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Functions and anatomy of the extensor digitorum longus muscle shown with 3D model animation.
Have you ever wondered what it takes to wiggle your toes? Probably not, as you mostly use this movement mainly when you lie down on your bed and are extremely bored or to show off your sparkly new footwear to your fashionista best friends. Although this video wasn’t made to make you a star on the catwalk, it will teach you something about some of the muscle movements of your toes. Specifically, the functions of the extensor digitorum longus muscle.
If you take a sneak peek at the model on the screen, you can easily see and clearly identify this muscle from an anterior perspective. This means that the extensor digitorum longus is located superficially on the anterior or ventral side of the leg. As a result, it’s part of the anterior compartment of the leg and it’s also the most lateral muscle out of the muscles in this compartment.
Since anatomy experts enjoy confusing students such as yourself, this compartment has several alternative functional names. You can refer to it as the dorsiflexor, or less commonly, the extensor compartment of the leg, because muscles located here are responsible for dorsiflexing or extending the foot. In order to proudly showcase your new shoes, there needs to be a sense of excitement. While I’m sure this will easily come to you, the only thing getting excited in the world of anatomy are nerves, whose job it is to stimulate our muscles.
The nerve responsible for causing the extensor digitorum longus to contract is the deep fibular nerve, also known as the deep peroneal nerve, which is now highlighted for you on the screen. Its fibers are derived from the fourth and fifth lumbar segments of the spinal cord with most of the muscles innervation coming from L5.
As you probably know by now, every muscle in your body works hard to pull and tug on the bones that it is attached to, ultimately, moving them. Therefore, if you can get your head around the muscle’s origins and insertions, figuring out the movements that it’s involved in will be child’s play.
As you can see on the screen, the extensor digitorum longus originates in three places. Most of it begins from the proximal three-quarters of the medial surface of the fibula and the superior portion of the anterior surface of the interosseus membrane of the leg which is this portion right here. A small part of the muscle also originates from here – the inferior surface of the lateral condyle of the tibia.
As the extensor digitorum longus travels inferiorly down the leg, it becomes tendinous just superior to the ankle, round about here. The muscle then slips underneath the superior and inferior extensor retinacula of the foot, subsequently dividing into four slips of tendon that travel distally down the dorsal aspect of the foot continuing towards the second to fifth toes. Each tendon terminates by inserting into both the middle and distal phalanx of each digit.
Now that we’re finished with the attachments of the extensor digitorum longus, I’d like to take a moment to explain another muscle associated with the extensor digitorum longus. If you look closely at the model on the screen, you’ll notice that right next to the extensor digitorum longus is this muscle which is the fibularis tertius. I’m going to isolate this muscle from the extensor digitorum so that we can give it our full attention for just a moment.
This fibularis tertius is considered a separate part of the extensor digitorum longus, commonly described as the latter’s fifth tendon. The origins of the fibularis tertius are the same as those of the extensor digitorum longus. The pathways of the two muscles towards the foot are also the same and upon reaching the ankle, they share a common synovial sheath that travels underneath the inferior extensor retinaculum. However, they differ in their insertions as the fibularis tertius attaches to the dorsal aspect of the base of the fifth metatarsal rather than the phalanges.
Now that we know where the extensor digitorum longus begins and ends, let’s take a moment to look at what joints this muscle acts upon. The joints in question here are the ankle joint, the subtalar joint, and the metatarsophalangeal joints of the lateral four toes. The muscle also acts on the interphalangeal joints of the same lateral four toes but very insignificantly, so I’ll only mention these for completeness.
The ankle joint, also called the talocrural joint, is the connection point between the distal ends of the tibia and fibula and the superior part of the talus. Just below that, we have the subtalar joint, also sometimes referred to as the talocalcaneal joint, which correctly implies an articulation between the talus and the calcaneus bones of the foot.
Moving onto the metatarsophalangeal joints of the second to fifth toes, you can see those right here, highlighted for you on the screen. They’re located between the metatarsals and the proximal phalanges of their respective toes, hence, the name, metatarsophalangeal. Sometimes, anatomy isn’t so difficult.
Now that we have all the building blocks, let’s put them all together and build together the movements that the extensor digitorum longus is involved in. The first and most important one involves extension of the lateral four toes while the second one is dorsiflexion of the ankle joint. Finally, the third movement which we’re going to explore today is eversion of the foot.
