Video: Flexor digitorum longus muscle (3D)

Hello everyone! Before we put our anatomy learning caps on, let me ask you something. What do you visualize when you think about the functions of a muscle? You're probably thinking about a red color bundle of fibers contracting or relaxing to result in some sort of movement, right? Well, when I think about the functions of muscle, I think about a pheasant, two feet and a dog – just like the ones you can see on your screen. What do these three images have in common?

At first, it might be difficult to see the connection but after you get past the dog's gracefulness and the pheasant's sense of fashion, you can observe that they're all curling and standing on their toes. One muscle contributing to these actions in humans is the flexor digitorum longus and in this tutorial, we'll learn all about its functions. But before we dive into the nitty-gritty, let me show you the muscle first and put it into context.

You can now see the flexor digitorum longus muscle highlighted on your screen in all its glory. As you can see from its location, this muscle is part of the posterior compartment of the leg. This compartment is in turn divided into two muscle groups – a superficial one and a deep one. The muscle in question – the flexor digitorum longus – is part of the deep muscle group which is situated below the superficial subcompartment. So, how important are the muscles of the posterior compartment including the flexor digitorum longus?

In short, very. Without them, you won't be able to walk, run, jump or balance yourself on your toes when reaching up to grab something off the top shelf. Now, let's get back to the flexor digitorum longus and find out a bit more about it.

For a muscle to perform its functions, it needs to know action you want to perform, how to do it and when. This type of information is carried by nerves and muscles are innervated by many of them. The flexor digitorum longus is no exception. It's innervated by the tibial nerve which you can now see on our 3D model. The tibial nerve is one of the terminal branches of the longest nerve in the human body – the sciatic nerve. It's useful to note the root values of the nerve associated with the muscle we are learning – in this case, the root values of the tibial nerve. Specifically, it's muscular branches to the flexor digitorum longus which are S1 and S2.

If you were to look for the flexor digitorum longus, you would find it in a very specific location inside the deep subcompartment of the posterior leg. It lies medial to the flexor hallucis longus and underneath the soleus which is one of your calf muscles. The muscle sits on the tibia and the tibialis posterior muscle so it really does lie quite deep inside your leg.

If we look at our friend on the screen, we can see the attachments of the flexor digitorum longus muscle. Firstly, it originates from the highlighted area over here which is the medial part of the posterior surface of the tibia. After passing inferiorly towards the medial malleolus, it curves around it, wraps underneath the foot then inserts onto the bases of the distal phalanges of the lateral four digits which are now highlighted for you on the screen. In other words, these are the second, third, fourth and fifth toes of your foot.

The flexor digitorum longus muscle doesn't work alone. There's a muscle on the sole of your foot called quadratus plantae which you can now see on your screen that inserts into the tendon of the flexor digitorum longus right over here. For now, I'll only tell you that the quadratus plantae helps flexor digitorum longus to pull the toes backwards but I'll return to this later on in the tutorial.

As I'm sure you are aware, knowing the attachments of a muscle is half the journey on the way to understanding its function so we're almost there. If you look at the origin and insertion of the flexor digitorum longus, I'm sure you can name the joints moved by this muscle. One is the talocrural more commonly known as the ankle joint, the second is the subtalar joint also known as the talocalcaneal joint, the third are the metatarsophalangeal joints of the second to fifth toes which are quite a mouthful to pronounce, and finally, the fourth are the interphalangeal joints located between the phalanges of the same toes.

The ankle joint which you can see here is the point where the distal ends of the tibia and fibula meet the superior part of the talus which is one of the bones of the foot. This joint is a synovial hinged joint meaning it can only move in a single plane of motion. In the case of the ankle, the possible movements are up called dorsiflexion and down called plantarflexion.

The subtalar joint also called the talocalcaneal joint is formed at the junction between the talus and the calcaneus which are two bones of the foot. It's also a synovial joint similar to the ankle allowing movements of the foot in one plane only towards the midline called inversion and away from the midline called eversion.

The metatarsophalangeal joints which are highlighted for you on the screen are formed at the meeting point between the metatarsals of the foot and the proximal phalanges. The interphalangeal joints on the other hand are of two types – the proximal ones which are between the proximal and middle phalanges of the foot and the distal ones which are between the middle and distal phalanges.

