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Anterior compartment forearm muscles

Origins, insertions, innervation, functions and related clinical anatomy of the muscles of the anterior compartment of the forearm.

Show transcript

Hello, everyone! This is Joao from Kenhub, and welcome to another anatomy tutorial where, today, we’re going to be talking about the anterior compartment forearm muscles.

So what we’re going to be doing on this tutorial is looking at this image here mainly, which shows the anterior view of the forearm and the hand as well.

Keep in mind this is the anatomical position, so we’re looking at the anterior view of the anatomical position of these structures here.

Now, what we’re going to be doing is describing the different origins, insertions, also innervation and functions of these muscles, and a little bit of related clinical anatomy that is worth knowing, especially during exams. And if you’re in different health professions or you’re a student in a different health professions, I think it’s worth to cover a little bit of clinical anatomy related to these structures.

Without keeping you waiting, we’re going to continue on to the first topic here.

The first point that I would like to make is that these muscles can be divided into two groups. One is found superficially, and we’re going to call them the superficial muscles, and we’re going to also find a deep layer known as, then, the deep muscles.

Another point I would like to make here is just give you an overview of the origin and insertions that these muscles will have. You can see here on this image that these muscles are usually or most of them are originating from the medial epicondyle of the humerus that you see a little bit here. This is the medial epicondyle, and notice that most of these muscles are originating from this point.

And they also have origins on other bones of the forearm.

Then, they will run towards the distal part of the ulna, here, on this direction, which is now covered mostly by these muscles, and also the radius. So these muscles are going to insert distally on these two bones as well as, you can see here, the hand and also the wrist.

I would like to also add a little bit of an overview of the different functions that these muscles will be performing, and they’re sometimes called the flexors of the forearm because they’re going to be flexing some of the joints in the forearm and hand.

However, other functions are also included, which will be covered later on on this tutorial. And a point I would like to make here is that their long tendons… Notice here that their long tendons can be easily followed and even palpated at the wrist joint, especially during flexion.

And we’re going to start off with the superficial layer that I’m going to isolate now. See that I just removed the deep layer, and now, to show you the superficial muscles of the anterior compartment of the forearm.

And now, these muscles include the flexor carpi radialis, the flexor carpi ulnaris, the flexor digitorum superficialis, the pronator teres, and the palmaris longus.

So we’re going to be covering all these muscles on the next slides.

Let’s start off with the very first one that you see here, highlighted in green. This muscle, if you remember from that list, is known as the flexor carpi radialis, and in terms of origin point, we’re going to see here—and remember also from one of the first slides here on this tutorial—that I mentioned that it’s going to be coming out of the medial epicondyle of the humerus. So this is going to be the origin point for the flexor carpi radialis.

Then, this muscle is going to run from the medial epicondyle to, then, insert at the bases of the second and third metacarpal bones, as you can see here. Notice that this is the third metacarpal bone, and this is the second metacarpal bone, and the tendon is splitting and inserting on both bases of these bones.

As for the innervation of the flexor carpi radialis, this muscle is going to be innervated by this nerve that you see here, highlighted in green, which is the median nerve. And the median nerve arises from the brachial plexus… which you can also see here the brachial plexus. Notice where the median nerve is coming from.

I would like to also include the different functions associated to the flexor carpi radialis, and we have here two images. One of them (the one on the left), you see the muscle in action, when it’s contracted, and then you see the one on the right that shows the muscle when it’s relaxed.

Now, when contracted, the flexor carpi radialis muscle leads to flexion, also known as palmar flexion (as you can see here on this image and indicated by this arrow) of your wrist, clearly seen here. So this is flexion or palmar flexion – in the direction of the palm of your hand in this joint here, the wrist joint.

Another function or action that this muscle is able to perform is known as radial abduction, which is here, indicated by this arrow.

And radial abduction is when you move the hand in the anatomical position (as you see here) to the sides of your body. And the movement happens in the direction of your radius, hence why we call it, then, the radial abduction.

