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Peeling a banana with your feet is definitely not the most common party trick you’ve ever seen, probably for a good reason. And though I probably wouldn’t be so keen to eat a banana peeled by someone’s foot, I would be interested to know how on earth they can move their big toe around like that. What muscles in our foot might help us to make this happen? If you’re also keen to figure this unusual talent out, maybe we can find our answer when we study the functions of the medial plantar muscles in 3D.

So in today’s 3D muscle function video, we’re going to be looking at the functions of three plantar muscles of the foot. And these are the abductor hallucis muscle – a relatively large plantar muscle found on the medial border of the foot; the flexor hallucis brevis muscle, which is found deeper on the plantar aspect of the foot; and finally, the adductor hallucis muscle that is characterized by its two heads, which have an unusual and somewhat variable anatomy.

As some of you have already guessed, all three of these muscles share a common trait and that they all act upon the hallux, which is also known as the great or big toe of the foot. But before learning about each of these muscles individual functions or actions, it’s important to remind ourselves first of their anatomy. So, let’s take some time to look at these three muscles individually, beginning first with the abductor hallucis muscle.

As you can see, on our awesome 3D model here, the abductor hallucis muscle is located along the medial aspect of the plantar surface of the foot. If you learned your plantar muscles by layers, you can make a note that this muscle is found on the first layer of the plantar muscles. The abductor hallucis has its origin or proximal attachment at the medial process of the calcaneal tuberosity right here. It also finds its origin on some other soft tissues of the foot, such as the flexor retinaculum and plantar aponeurosis. Its long muscular belly courses along the medial aspect of the foot eventually tapering off to insert at the base of the proximal phalanx of the hallux or big toe.

Moving deeper into the plantar muscles of the foot now, let’s take a look at our second muscle, the flexor hallucis brevis muscle. This muscle is slightly more lateral than the abductor hallucis and belongs to the third layer of the plantar muscles. As you can see on our model, the flexor hallucis brevis has a split proximal attachment or origin which I should note is quite variable in its anatomy. Its medial limb arises from an attachment to the tendon of the tibialis posterior muscle or occasionally from the medial cuneiform bone.

The lateral limb is more controversial and you’ll see different sites of attachment depending on which textbook you’re using. And as you can see here, the lateral limb can have an attachment to the lateral cuneiform bone of the foot. Alternatively, it may be attached to the medial plantar surface of the cuboid bone or even to the plantar calcaneocuboid ligament.

Both short limbs come together to form a long thick muscular body which courses towards the hallux, and as you can see, the flexor hallucis brevis has a bifurcate or fork-like tendon of insertion which inserts into either side of the proximal phalanx of the big toe by the sesamoid bones of the hallux.

Okay, it’s time now to turn our attention to our final muscle of interest today which is the adductor hallucis muscle. And like the flexor hallucis brevis, this muscle also belongs to the third layer of the plantar muscles. So, I’m pretty sure that the first thing that you’ve noticed about the adductor hallucis is that its composed of two heads which are known as the oblique and the transverse heads.

The oblique head is somewhat variably attached to several bones of the foot including the bases of the second through fourth metatarsals as well as variably attaching to the cuboid and lateral cuneiform bones of the tarsus. It may also alternatively attach to the tendon of the fibularis longus muscle.

The transverse head is somewhat smaller than its oblique brother and it originates from the plantar metatarsophalangeal ligaments of the third through fifth digits. And this can also be variable with attachments to the second through fourth digits and this is sometimes referred to as its origin. Both heads of the adductor hallucis course towards the metatarsophalangeal joint of the big toe where they both collectively form a common tendon with the flexor hallucis brevis which inserts onto the lateral aspect of the base of the proximal phalanx of the big toe.

And with that, we’ve discussed all three medial plantar muscles of the foot. Of course, when we discuss the anatomy of any muscle, it’s important for us to identify which joints these muscles act upon and luckily for us, all three of our muscles primarily act upon the same joint, which is the metatarsophalangeal joint of the hallux or the big toe.

Of course, our muscle isn’t of much use to us if it doesn’t have a nerve supply or innervation. So we’re going to be talking about a couple of these nerves right now.

