Video: Flexor hallucis longus muscle (3D)

Hey there! Have you ever wondered what it takes to bend your big toe and try to crumble up a towel with it? You just need a small amount of patience and a great amount of interest in anatomy in order to find out more about this seemingly inconspicuous movement.

The muscle allowing you to do this is the flexor hallucis longus, and in this tutorial, we'll learn all about its functions. You can see it now isolated for you on the screen. If you look closely at its location, you can see that the flexor hallucis longus is located quite deep at the back of your leg with several muscles covering it. Therefore, it shouldn't be a surprise to find out that it is part of the deep muscle group of the posterior compartment of the leg. The other group which is part of this compartment is the superficial group of muscles and they're the ones covering the flexor hallucis longus.

Let's return to the flexor hallucis longus to find out more about it. You can see it again isolated on the screen in all its glory. To help you bend your big toe and allow you to perform the functions you'll hear about, the flexor hallucis longus needs a nerve supply. This muscle is innervated by the tibial nerve which you can see right now highlighted on your screen. The tibial nerve is one of the terminal branches of the longest nerve of the human body which is the sciatic nerve. It's always useful to note the root values of the nerve associated with the muscle we're learning – in this case, the root values of the tibial nerve, specifically, its muscular branches to the flexor hallucis longus – the S2 and S3.

Before diving into the heart of this video, firstly, let's take a closer look at the attachments of the flexor hallucis longus. I'm sure you're already aware of the idea that form follows function. So if you understand where a muscle begins and ends, you can mostly likely deduce what it does.

According to our friend on the screen, we can see that the flexor hallucis longus originates here along the inferior two thirds of the posterior surface of the fibula as well as the adjacent interosseus membrane. In this journey, the flexor hallucis longus passes posterior to the distal end of the tibia and travels through a groove located on the posterior aspect of the talus bone. From here, it crosses onto the calcaneus bone and passes through another groove located under the sustentaculum tali or talus shelf of the calcaneus. The tendon then continues along the length of the plantar aspect or the sole of the foot before it inserts into the base of the distal phalanx of the great toe. It passes between two sesamoid bones inside the tendons of the flexor hallucis brevis which protect the tendon of the flexor hallucis longus.

Now that the hard part is over and we've seen the origins and insertions, I'm sure it's very easy for you to identify the joints moved by the flexor hallucis longus and, more importantly, the directions of those movements. The first and most obvious joint is the talocrural joint more commonly known as the ankle joint which you can see now highlighted for you on the screen. It's the point where the distal ends of the tibia and fibula meet the superior part of the talus.

The second joint is the subtalar or talocalcaneal joint which is formed at the junction between the talus and the calcaneus which are two bones of the foot. A third joint affected by the flexor hallucis longus is the metatarsophalangeal joint of the great toe. It is formed by the union of the first metatarsal and proximal phalanx of the big toe itself.

While we're at the hallux – which is another name for the great toe – let's have a look at the fourth and last joint moved by the flexor hallucis longus. It's this joint over here – the interphalangeal joint – which is formed by the union of the proximal and distal phalanges of the big toe.

Now that we know what joints are involved, let's have a look at what movements are produced by the flexor hallucis longus at these four joints.

The muscle is responsible for toe flexion around the metatarsophalangeal and the interphalangeal joints of the great toe, plantarflexion of the foot around the ankle joint, 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.

The first and most important function the flexor hallucis longus muscle is involved in is toe flexion which happens around the metatarsophalangeal and interphalangeal joints. In order to see it, we'll ask our friend on the screen to give us a hand and to wiggle his great toe, but before he does, we'll take the very best front seats so we have a close view of the action and not miss anything.

When your foot is off the ground and you wiggle your big toe, the flexor hallucis longus contracts during the movement to bring the hallux downwards, otherwise called flexion, as you can see now being demonstrated on the screen. Why would you want to wiggle only your great toe anyway? Actually, the importance of flexion of the big toe extends to walking and the gait cycle. Although this action is second nature to you and feels intuitive, every little detail has to be in place so you don't fall flat on your face. 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 hallucis longus contracts like you see now, the hallux is curled to keep a firm contact with the ground. A similar situation happens when the flexor digitorum longus muscle contracts curling the other four toes.

Essentially, the great toe grips onto the floor by working in harmony with the flexor digitorum longus muscle which acts on the other four toes, these muscles help provide you with stability and prevent you from falling especially on uneven surfaces. You might have to wave goodbye to what this man is doing if your toes were not helping to grip the ground you're walking on.

Let's continue now with plantarflexion, the second movement that the flexor hallucis longus is involved in. In order to see this movement which happens around the ankle joint, we'll tell our friend here to rotate slightly so we see a posteromedial view of the leg. As the name itself suggests, plantarflexion involves the movement of the foot in the direction of the plantar side of the foot. You probably know it more simply as the sole or the underside of the foot. Therefore, when the flexor hallucis 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 to take a peek into your neighbor's garden.

Although it is a plantarflexor, the flexor hallucis longus is not the plantarflexor – meaning, it is not the primary contributor to this movement. In fact, it provides a relatively small amount of force to this action. It rather works to assist the larger muscles of the posterior compartment namely the gastrocnemius and soleus muscles to plantarflex the foot around the ankle joint.

So, there we go, two movements that the flexor hallucis longus is involved in – flexion of the great toe and plantarflexion. Now, let's move on to the third and final function of this muscle. We're almost done here.

Foot inversion, also called supination, involves the movement around the subtalar joint in the direction of the midline. In order to see this movement, we'll annoy our friend over 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 hallucis longus assists in titling the foot towards the midline, like you can see happening right now on your screen. Let's see a closer view of the action 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 we need it for? Well, you can think of it as a protection mechanism for your ankle. The 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.

While the flexor hallucis longus is a weak invertor and only assists with this movement in coordination with the other invertors and evertors of the foot, it still helps you to stay balanced when walking or balancing on one foot. It does this by constantly correcting the position of your foot to help present your line of gravity. Thanks to muscles like your flexor hallucis longus, you balance perfectly and don't fall flat on your face.

So, there you have it, the functions of the flexor hallucis longus muscle.

Before we bring this to an end, let's quickly recap what we've learned and see the movements one more time.

The flexor hallucis longus acts on four joints – the ankle, the metatarsophalangeal joints, the interphalangeal joint, and the subtalar joint. We've seen that it is capable of producing great toe flexion at the metatarsophalangeal and interphalangeal joints by pulling the hallux backwards, like when you're wiggling or toe or when you need to grip the ground such as during a walk or when standing. This muscle is also capable of weakly contributing to the plantarflexion of the ankle by moving the foot backwards towards the sole of the foot.

Finally, we saw that the flexor hallucis longus can assist other muscles in pulling 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 wiggling your great toe could be so complicated?

I hope you enjoyed watching. Please be sure to check out our other 3D muscle function videos and lots more at kenhub.com. See you next time!