You can also call it radial deviation, and if the arrow was moving in the opposite direction (so going this way, from left to right) you could, then, call it ulnar abduction or ulnar deviation. And that has to do, of course, with the fact that the movement happens in the direction of the bone that is hidden now from here, the ulna.

We are ready to move on to the next muscle that you see here, highlighted in green, and this is going to be the flexor carpi ulnaris.

In terms of origin points, this muscle will also be originating here from the medial epicondyle of the humerus.

The other origin point for the flexor carpi ulnaris is going to be the olecranon of the ulna, and you can see a little bit here that the muscle is going to the back of this bone, the ulna, where it’s going to be originating from as well.

Now, the insertion point for the flexor carpi ulnaris is going to be a couple that we need to remember. If you notice here, the tendon is going to be using the pisiform bone as a sesamoid bone towards its insertion points, which will be, then, the hamate bone and also the base of the fifth metacarpal bone, which you can see here.

So this is the base, or this is the fifth metacarpal bone, and you notice here that the muscle is going to be inserting at the base of the fifth metacarpal bone.

When it comes to the innervation of the flexor carpi ulnaris, this muscle is going to be innervated by this nerve that you see here, highlighted in green, which is the ulnar nerve, which also arises from the brachial plexus.

Now, the flexor carpi ulnaris also has few functions or actions that we need to talk about, and here on these images, you’re also looking at (on the left side) the muscle in action and on the right side when it’s peacefully just staying there with no action going on.

Now, the muscle or this muscle, the flexor carpi ulnaris, is the most medial of all superficial flexors and is mainly responsible for the contour of the ulnar side of your forearm.

Like we see on all the flexors of the forearm, the flexor carpi ulnaris, when contracted, will lead to, then, palmar flexion, which you can see represented on this image by this arrow as well. So palmar flexion at the wrist joint.

Another movement the flexor carpi ulnaris is responsible for is ulnar abduction, which you can see now, represented by this arrow as well.

Now, the next muscle that we’re going to be talking about is now seen, highlighted in green, and this is known as the flexor digitorum superficialis.

Now, in terms of origin point, as you can see from this image even, the muscle has a widely spread origin point attached to not only the medial epicondyle of the humerus, as you can see here, but it’s also attached to the ulna, on the coronoid process of the ulna as well as the radius. Notice here, distally from the radial tuberosity.

When it comes to the insertion points for the flexor digitorum superficialis, its four insertion tendon split into two smaller end tendons, as you can see here (the split happening right about here). And each of these tendons will, then, insert on both sides of the middle phalanges of the second through fifth fingers.

And one reminder here on how we count the fingers, give these numbers to the fingers – so your thumb is going to be number one, then you count from one all the way to five to your little finger.

Now, in terms of the innervation for the flexor digitorum superficialis, we’re going to see that this muscle is innervated by the median nerve.

Now, we’re going to be talking about the different functions or actions associated to the flexor digitorum superficialis, and you can get two clues already on the screen. One is this image on the right side that’s showing what happens when the muscle contracts, and the other clue is always on the name.

Remember this: the name says a lot about a muscle, and “flexor,” meaning that there is going to be some flexion. “Digitorum” means “fingers” in Latin, so there is going to be flexion of the fingers on your hand.

So the flexor digitorum superficialis muscle is going to perform palmar flexion of the fingers, as you can see here, from the second to fifth fingers. So the thumb is not affected because there is no tendon. Notice here. This is your thumb. There is no tendon of the flexor digitorum superficialis running through the thumb, and for that reason, there’s not going to be any movement caused by the flexor digitorum superficialis on your thumb.

We’re now ready to move on to the next muscle that you see here, highlighted in green. This is known as the pronator teres.

Now, in terms of origin points, this muscle is known to originate from also the medial epicondyle of the humerus, as you can see here, and the coronoid process of the ulna.

