal bone. Notice here, this is the second metacarpal bone, and the base would be a bit proximal here, and this is the insertion point for the extensor carpi radialis longus.

Moving on to, now, the actions associated or functions associated to the extensor carpi radialis longus, as you see now, here, this image of the muscle in action, this muscle is a weak flexor of the forearm at the elbow joint, and when contracted, it aids in movements within the wrist joint, such as dorsal extension, and assisting fist closure, and abduction or radial deviation of the hand—so basically, this movement that is represented here by this arrow, when you move your hand to the lateral side, to the side of the radius. This is why we call it also radial deviation.

We’re moving on to the next one, this seen here, highlighted in green is now known as the extensor carpi radialis brevis. This one is shorter and thicker than the extensor carpi radialis longus. And this one will be arising from the common extensor tendon of the superficial extensors at the lateral epicondyle of the humerus, as you can see here.

One important clarification, one important point that I wanted to make here on this tutorial, that sometimes, these muscles of the posterior compartment are also referred to as extensors of the forearm. So sometimes, the superficial group is also known as the superficial extensors of the forearm, as the deep group is also known as the deep extensors of the forearm.

Now that this clarification is made, we’re going to move on, now, to the insertion point of the extensor carpi radialis brevis. And this muscle will be attaching to the posterior side of the base of the third metacarpal bone, as you can see here. So this is the base of the third metacarpal bone or the bone that is in direct connection to, then, the middle finger.

Now, also a word on the different functions or actions associated to the extensor carpi radialis brevis, and like the longus, this muscle is also a weak flexor of the forearm at the elbow joint. And when contracted, it will also help with movements within your wrist joint, namely, then, dorsal extension or assisting fist closure and also abduction of the hand (also known as radial deviation) which you see here represented by this arrow—so when you move your hand to the lateral side or towards your radius. That’s why we call also radial deviation, because there is also ulnar deviation, which is when you do the opposite movement.

We’re now ready to move on to the next muscle that you see here, highlighted in green. This one is known as the extensor digitorum. And this muscle will be originating from the lateral epicondyle of the humerus, which is this area here of the bone, and you notice where the muscle is originating from.

And as you can see, in terms of insertion points, you notice that the muscle will be dividing here distally into four tendons or four smaller tendons which insert at the posterior surface of the middle and distal phalanges of the second all the way to your little finger or the fifth finger.

And notice here that these are the middle phalanges, and these will be, then, the distal phalanges, and notice how the muscle is attaching in these areas of your digits.

We’re going to move on to, then, the functions or actions associated to the extensor digitorum. And this muscle’s contraction will cause dorsal extension at the wrist joint, as you can see represented by this arrow and the movement that you see here on this image. So this is dorsal extension at the wrist joint.

And furthermore, the extensor digitorum has a role in extension of several joints, including the metacarpophalangeal, the proximal interphalangeal, and distal interphalangeal joints of the second to fifth fingers.

And if I use this image here, I can show you that the metacarpophalangeal joints are found between the metacarpal bones and the proximal phalanges, and the proximal interphalangeal joints will be, then, found between the proximal phalanges and the middle phalanges of the fingers, and of course, the distal interphalangeal joints will be found between the middle phalanges and the distal phalanges of the fingers. And these will be, then, moved or extended by the extensor digitorum.

And due to the fact that its insertion tendons are partially connected to each other through the dorsal aponeurosis, when one finger extends, it will simultaneously lead to slight extension of neighboring fingers.

Next muscle that we’re going to be talking about that you see now, highlighted in green, this is the extensor digiti minimi. And the extensor digiti minimi is a slender muscle of the forearm and will be originating from the lateral epicondyle of the humerus, as you can see here.

The muscle goes all the way to, then, insert on the dorsal aponeurosis of the little finger. Its muscle belly often blends with the extensor digitorum.

As for the functions or related actions to the extensor digiti minimi, this muscle will be able to cause extension at the wrist joint as well and also extension of the little finger, as you can see here, and this all has to do with the fact that this muscle will be inserting on the dorsal aponeurosis of the little finger. So when contracted, it can cause, then, extension of the little finger.

The next muscle that we’re going to be talking about now, seen here, highlighted in green, is known as the extensor carpi ulnaris muscle. And the extensor carpi ulnaris is a skeletal muscle that is located on the ulnar side of your forearm.