Let’s take each movement one by one and see what’s the deal with each of them.
The first movement I’m going to explore with you is an extension of the lateral four toes. Every time you extend your second to fifth toes to stretch or perform some activity, this movement happens around the metatarsophalangeal and interphalangeal joints. Extension of the toes involves the motion in an inferior to superior direction, decreasing the angle between the toes and the dorsum which is commonly known as the top part of your foot. Therefore, when your toes are flexed and your extensor digitorum longus muscle contracts, it pulls your second to fifth toes upwards back into a neutral position. If this movement continues because you decided to stretch your toes even more, the lateral four toes are pulled even more superiorly, further reducing the angle between them and the dorsal part of the foot. This specific exaggerated extension is also called hyperextension or dorsiflexion of the second to fifth toes.
As you might imagine, the idea of extending your toes can be quite strange but if you think about it, it makes a lot of sense. First of all, you probably know that muscles work as antagonistic pairs performing opposing actions. In simple terms, if you did not have an extensor digitorum longus, your toes would end permanently flexed or curled up like this. Yes, it’s all very cute and all but it’s not the most useful or functional position for performing your daily activities, is it?
Your toes also play a crucial role in both balance and forward motion. As we push off the ground, our toes extend or dorsiflex, just as you can see here. In cooperation with the extensor hallucis longus, this extension of our toes pulls the plantar fascia tight, stabilizing the longitudinal arches of your foot. This allows greater forward propulsion by creating a rigid lever during the push off phase of walking or running. In addition, the stiffening of the plantar fascia caused by extension of the toes provides improved shock absorption when the foot hits the ground later on.
So there you have it, the first and most significant action of the extensor digitorum longus – extension of the lateral four toes.
Now, let’s see the second movement that this muscle is involved in – dorsiflexion. This action involves moving the foot from an inferior to superior position towards the dorsum of the foot which is simply known as the back or the top part of the foot. In anatomy jargon, you’re flexing your foot towards its dorsum hence the name dorsiflexion. Since our friend on the screen is so patient with us, let’s take advantage of him a bit more. I am sure he won’t mind at all.
In your day-to-day life, you mostly have your foot on the ground in a neutral position, so we’ll start from here. Therefore, when you decide to dorsiflex the foot, the angle between the foot and the anterior portion of the leg is reduced bringing them closer together.
When do you need to dorsiflex your ankle? Surprisingly, on a daily basis. For example, your ankle passes through phases of dorsiflexion when you walk or run, or when you’re on a hill or an uphill road. During climbing, dorsiflexion allows you to lean forward and tip your center of gravity so that you don’t fall backwards. That would certainly be a bad day for this guy.
In terms of dorsiflexion, there’s a very important aspect that you need to be aware of. The extensor digitorum longus is not the most powerful dorsiflexor of the anterior compartment of the leg but rather assists the larger tibialis anterior to perform this action.
Last, but not least, the third function of the extensor digitorum longus which we mentioned before was eversion of the foot which occurs at the subtalar joint. This movement is articularly relevant when examining the fibularis tertius muscle which I introduced to you earlier. When the foot is in a neutral position, eversion involves the titling of the foot outwards or away from the midline. As you saw in the animation, it is a relatively limited movement due to this bony obstruction here, which is the lateral malleolus of the fibula.
Why do we need to evert our feet, you ask? Well, eversion occurs more than you might think. For example, during the midstance phase of walking or running, our evertors work to help us keep balanced, as you can see here. They also help to protect our ankle joints by assisting an inverted foot return to an optimal position before hitting the ground again when running.
So, there you have it – the functions of the extensor digitorum longus muscle. It wasn’t that bad, was it?
Before parting our separate ways, let’s recap what we’ve learned about the extensor digitorum longus.
In terms of movements, the extensor digitorum longus is capable of producing extension and even hyperextension of the second to fifth toes by pulling them upwards around the metatarsophalangeal and interphalangeal joints. In addition, it also contributes to dorsiflexion by helping other powerful muscles to move the foot towards its dorsum part around the ankle joint. And, finally, the extensor digitorum longus along with the fibularis tertius helps to evert the foot which occurs at the subtalar joint. Luckily for you, the next time you wiggle your toes, you’ll see more than four small toes moving back and front.
I hope you enjoyed this tutorial about the functions of the extensor digitorum longus muscle. See you soon and all the best!