And from the medial epicondyle and coronoid process of the ulna, it courses under the brachioradialis muscle and then attaches to the lateral side of the radius, as you can see here on this image as well.

Now, in terms of innervation, the pronator teres is going to innervated by this nerve here as well, the median nerve.

We’re going to also talk about the different functions or actions associated to the pronator teres that you see here now on this image in action, and again, the name tells the main function.

Together with the pronator quadratus muscle, the pronator teres is the most important muscle for pronation of the forearm, which you see here also, indicated by the movement of this arrow.

And pronation of the forearm means that if you show a thumbs up, you can see the thumb moving medially towards the midline of your body.

And also like the other flexors of the forearm, it leads to flexion of the elbow joint.

Next muscle that we’re going to talk about, here, seen highlighted in green as well, one of the superficial muscles of the anterior compartment of the forearm is known as the palmaris longus.

Also an easy one to remember in terms of origin point, the palmaris longus is a very slender muscle that originates from the medial epicondyle of the humerus, as you can see here.

And one important note about this muscle is that it is highly variable, and in around 20% of people, it is missing on one or both arms completely.

Now, when it comes to the insertion point for the palmaris longus, this muscle inserts at the flexor retinaculum and also the palmar aponeurosis of the hand.

And as you can see here on the image, the muscle goes from the medial epicondyle all the way to the insertion points, which will be the flexor retinaculum, which is a bend that crosses your wrist, and the palmar aponeurosis, which you see a portion of it, which is, then, the insertion point for the palmaris longus.

When it comes to the innervation of the palmaris longus, this is also an easy one. Remember, this is going to be innervated by this nerve that you see, now highlighted in green, the median nerve.

And as you would expect, next, we’re going to be talking about the different functions or actions associated to the palmaris longus, which is also seen here in action. And when it contracts, the palmaris longus will lead to, then, (what you see here clearly) palmar flexion as well as tension of the palmar aponeurosis.

Now that we just covered the superficial layer of the anterior compartment of the forearm, I would like to just give a quick word on clinical anatomy about these muscles, or related clinical anatomy.

What I wanted to add about clinical… related clinical anatomy, about the superficial flexors is that chronic false strain of these muscles often leads to inflammation and increased connective tissue at their origin points at the medial epicondyle of the humerus.

This happens especially to golfers due to the constant flexion of the elbow and also the wrist. And for that reason, this condition is also called “golfer’s elbow.” And classic symptoms include pain and trouble performing routine tasks.

Now that we talked about the superficial muscles of the anterior compartment of the forearm, I have here now, again, the image that shows all the muscles of the anterior compartment of the forearm to now use the magic of editing to remove the superficial layer and then expose the deep layer or the deep muscles of the anterior compartment of the forearm.

And before we talk about them, I would like to list them for you. We’re going to be talking about the flexor digitorum profundus, the flexor pollicis longus, and the pronator quadratus.

And I would like to use this image to just quickly show them to you. So, you have here the flexor digitorum profundus, the flexor pollicis longus, and then under them, you see this square-shaped one, which is the pronator quadratus.

Let’s start off with the very first one on the list that you see now, highlighted in green. This is the flexor digitorum profundus. It is the only flexor of the distal joints of the fingers and therefore very important for moving those joints.

And when it comes to the different origin points, it will be originating at the proximal half of the anterior ulna and also the interosseous membrane, which you can see a little bit. Notice here, this is the interosseous membrane, which is found between—interosseous, meaning between—two bones which are the ulna and also the radius. So this muscle will be originating also from this membrane, the interosseous membrane of the forearm.

Now, it also has an insertion point, and as you can see, its four tendons run through the carpal tunnel, which is this tunnel here that you find on your wrist. And also, it runs between the split and tendons of the flexor digitorum superficialis that we talked about before, at the height of the middle phalanges.

And distally, these tendons of the flexor digitorum profundus will be inserting at the palmar side of the distal phalanges of the second all the way to the fifth fingers, as you can see here clearly on this image.

