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Functions of the fibularis longus muscle

Functions and anatomy of the fibularis longus muscle shown with 3D model animation.

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Today, we’re going to be talking about one of the leg muscles which is especially important for those of you who enjoy running to keep fit. It’s one of the muscles which helps to lift your heel off the ground during push-off and then keep the sole of your foot in the correct position to make sure it lands optimally onto the ground again on landing. What muscle could I be talking about?

Since you saw the title of this tutorial when you clicked play, you already known the answer to that question. That’s right. Today, we’re talking about the functions of the fibularis longus muscle.

As always, before we learn about the various functions of any muscle, let’s take a few moments to remind ourselves of its anatomy. Many of you know that the fibularis longus muscle is also sometimes referred to as the peroneus longus which comes from the Greek word, perone, which has the same meaning as the Latin word fibula which means “pin or clasp”. Both terms are perfectly fine to use, however, we’ve noticed the term fibularis longus seems to be the more common of the two in most textbooks as well as the official name listed in the Terminologia Anatomica.

The fibularis longus muscle belongs to the lateral compartment of the leg, which contains just one other muscle, the fibularis brevis muscle. These muscles are also known as the evertors of the foot, but we’ll discuss more about what that term means in just a short while.

As its name suggests, the fibularis longus is the longer of the fibularis muscles as well as the more superficial of the two. It also arises more proximally along the shaft of the fibula compared to its smaller brother, the fibularis brevis. The fibularis longus muscle has its proximal attachment or origin from the head of the fibula as well as from the proximal two-thirds of the lateral aspect of its shaft. The long muscle belly continues almost the entire length of the leg where it then tapers off as a long tendon which wraps around the lateral malleolus of the fibula in a pulley-like groove known as the lateral malleolus sulcus, which it shares with the tendon of the fibularis brevis.

The tendon of the fibularis longus then continues along the lateral aspect of the calcaneus bone below a small prominence known as the fibular trochlea. It then continues within another well-defined groove located on the infralateral surface of the cuboid bone which guides the tendon onto the plantar aspect of the bone. From here, it then crosses the plantar aspect or the sole of the foot where it attaches to the lateral aspects of the medial cuneiform and the first metatarsal bones.

Like all muscles, the fibularis longus needs a source of innervation to tell it when to contract and to do its job. In this case, our nerve of interest is the superficial fibular nerve which is the branch of the common fibular nerve.

Now that we’re familiar with the origin and insertion points of the fibularis longus muscle, we can identify which points this muscle acts up on. The first is the ankle joint, also known as the talocrural joint, which is formed by the articulation of the medial and lateral malleoli of the tibia and fibula where the superior surface of the talus bone.

The next joint of interest is the subtalar joint, also known as the talocalcaneal joint, which as this name suggest is located under the talus at the articulation of the talus and the calcaneus bones.

And finally, as the long tendon of the fibularis longus reaches right across the plantar surface of the foot, it also has some effect on the small joints between the tarsal and the metatarsal bones, which are known as the intertarsal and tarsometatarsal joints.

Now that we are familiar with the morphology of the fibularis longus muscle, let’s now turn our attentions to its functions, several of which are extremely important for the basic movements for proper gait or walking. As I mentioned earlier, the fibularis longus as well as its smaller brother, the fibularis brevis, are known as the evertors of the foot. So with that in mind, our first function of interest here is going to be eversion of the foot, which occurs at the subtalar joint.

In general, this involves the tilting of the sole of the foot away from the midline. As you can see in the animation, from the anatomical position, eversion of the subtalar joint is a relatively limited movement of about twenty degrees at most with the lateral malleolus of the fibula being the primary limiting element for further movement.

More commonly however, this function of the fibularis longus is less about elevation of the lateral border of the foot as we just demonstrated, but rather to prevent or correct excessive inversion. This basically means that from an inverted position, the fibularis longus works to move the subtalar joint back to the neutral position where it is less prone to injury and leaving the foot more suited for shock absorption. This is extremely important in activities such as running and jumping. It also helps to transfer body weight more from the lateral to the medial side of the foot and even towards the opposite foot when walking.

Damage to the tendon of the fibularis longus or to the superficial fibular nerve can result in an oversupinated foot where instead of your foot sitting like this, they look more like this where your weight rolls onto the lateral edges of your feet. This condition can sometimes occur in runners, for example, and causes reduced shock absorption which can lead to pain in the feet and the lower leg as well as increased susceptibility to ankle sprains.

The fibularis longus also assists the large posterior muscles of the leg in plantarflexion of the foot at the ankle joint drawing the heel upwards towards the posterior leg which causes the toes to point downwards, which as I mentioned at the beginning of this tutorial, happens during the toe-off phase when walking or running.

The last function which I’d like to discuss with you now involves the role of the fibularis longus in support of the transverse and lateral longitudinal arches of the foot. As you can see on our 3D model, the tendon of the fibularis longus muscle forms a sort of a transverse strap or sling across the plantar aspect of the foot. In the case of the transverse arch, the fibularis longus works to pull the medial border of the transverse arch towards the lateral border, promoting the concave form of the arch.

When we stand in our tippy toes or during the toe off phase of walking or gait, the fibularis longus also comes strongly into action helping to maintain the lateral longitudinal concavity of the foot.

So next time you’re toe standing doing the jailhouse rock or even your best Jacko impersonation, remember that your fibularis longus is working hard to keep your arches while doing it.

And that’s it - the three main functions of the fibularis longus. Let’s review them once again before we finish.

The first function of the fibularis longus which we mentioned was eversion of the foot which we learned involves a lateral tilt of the foot at the subtalar joint. The second movement spoke of the role of the fibularis longus in assisting the large posterior muscles of the thigh with plantarflexion of the foot which of course occurs at the ankle joint. And the third and final function of the fibularis longus which we spoke about concerns its very important role in support of the transverse and lateral longitudinal arches of the foot especially when the foot is plantarflexed.

And that concludes our tutorial for today. Be sure to check out our other 3D muscle function videos on our website and I look forward to seeing you next time!

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