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Functions of the plantaris muscle

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

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Today, we're going to discuss the functions of a tiny muscle of your leg which some considered to not really have a function – a so-called vestigial muscle – which apparently holds no significant role in humans. But is this true? Stay with me and we'll try to get to the bottom of this mystery as we discuss the functions of the plantaris muscle.

The plantaris is quite an interesting muscle which is present in ninety percent of individuals. It is one of those muscles which we often overlook due to the fact that that it is relatively small, somewhat variable in its anatomy, and just a little enigmatic in its functions, or the lack thereof, some would say. But let's find out more, beginning first with the anatomy of the muscle.

As you can see in our 3D model, the plantaris is a relatively small muscle located in the posterior compartment of the leg. To be even more precise, we can describe it as one of the superficial muscles of the posterior compartment of the leg as it lies deep only to the larger overlying gastrocnemius muscle. As I mentioned, the plantaris can present some variation in its size and attachment points but let's take a look at what most would consider its most commonly observed morphology.

The origin, or proximal attachment of the plantaris, is most often located here towards the inferior end of the lateral supracondylar line of the femur which is adjacent or slightly superior to the origin of the lateral head of the gastrocnemius muscle. It also may present an attachment to the oblique popliteal ligament located posterior to the knee joint. The muscular belly of the plantaris is also quite variable in both its thickness and its length. Most often, however, it usually presents itself as a relatively small and thin tapering slip of muscle, perhaps equivalent in size to your middle or index finger.

From its muscular belly, the plantaris continues as a long thin tendon initially running between the gastrocnemius and soleus muscles. It proceeds in an inferomedial direction as it courses through the leg passing medially along the medial border of the gastrocnemius muscle before inserting directly into the posterior surface of the calcaneus bone of the foot. It may also indirectly insert into this location by first joining the adjacent calcaneal tendon.

The long tendon of the plantaris is, in fact, so fine and slender in appearance, it is sometimes confused as a nerve by some of the more junior anatomists during dissection. For this reason, you'll often hear your anatomy demonstrator refer to it as the freshman's nerve. Due to its slender nature, the tendon of the plantaris is prone to damage or failure especially during exercise or sporting activities such as tennis, soccer or running. This most commonly occurs to individuals in their forties and above due to natural degradation of tendon integrity and elasticity.

Rupture of the plantaris tendon is often experienced as a painful snap, often felt in the calf portion of the leg resulting in pain, swelling, and bruising of this part of the leg. As the plantaris muscle is often overlooked as a muscle of the posterior compartment of the leg, this injury is often misinterpreted as a tear or strain of one of the larger triceps surae muscles.

Now all muscles need some form of innervation even the small and less well-known ones such as the plantaris. This innervation is delivered to the muscle by the tibial nerve, which is known as one of the terminal branches of the larger sciatic nerve. Interestingly, the muscular branches of the tibial nerve to the plantaris are often shared with those supplying the lateral head of the gastrocnemius muscle, and if you're curious, the nerve root values for the branches of the tibial nerve to the plantaris muscle are S1 and S2.

Having now explored the anatomy of this unusual muscle, we can see that the plantaris crosses two joints, just like its big brother, the gastrocnemius muscle. These are, of course, the knee joint, formed by the articulation of the distal end of the femur with the superior articular surface of the tibia, as well as the ankle joint, also known as the talocrural joint, which is formed by the distal ends of the tibia and fibula with the superior surface of the talus.

Now to the part of this tutorial which we've all been waiting for. It's time to find out what exactly are the functions of the plantaris muscle.

As we just mentioned, the plantaris muscle has similar attachment points to the overlying gastrocnemius muscle and subsequently also works on the same joints. So, it should be no surprise that they also share similar functions. The first of these is weak flexion of the leg at the knee joint. And when I say weak flexion, I mean, weak flexion. Yes, some sources describe the plantaris as assisting in this movement, however, in reality, the contribution of the plantaris to flexion of the knee joint is almost negligible, relative to that produced by the much larger gastrocnemius and soleus muscles.

The second function of the plantaris muscle is weak plantarflexion of the foot occurring at the ankle joint. And, again, we are putting emphasis on the weak part of this statement. As you can see in the animation, plantarflexion is a movement generally described as the pulling of the posterior aspect, or heel of the foot, up towards the posterior leg causing the toes to point downwards towards the sole or plantar aspect of the foot.

So, flexion of the leg at the knee joint and plantarflexion of the foot at the ankle joint are what we could describe as the classic functions of the plantaris muscle. The issue with describing these two movements as defined functions is because with or without the plantaris muscle, we're perfectly able to bend or flex our knees and plantarflex our foot to stand on our tippy toes. So where does that leave us? If we can live without our plantaris muscles, could there be potentially be other functions that play here?

One proposed theory is that the plantaris may also act as a muscle of proprioceptive function. Proprioceptive function – what's that you ask? Well, proprioception is the unconscious awareness of our brain to the position of a particular body part or joints during motion. This is important as it helps our brain to protect our joints, muscles and tendons by monitoring the coordination of our movement. Some of this type of information is provided to the brain by proprioceptors found in skeletal muscle which transmit information regarding muscle length and tension.

Coincidentally, the plantaris muscle has been shown to be relatively densely populated with these proprioceptors suggesting that it may play a role as a spatial monitor of the triceps surae group and ankle joint. That being said, just like the movements of flexion and plantarflexion, this potential proprioceptive role of the plantaris is not an essential function of this muscle. This is due to the fact that no impairment of joint stability or function has been identified in either those who do not have a plantaris or in those who've injured this muscle.

To finish off functions of the plantaris, I'd like to explore a kind of unintentional clinical function of this muscle. In particular, it's a long, thin tendon. As we've learned, the plantaris muscle is not essential for any of the previous outlined functions which we mentioned, therefore, its removal has no effect on the lower limb function.

We also know that the tendon of the plantaris muscle is readily accessible due to its relatively superficial position within the medial posterior compartment of the leg. This makes it an ideal candidate for harvesting for use in reconstructive surgery procedures especially in the upper limb. Tendon autografts like these are useful considering they have similar biomechanical properties and a human body tends to accept its own tendons as opposed to those of a donor or synthetic alternatives. When harvested, the long tendon of the plantaris muscle can ideally serve up to three wrist-to-fingertip grafts or a single long forearm-to-fingertip graft when necessary.

And that's it! We've explored the functions of the plantaris muscle. Before I leave you, let's recap quickly the roles of the plantaris muscle. We first stated the two classic functions of the plantaris muscle which were assisting with the gastrocnemius muscle by means of weak flexion of the leg at the knee joint, in addition to also assisting with plantarflexion of the foot at the ankle joint. We then looked at another potential function of the plantaris which centered around a role as a muscle of proprioceptive function. And, finally, we spoke briefly about the use of the plantaris tendon in reconstructive surgery.

I hope you enjoyed our short tutorial today on this plantaris muscle. Please be sure to check out our other 3D muscle function videos on our website, kenhub.com, and until next time, thanks for watching!

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