Hello everyone! This is Joao from Kenhub. Welcome to another tutorial where, this time, I’m going to be talking about the deep flexors of the forearm.
As you can see on the title, “deep flexors,” you have to have also a superficial group, right? And we did talk about them on a different tutorial here at Kenhub, so you can also check it out if you want to have a little bit more knowledge on the superficial flexors. And you can also see here on this image the superficial flexors, but today, we’re going to be focusing on the deep flexors, which is, there, found under the superficial flexors.
Now, all of these muscles, these superficial and deep flexors, they’re found on the ventral forearm. That’s very important to know. And the other thing that you need to know is that they run under the flexor digitorum superficialis, very closely to the radius and ulna, and for that reason, they are difficult to palpate. You can palpate the superficial layer or the superficial flexors, but the deep flexors are, of course, covered by a superficial layer. For that reason, it’s quite difficult to feel them on your forearm.
Now, the deep flexors are a group of three muscles, keep that in mind, and the three muscles that we’re going to be covering their origins, insertions, and innervation on this tutorial are the flexor digitorum profundus, the flexor pollicis longus, and finally, we’re going to look at the pronator quadratus.
So first on our list of muscles that we’re going to discuss is the flexor digitorum profundus. And every time you’re asking about what is the function of a certain muscle, what does it do in your body, always look at the name. It might help you out, figuring out the function of a certain muscle. And this is a perfect example. Flexor means that this muscle is going to be flexing, but what is going to be flexing? The fingers, because “digitorum” means “fingers,” and “profundus” is “deep.” So this is the deep flexor of the fingers. And this muscle is also considered an extrinsic muscle of the hand because it acts on the hand itself as flexing the fingers, but it has its muscle belly outside the hand so on the forearm as you can see here. That’s why we call it an extrinsic muscle of the hand.
Now, in terms of origin points, there are two that you need to remember. One, you can see right about here on the bone, this bone right here. This is the ulna, and more specifically, the anterior surface of the ulna, which will serve as origin point for the flexor digitorum profundus. Another area that will also serve as an origin point is this membrane that you see here on the forearm. This is known as the interosseous membrane, and it will also serve as an origin point for the flexor digitorum profundus.
Now, let’s move on to the insertion point. You can see that there are several because its four tendons run through the carpal tunnel as you can see here. It’s running through the carpal tunnel and between the split end tendons of the flexor digitorum superficialis at the height of the middle phalanges. And then distally, as you can see right on the tips of… almost at the tips of the fingers, on the distal phalanges of the second to fifth fingers, it is where this muscle is inserting on. So the distal phalanges of the second to fifth fingers will serve as insertion point for the flexor digitorum profundus.
The third thing that we need to remember about the flexor digitorum profundus is its innervation. And it’s important to remember, as this muscle is spreading all over the hand and all over the forearm, we need to remember that it needs to be supplied by two—two—nerves. And these are the median nerve which will be supplying the second and third fingers, then another nerve, the ulnar nerve, which will be supplying from the fourth and fifth fingers.
Now, let’s move on to the second muscle on our list: the flexor pollicis longus. And looking at the name again, we can find out what this muscle is going to be doing. And it’s going to be flexing your thumb. Flexor, of course, meaning that the muscle is flexing and “pollicis” means “thumb,” so this is a flexor of the thumb. And it lies at the same plane as the flexor digitorum profundus as you can also see a little bit better on this image. That is lying... these two muscles are lying on the same plane.
And in terms of origin points, you need to remember two. One is happening here on this bone, as you can see. And this is the anterior surface of the radius, and it’s serving as origin of the flexor pollicis longus. Another one that we also saw for the flexor digitorum profundus, the interosseous membrane on the forearm that will serve as the origin point for the flexor pollicis longus. Now, sometimes, you also find another origin at the medial epicondyle of the humerus.
Now, let’s move on to the insertion point of the flexor pollicis longus. And as you can see, its tendon—notice here that the tendon runs through the carpal tunnel and inserts at the palmar side, right about here, the palmar side of the distal phalanx of the thumb.
Now, moving on to the last part of the flexor pollicis longus, also the innervation of this muscle, this muscle is going to be supplied by one nerve this time, and this is the median nerve.
Now, our list of deep flexors of the forearm is quite short—only three muscles. And now we’re ready to move on to the last one on the list: the pronator quadratus that can be seen here highlighted in green on both of these images. Now, you can see that they look quite different. So the muscle looks slightly different on both of these images, because on the left one, you see it on the functional position or the functional view of the muscle, while on the right one, it’s relaxed. The muscle is relaxed here.
Now, I want to add a few pieces of information that are quite important to know about the pronator quadratus. One is that it’s found on the distal portion of the forearm, as you can clearly see here on these images. And looking at the name again, it acts to pronate the forearm. It’s a pronator, so meaning that it will turn the forearm so you’re your palm faces downward.
Now, in terms of positioning of this muscle, this is the deepest muscle of the forearm, especially on the ventral side. This is the deepest of all the flexor muscles. Now, the muscle has a squared shape. It’s true. And it happens because of its origin and insertion. Now, the origin point is going to happen on the anterior surface of the ulna. And then the muscle is going to extend horizontally and attach to the neighbor bone, which is the radius. So it’s going to insert on the anterior surface of the radius.
Moving on to the last portion of the pronator quadratus, the innervations of this muscle, and you only need to remember that the pronator quadratus is supplied by the median nerve.
So I would like to use this tutorial to add an important note about the innervation of the deep flexors of the forearm. Let’s say this is a summary and a reminder of what we talked about throughout this tutorial. And one thing that you know is that all flexors or almost all flexors of the forearm are supplied by this nerve that you see here highlighted in green, the median nerve. But there is an exception in terms of the deep flexors of the forearm. The flexor digitorum profundus muscle receives what is called as a double innervation though the median and ulnar nerves—very important thing for you to remember.
Throughout this tutorial I mentioned a few times about certain structures that pass through the carpal tunnel. And I wanted to remind you that, at the wrist, the tendons of the flexor digitorum profundus and flexor pollicis longus run through the carpal tunnel, this is a reminder
And if you know, the carpal tunnel is none other than the passage that is formed by the carpal bones dorsally. And then on the anterior side you find a tight densification of the antebrachial fascia, this portion here highlighted in green, known as the flexor retinaculum, which forms this passage where all these structures are going to go through.
Now, along with these tendons, the carpal tunnel also contains the median nerve. And another important structure that passes through the carpal tunnel is the tendons of the flexor digitorum superficali,s which we talked about also on another tutorial where we covered the superficial flexors of the forearm.