Imagine that you and your friend are walking along a beautiful beach somewhere wonderfully tropical and perfect. You're taking in the sea air enjoying the feeling of the warm fine sand between your toes. Looking back to see your trail of footprints in the sand, you notice something odd. Your friend's footprints are perfectly formed and beautifully shaped. Yours, however, look somewhat more rounded, kind of blobby and, well, not so nice. Why is this? What's happened to your feet? Curious? Stay with me and I'll explain a possible reason for this and a lot more as we learn about the functions of the tibialis posterior muscle.
Before we explore the different functions of the tibialis posterior, let's first remind ourselves of its anatomy. As you can see on the screen, this muscle is found in the deepest portion of the posterior compartment of the leg between the two bones of the leg which are the tibia and the fibula. At its proximal end, the tibialis posterior has three attachment sites. It originates from the posterior surface of the tibia, the posterior surface of the fibula, and the interosseus membrane of the leg that lies between the tibia and fibula.
Moving down to the distal end of the leg, we can see that the tendon courses through a groove posterior to the medial malleolus of the tibia along with the tendon of another muscle – the flexor digitorum longus. This is an important landmark as this groove, the medial malleolar sulcus, acts as a pulley and is necessary for the functions of the tibialis posterior to be carried out.
From the medial malleolar sulcus, the tendon passes to the plantar aspect of the foot where it attaches to several bones, initially, the navicular and the medial cuneiform bones before proceeding laterally to the intermediate and lateral cuneiform bones as well as the second through fourth metatarsals. It may also give an attachment to the cuboid bone.
Looking at these attachments is important as it allows us to see which joints will be involved with the movement of this muscle and, in this case, we have two primary joints which the tibialis posterior works on. The first is the talocrural joint commonly known as the ankle joint. This is a hinge joint formed by the malleoli of the tibia and fibula which articulate with the talus bone of the foot. The other joint we need to be aware of here is the subtalar joint which is also known as the talocalcaneal joint as it involves the talus and calcaneus bones. It's worth mentioning that the tibialis posterior also works on several other small joints located between the tarsal bones of the foot which are known as the intertarsal joints.
Like all muscles, the tibialis posterior needs some innervation to tell it how to behave and here our nerve of interest is the tibial nerve which is derived from the L4 and L5 roots of the lumbar plexus. The tibial nerve is the larger of the two main branches of the sciatic nerve.
Now let's turn our attention to the functions of the tibialis posterior. The first and perhaps most commonly known function of the tibialis posterior is inversion or supination of the foot which effectively means to roll the plantar surface or the sole of your foot towards the midline. Let's take another look at this movement from a posterior perspective to get a better view.
As you can see with this particular movement, the tibialis posterior acts primarily on the subtalar joint, however, there's also movement at several other intertarsal joints such as the talonavicular and calcaneocuboid joints. Full inversion of the foot can obviously only occur when the foot is off the ground, however, when the foot is on the ground, dynamic contraction of the tibialis posterior also plays a role in the support of the medial longitudinal arch of the foot during weightbearing. Effectively, this means the tibialis posterior helps to hold up the medial arch of the foot as weight is applied down on it during activities such as walking, running and jumping.
While standing especially on one foot, the tibialis posterior muscle is also involved with depressing the lateral side of the foot and pulls medially on the leg as needed to counteract the lateral leaning for balance. Injury to the tibialis posterior tendon caused by a fall, chronic overuse, or even obesity can cause the medial arch to collapse resulting in what is commonly known as flat foot which may leave you with those big wide footprints just like we say at the beginning of our tutorial. This occurs when damage to the tendon causes it to be stretched which the tibialis posterior can't compensate for. Failure of the tibialis posterior muscle can also cause overpronation of the foot, similar to what you can see in this image.
Finally, there's one last function I want to mention before we finish off and that function is plantarflexion of the foot at the ankle or talocrural joint causing the foot to flex downwards and bringing the toes away from the leg. Although the tibialis posterior is not a major plantarflexor like the overlying gastrocnemius or soleus muscles, it is important in stabilizing the foot during plantarflexing actions such as walking on your tippy toes.
And with that, we've reached the end of our tutorial for today, but before I leave you, let me quickly recap the functions of the tibialis posterior muscle.
We began with inversion or supination of the foot at the subtalar joint which we saw involved turning the plantar aspect or sole of the foot towards the midline. After that, we spoke about the role of the tibialis posterior in supporting the medial longitudinal arch of the foot as well as what happens when this fails. And, finally, we finished with a brief note on how the tibialis posterior assists with plantarflexion of the foot at the ankle joint.
So that brings us to the end of our tutorial on the functions of the tibialis posterior muscle. I hope you've enjoyed the tutorial and, if so, don't forget to go to our homepage and subscribe for more videos like this one.