So we have two nerves of interest for our group of muscles today. They are the medial and lateral plantar nerves, both of which are terminal branches of the tibial nerve. The medial plantar nerve innervates the abductor hallucis muscle in addition to the medial part of the flexor hallucis brevis. And the lateral plantar nerve on the other hand, innervates the lateral part of the flexor hallucis brevis in addition to the adductor hallucis.

So now that we’ve certainly refreshed our memories about the anatomy of the abductor hallucis, flexor hallucis brevis, and the adductor hallucis muscles, it’s time to put our knowledge to work and explore the functions of each of these muscles. And, of course, we’re going to start with the abductor hallucis muscle.

So let’s start with the most obvious function which, true to this muscle’s name, is abduction of the hallux occurring at the metatarsophalangeal joint. And, I guess, the most important thing to start with here is to describe what exactly abduction means especially when we’re talking about the foot.

So, normally, when we talk about abduction, it means to move a body part away from the midline or laterally and conversely, adduction, means to move towards the midline or medially. The foot is one exception to this because in this instance, the terms abduction and adduction are used in relation to an axis through the second digit. This means that abduction is a movement away from the second digit and adduction is the opposite bringing something towards the second digit.

So with all that in mind, let’s look at our animation one more time. And as you can see now, in the case of the big toe, abduction here is a medial movement away from the second toe.

Another function of the abductor hallucis is to assist in the flexion of the big toe, once again occurring at the metatarsophalangeal joint of the hallux. And last but certainly not least the final function of the abductor hallucis is to support the longitudinal arch of the foot. Now to show you what this means, I’m going to ask our model to put some weight down on this foot and lower his longitudinal arch. With contraction of the abductor hallucis, the bones along the medial length are pulled together causing the arch to rise. And this is extremely important when walking and running as it ensures proper way to distribution and support across the foot.

Okay, let’s continue on now and explore the functions of our second muscle of interest – the flexor hallucis brevis. And true to its name, the primary function of this muscle is flexion of the hallux, or the big toe, at the metatarsophalangeal joint. So let’s take another look at this movement but from a lateral perspective so you can get a better view of the flexor hallucis brevis in action. So in addition to flexion of the big toe, this muscle also helps to support the longitudinal arch of the foot.

And time now to look at our final muscle – the adductor hallucis. So just like the previous examples which we’ve examined, the name of our muscles gives us a clue to one of its functions and this means that our first movement which we’re going to look is adduction of the hallux, or the big toe, at the metatarsophalangeal joint.

So remember when it comes to the foot, abduction and adduction is always considered in reference to the long axis of the second toe and we saw earlier that abduction of the big toe caused it to move medially away from the second toe and the adductor hallucis counteracts that and instead adducts the big toe laterally towards the second digit.

The adductor hallucis also assists in flexion of the hallux once again occurring at the metatarsophalangeal joint. And finally, this muscle also assists in supporting both the transverse and longitudinal arches of the foot and it’s the transverse head of the adductor hallucis that helps to support and stabilize the transverse arch of the foot as you can see here, while the oblique head works to support the longitudinal arches of the foot in a similar fashion to that same with the flexor hallucis brevis.

And that’s it! We’ve covered all the functions of our three medial plantar muscles. I hope you enjoyed learning about their intricate movements.

To finish up, let me quickly recap what we talked about today. So we first looked at the abductor hallucis muscle and its three functions which included abduction of the hallux at the metatarsophalangeal joint which works to move the big toe away from the second toe, flexion of the hallux which is also at the metatarsophalangeal joint which causes the big toe to bend in the plantar direction, and, finally, the adductor hallucis muscle also provides support for the longitudinal arch of the foot.

Our second muscle of interest was the flexor hallucis brevis. And this muscle was also involved in both the flexion of the hallux at the metatarsophalangeal joint and again from a lateral perspective so you can get a better view of the flexor hallucis brevis in action. So, in addition to flexion of the big toe, this muscle also helps to support the longitudinal arch of the foot.

And, finally, we looked at the adductor hallucis muscle, and its functions were adduction of the hallux at the metatarsophalangeal joint pulling the big toe in the direction of the second toe and flexion of the hallux, once again, occurring at the metatarsophalangeal joint. The transverse head of the adductor hallucis helps to support the transverse arch of the foot while the oblique head of the adductor hallucis assists in support of the longitudinal arch of the foot.

And that’s it! We’ve explored all the medial plantar muscles of the foot. Time now to treat yourself with a banana! Just kidding.

Thanks for watching and happy studying!