Now, we’re also going to be covering the innervation of the digitorum profundus, the flexor digitorum profundus, and one interesting thing that you’re going to see here that is different from the other muscles that we saw is that the flexor digitorum profundus is innervated by two nerves.

One is the median nerve, which will be innervating especially the second and third fingers or the section of the muscle that runs to the second and third fingers, and also, we’re going to see that the ulnar nerve is going to do the same but for the fourth and fifth fingers.

We’re ready to move on to, now, the different functions or actions associated to the flexor digitorum profundus, which you now see here in action.

And due to its insertion points, this muscle, when contracting, is responsible for palmar flexion of the distal phalangeal joints of the second to fifth fingers.

The next muscle that we’re going to be talking about is now seen highlighted in green, and this is the flexor pollicis longus.

In terms of origin points, we’re going to see that it starts at the anterior side of the radius, as you can clearly see here its attachment to the radius, and also that membrane that we talked about, the interosseous membrane of the forearm.

Now, the tendon of the flexor pollicis longus also runs through the carpal tunnel, as you can see here, and then inserts at the palmar side of the distal phalanx of the thumb.

Now, we’re moving on to talk about the innervation of the flexor pollicis longus, which is this nerve that you now see highlighted: the median nerve.

I’d like to also talk a little bit about the different functions or actions associated to the flexor pollicis longus, and you can clearly see here also a clue on the image, another clue on the name. Pollicis means “thumb,” so this muscle is going to be responsible for palmar flexion of the thumb, as you can see, and also opposition of the saddle joint.

In addition, like the other flexors of the forearm, the flexor pollicis longus will also be supporting palmar flexion of the wrist joint.

We’re going to move on to the last one on that list that we saw about the deep layer of the anterior compartment of the forearm—this one that you see here, highlighted in green, has a square shape, and is known as the pronator quadratus.

Now, in terms of origin points, this muscle is going to originate at the distal end of the ulna, as you can see here. So this is the origin point. And then it will be extending horizontally to, then, insert at the radius, on the anterior margin of the radius, as you can see here, giving the muscle a square-shaped appearance—hence the name quadratus, which in Latin means “squared.”

And one important point to add here is that this muscle is the deepest muscle of the anterior forearm.

The next point that we need to add here is the innervation of the pronator quadratus, and this muscle is going to be innervated by the median nerve.

A word on the different functions or actions associated to the pronator quadratus, together with the pronator teres muscle, the pronator quadratus is the most important muscle for pronation of the lower arm, as you can see here.

Before we conclude this tutorial, I would like to make a few clinical anatomy or related clinical anatomy points, and one of them is lesion of the anterior interosseous nerve, which is a branch of the median nerve. This leads to what is known to be as anterior interosseous syndrome or Kiloh-Nevin syndrome.

The affected patients usually complain about pain in the forearm and hand weakness.

The most characteristic sign though is the inability of forming the okay sign with your fingers or the pinch sign, and this happens due to the fact that the flexor digitorum profundus and also the flexor pollicis longus are the only muscles which are able to bend the fingers at their distal interphalangeal joints.

Now, you see here the connection that if the anterior interosseous nerve is damaged, then there will be improper innervation of these muscles, and that’s how you can check whether they are working properly or whether this nerve is damaged somehow.

Another related clinical anatomy point I would like to make here is actually an important and frequent complication at the anatomy of the forearm, which is known as the carpal tunnel syndrome.

And the carpal tunnel is located at the wrist, as you can see here—I mentioned this several times throughout this tutorial—and this is where the tendons of the flexor digitorum superficialis, flexor digitorum profundus, and also the flexor pollicis longus, the tendon will be also passing here, as well as the median nerve will be passing through this carpal tunnel.

So any kind of chronic stress in this region will lead to, then, compression of these structures, especially the median nerve. This is what causes, then, what is known to be as the carpal tunnel, which is usually diagnosed when it leads to pain and parasthesia radiating into the whole arm.

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