Now let's take a closer look at the movements of the ankle, subtalar, metatarsophalangeal and interphalangeal joints that the flexor digitorum longus is involved in.

This muscle is responsible for plantarflexion of the foot around the ankle joint, toe flexion around the metatarsophalangeal and interphalangeal joints, and foot inversion around the subtalar joint. So, there are three movements in total. Let's take these movements one at a time and understand what's happening with the help of our model here on the screen.

In order to see plantarflexion, we'll tell our friend here to rotate so we see a posteromedial view of the leg. As the name itself suggests, plantarflexion involves the movement of the foot towards the plantar side of the foot. It's also known more simply as the sole or underside of the foot. Therefore, when the flexor digitorum longus contracts like it does now, the foot is pulled or flexed backwards – an action that you do every time you jump or stand on your toes in order not to be blocked by your uncle's head in every family photo.

It's important to realize that the flexor digitorum longus is not the only plantarflexor of the foot. This muscle works together with other ones especially the large and powerful superficial muscles of the posterior compartment of the leg to flex the foot around the ankle joint.

Let's continue now with the second movement that the flexor digitorum longus is involved in but this time around the metatarsophalangeal and interphalangeal joints.

In order to see toe flexion, we'll ask our friend on the screen to give us a hand and to wiggle his toes. But before he does, we'll rotate our model slightly so we have a full view of the action and don't miss anything.

When your foot is off the ground and you wiggle your toes, the flexor digitorum longus contracts during the movement to bring the second to fifth toes downwards, otherwise called flexion as you can see now being demonstrated on your screen. Wiggling only your lateral four toes is a nice party trick but it doesn't compare to the importance of toe flexion. For example, when you walk, your foot is on the ground under the load of your own body weight. In this position when the flexor digitorum longus contracts like you see now, the toes are curled to keep a firm contact with the ground. Essentially, they grip onto the floor providing you with stability and preventing you from falling especially on uneven surfaces.

I told you previously that the quadratus plantae muscle helps the flexor digitorum longus muscle to flex the toes. On its own, the tendency of the flexor digitorum longus would be to pull the toes and foot in the medial direction. However, by inserting into the tendon of flexor digitorum longus, the quadratus plantae aligns the pull in such a way that it takes place in a straight line rather than medially resulting in a flexion of the toes straight backwards.

So there we go, two movements that flexor digitorum longus is involved in – plantar and toe flexion. Now, let's move on to the third and final function of this muscle.

As you are aware of by now, foot inversion, also called supination, happens around the subtalar joint. In order to see this movement, we'll annoy our friend here some more by asking him to rotate slightly so we see a posteromedial view of the leg. In this position, contraction of the flexor digitorum longus brings the foot towards the midline in a movement called inversion like you see happening right now on your screen.

Let's see a closer view of this action and from a different perspective. Here's the movement once again but looking down the axis of movement for foot inversion. Foot inversion is quite an odd movement so what do you need it for? You can think of it as a protective mechanism for your ankle. This joint is at its most vulnerable when the foot is forced sideways so inversion prevents excessive eversion by pulling the foot in the opposite direction. Together with other muscles, it also helps you to stay balanced when walking or balancing on one foot by constantly recentering your line of gravity. Thanks to muscles like your flexor digitorum longus, you balance perfectly like our friend here and don't fall flat on your face.

So there you have it, the functions of the flexor digitorum longus muscle. Before we bring the tutorial to a close, let's quickly recap what we've learned and see the movements one more time.

The flexor digitorum longus acts on four joints – the ankle, the metatarsophalangeal joints, the interphalangeal joints, and the subtalar joint. We have seen that at the ankle, this muscle is involved in plantarflexion by moving the foot backwards towards the sole of the foot. This muscle is also capable of producing toe flexion at the metatarsophalangeal and interphalangeal joints by pulling the second to fifth toes backwards like when you're wiggling your toes or when you need to grip the ground with your toes such as when walking or standing. Finally, we saw that flexor digitorum longus can pull the foot towards the midline in a movement called foot inversion. Here it is once again but from a different view.

Who would have thought that three pictures of a pheasant, two feet and a dog could begin your learning about the functions of the flexor digitorum longus? I hope you enjoyed this tutorial, thanks for watching and see you next time.