And you can see here now, clearly, on this image, this is part of the ulna, as you can see a bit behind the muscle. And this would be, then, the radius.

It has its origin point at both the lateral epicondyle of the humerus, which you see here.

You can also say that the muscle has two heads, and one of the heads will be originating from the lateral epicondyle of the humerus, while the other head, which is known as the ulnar head, will be, then, originating from the posterior surface of the ulna, as you can see here.

Now, the muscle goes all the way to, then, insert at the base of the fifth metacarpal bone, as you can see here on this image. This is the fifth metacarpal bone, and the base of the bone is found right about here where, then, the muscle will insert.

Now, we’re moving on, talking about the different functions or actions associated to the extensor carpi ulnaris, and you see two images here of the muscle.

The one on the left shows the muscle contracted, while the one on the right side is showing just the muscle with no contraction.

And as the name suggests, extensor carpi ulnaris, meaning that it’s going to extend the wrist, this muscle is also responsible for extension at the wrist joint, which you see here, represented by this arrow.

And in addition, the contraction of the extensor carpi ulnaris will pull the hand medially, what we call ulnar abduction, which I mentioned before, and you see here, represented by this arrow—so when you move your hand towards the medial side of your body or towards your ulna.

And this is what we call, then, ulnar deviation as well.

Now, as I promised, we’re going to have a little word on related clinical anatomy related to, then, the superficial muscles of the posterior forearm.

And due to incomplete migration of the forearm muscles during embryogenesis, approximately two to three percent of all people have what is known to be as a little brother of the extensor digitorum, which is known as the extensor digitorum brevis manus.

And this accessory muscle is normally harmless but can compress the interosseous nerve during hypertrophy. And this will, then, cause pain in the wrist joint.

We’re now ready to move on to the next group, and right now, I’m showing you all the muscles of the posterior compartment, but I’m going to isolate now the deep muscles of the posterior compartment, also known as deep extensors of the forearm.

Now, these are five muscles that I’m going to list now: the supinator, the abductor pollicis longus, the extensor pollicis brevis, the extensor pollicis longus, and the extensor indicis.

Now, another easy thing that we need to remember here is that all these muscles will be innervated by this nerve that you see here, highlighted in green, and if you remember well, yes, this is the radial nerve.

And knowing that, we’re going to move on to the very first one on the list here, that you see highlighted in green. This is known as the supinator muscle.

And the supinator is a broad muscle in the posterior compartment of the forearm that is curved around the upper third of the radius, as you can clearly see here on this image.

Now, in terms of origin points, it has a complex origin tendon arising from four places. One is the lateral epicondyle of the humerus. Another one will be the radial collateral ligament, the anular ligament of the radius, and another origin point will be the supinator crest of the ulna, as you can see here on the image.

Now, the muscle will, then, be inserting around the radius on the posterior surface of the radius.

In terms of functions or actions associated to the supinator, you can see it here in action. And the name says it all. The function of the supinator is to pull the radius causing, then, lateral rotation of the radioulnar joints, as you can see here indicated by this arrow. This is what we call, then, supination.

Ready to move on to the next muscle that you see here, highlighted in green. This one is the abductor pollicis longus, and this muscle has three origin points that you need to remember. One being the posterior side of the radius, the other one being the posterior surface of the ulna, which you can clearly see here, and the other one will be, then, the interosseous membrane or a membrane—as the name indicates, “interosseous” meaning between bones—a membrane which you should find between the radius and the ulna. And this muscle will, then, be originating from the interosseous membrane.

The abductor pollicis longus will go all the way to, then, insert at the base of the metacarpal bone of the thumb, as you can see here.

Often, the insertion tendon splits into two and additionally attaches to the trapezium, which you can also see here the trapezium bone.

Now, when we talk about different functions or actions associated to the abductor pollicis longus, and the insertion points told us a lot. The abductor pollicis longus will be pulling the thumb forward at the saddle joint or the joint between the trapezium and the first metacarpal bone.

This movement is what we would call, then, abduction, which leads to also a lateral movement of the radius at the wrist joint at the same time, which is called, then, radial abduction.

And you can see these movements indicated by this arrow here.

We’re now ready to move on to the next slide, the next muscle here. This one is going to be, then, the extensor pollicis brevis.

And this one is found also on the dorsal side of the forearm, as you would expect. And in terms of origin points, it begins more distally at the posterior side of the radius, which you can still see here from this image.

And also that membrane that we talked about before, that you see now, here, on this image, this is the interosseous membrane, and you can see how it originates from the interosseous membrane of the forearm.

Now, the extensor pollicis brevis muscle is going to, then, insert at the base of the proximal phalanx of the thumb, as you can see here.

Based on the name, you could guess a bit on the functions or actions associated to the extensor pollicis brevis. The extensor pollicis brevis will be causing extension of the thumb in the saddle and metacarpophalangeal joints.

And these joints can be also seen here. So this is the saddle joint, between the trapezium bone and the first metacarpal, and the metacarpophalangeal joint of the thumb, which is found right about here, between the metacarpal bone and the proximal phalanx of the thumb.

And you can see this movement indicated here by this arrow.

This also leads to radial abduction of the wrist.

Moving on to the next muscle on our list, this one that you see here, highlighted. It is the extensor pollicis longus, and this muscle arises medially at the posterior side of the ulna, as you can clearly see here on this image. And another origin point will be the membrane that I talked about: the interosseous membrane, which you can also see here on this image, which can serve as well as an origin point for the extensor pollicis longus.

When it comes to the insertion point for this muscle, it will be inserting at the base of the distal phalanx of the thumb, as you can see here.

And there, at the insertion point, it will form the dorsal aponeurosis, covering the thumb medially, laterally, and also dorsally.

And this sheet-like tendinous expansion will serve mainly as an insertion point for muscles and also some of the ligaments.

When it comes to the functions associated to the extensor pollicis longus, this muscle can cause extension of the thumb in the distal interphalangeal joints, seen here, indicated by this arrow, and contraction of this muscle can also cause dorsal extension of the hand.

The next muscle that we’re going to be covering is seen now, highlighted in green. This is known as the extensor indicis, and in terms of origin point, the muscle is going to, then, originate from the posterior side of the ulna, as you can clearly see here on this image, and also the interosseous membrane will be serving as an origin point for the extensor indicis.

Now, the muscle goes and then inserts distally at the dorsal aponeurosis of your index finger, as you can see here.

Now, in terms of functions or actions associated to the extensor indicis, the name says it all—extensor of the index finger. So the extensor indicis pulls the index finger while causing, then, extension at the metacarpophalangeal joint, the proximal interphalangeal and distal interphalangeal joints, as you can see here, represented by this arrow.

And keep in mind the metacarpophalangeal joint is found here, just between the metacarpal bone of the index finger and also the proximal phalanx of the index finger.

The proximal interphalangeal joint is found right about here, between the proximal phalanx of the index finger and also the middle phalanx of the index finger, and the distal interphalangeal joint is found just right here, between the middle phalanx of the index finger and the distal phalanx of the index finger.

So when this muscle contracts, it’s going to be able to extend all of these joints.

Contraction of this muscle can also lead to extension of the hand.

And now that we covered all the muscles, I would like to talk a little bit about different clinical aspects or one clinical aspect related to the deep muscles of the posterior forearm.

Now, the narrow course of the deep branch of the radial nerve between the two layers of the supinator muscle will carry a high risk of entrapment. And this is a condition referred to as the supinator syndrome.

And since the deep branch of the radial nerve supplies the extensors of the forearm, the affected patients will, then, additionally exhibit what is known to be as weakened ability to extend the hand and also their fingers.

Now, a bit of a fun fact here on this image, that if you extend your thumb, as you see on the image, you see this triangular depression just below the tendons. This is what we call anatomical snuff box or radial fossa.

The anatomical snuff box is going to be framed by the radius as well as both tendons of the extensor pollicis longus dorsally and the extensor pollicis brevis on the palmar side.

There are two clinical significance related to this structure that I would like to add here on this tutorial.

If you palpate the anatomical snuff box, you can feel the pulse of an important artery, the radial artery, because it runs in this region.

Also, the anatomical snuff box is used to palpate or feel the scaphoid bone as a quick way for diagnosing a potential scaphoid bone fracture.

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