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Functions and anatomy of the central muscles of the foot shown with 3D model animation.
Hey there! Who said that cleaning has to be a chore and boring? It’s definitely not if you try and do it only with your foot, although it might become frustrating after a while. However, if you find this amazing, just think what a well-oiled machine your foot must be to even be able to attempt such a feat. Are you curious to find out how this is possible? I certainly am. So, stay around to find out more about a group of five muscles that help making this cleaning stunt humanly possible. Together, they’re known as the central plantar muscles of the foot and we’ll use all the help we can get from our 3D model today to learn all about their functions.
So, which are the five central plantar muscles that we’ll be looking at? Firstly, we have the flexor digitorum brevis which is the most superficial one and runs the entire length of the plantar aspect of your foot slightly covering the other muscles. Then underneath in the same second layer, we have two more muscles – the quadratus plantae which has two heads and the lumbricals which is actually a group of muscles consisting of four smaller ones.
Then we have the final two groups of muscles which are both located in the deepest or fourth layer, so it takes some digging to get to them, hence these are the plantar interossei which are a group of three smaller muscles. And finally, we have the dorsal interossei which are also a group of small muscles.
Therefore, while we initially started with five central plantar muscles, there are in fact thirteen in total. Quite a lot can be squeezed into a middle band running down the center of the sole of your foot, right?
By the way, while we’re talking about nomenclature, let’s clarify what we mean by central plantar muscles. As you can probably guess, they’re called plantar because – surprise! surprise! – they’re located on the plantar side of your foot, and they’re called central because all of them are sandwiched between the medial and lateral plantar muscles of the foot.
But before we divide into the functions and movements of each one of these muscles, hold your horses, let’s recap their individual anatomy first.
The flexor digitorum brevis, which our friend on the screen is proudly showcasing, is located quite centrally on the plantar side of the foot between the abductor hallucis and the abductor digiti minimi muscles and directly underneath the plantar aponeurosis. It’s important to note that the plantar muscles aren’t simply located willy-nilly, but rather in layers based on how they will appear in dissection. As you already know, the flexor digitorum brevis is the most superficial out of all of them, hence, you guessed it, it is located in the first layer.
The flexor digitorum brevis originates in the medial process of the calcaneal tuberosity which is right here as well as some soft tissues of the foot such as the plantar aponeurosis. From its origin, the flexor digitorum brevis courses towards the lateral four toes where it divides into four tendons at the level of the metatarsals. Then each flexor digitorum brevis tendon travels together with a slip of tendon from the flexor digitorum longus all the way down to the bases of the proximal phalanges through tunnels called tendinous sheaths.
And both tendons continue their course until the level of the middle phalanges where each tendon of the flexor digitorum brevis bifurcates into two attaching to both sides of the shaft of the middle phalanx of the corresponding toe.
So moving deeper into the central plantar muscles of the foot, let’s have a look at our second muscle which is the quadratus plantae which you can now see on the screen in all its glory.
So this muscle belongs to the second layer of the central plantar muscles and is located centrally directly underneath the more superficial flexor digitorum brevis. As we can see clearly on our awesome 3D model, the quadratus plantae has two heads. The larger medial head originates from the medial plantar surface of the calcaneus which is this one over here. The smaller flat and sometimes tendinous lateral head originates close by but from a point distal to the lateral process of the calcaneal tuberosity. And the lateral head may also have an attachment to the long plantar ligament.
From its origin, the quadratus plantae muscle travels through the central compartment and inserts into the posterolateral margin of the tendon of the flexor digitorum longus muscle right where it divides into its four tendons running towards the lateral four toes. Therefore, it’s important for you to keep in mind that the quadratus plantae is quite unusual because it only has one bony attachment rather than the typical two which you are used to.
The quadratus plantae is not the only muscle related to the tendon of the flexor digitorum longus. If we move a little bit more distal along the length of the foot, we can see another group of muscles attached to this tendon which are collectively called the lumbrical muscles.
Also located in the second layer, the lumbrical muscles are composed of four small muscles which are numbered one to four from the medial to the lateral side of the foot. All of the lumbrical muscles originate from the flexor digitorum longus tendon which you can see right here. To be more specific, the second through fourth lumbrical muscles originate from the angles located between the digital tendons of the flexor digitorum longus attaching to the sides of adjacent tendons. The first lumbrical, however, has its proximal attachment only along the medial border of the first tendon.
All four lumbricals travel forwards towards the second through fifth toes medial to the tendons of the flexor digitorum longus before finally inserting into the medial side of the proximal phalanx of the lateral four digits. And you may also see them described as inserting into the medial aspect of the dorsal or extensor expansion of the same digits. Remember the extensor expansion is located on the dorsal side of each toe, we’ll see why this is important in just a little while.
Okay, moving deeper into the central plantar compartment, let’s look at our fourth group of muscles – the plantar interossei.
So, three small muscles belong to this group and they’re all located in the fourth layer of plantar muscles. The plantar interossei are numbered first to third from medial to lateral similar to the lumbricals, and they originate from the base and medial aspect of the shaft of the third through fifth metatarsal bones and from here they course along the length of each metatarsal before finally inserting into the medial sides of the bases of the proximal phalanges of the same digits. They also attach to the dorsal or extensor expansion of these digits similar to the lumbrical muscles.
Sharing the same layer as the plantar interossei are the dorsal interossei – the fifth and final group of muscles belonging to the central plantar muscles which you can now see highlighted for you on the screen – and this group consists of four muscles, all of which are located in the fourth layer of the central compartment of the foot.
As you’re already used to by now, the four dorsal interossei are numbered from medial to lateral, one to four, and each one of them has two heads which originate from opposing sides of the first through fifth metatarsal bones. So, unlike the plantar interossei which were unipennate, the dorsal interossei are bipennate or have two muscular bellies.
From its origin, each muscle travels between each adjacent metatarsals with the first dorsal interosseus finding its insertion or distal attachment point at the medial aspect of the base of the proximal phalanx of the second toe. The second through fourth dorsal interossei, however, insert along the lateral aspects of the bases of the proximal phalanx of the second through fourth digits. Therefore, the second toe is the most famous, since it is the only one with two dorsal interossei muscles – one on each side. And similar to the lumbrical and plantar interossei, the dorsal interossei also attach to the extensor hood of their respective digits.
Okay now that we’ve seen the origins and insertions of the central plantar muscles, it’s probably easy for you now to name the joints that they act on. So, let’s go through them quickly since there are only two of them.
The first group of joints which all of these five muscles act upon either directly or indirectly through the flexor digitorum longus are the metatarsophalangeal joints of the foot. As you can see on the model and guess from the name, these joints are formed by the articulation of the metatarsal bones with the proximal phalanges of the digits.
The second collection of joints that all five central plantar flexors act on are the interphalangeal joints. Once again, it’s super easy for you to see the location of those joints from their names between the proximal, middle, and distal phalanges of each digit.
Okay so, so far we’ve already looked at attachments and joints so, of course, you’re an expert now on these areas, and there’s one last essential aspect to learn about before diving into the functions of the central plantar muscles. And that is innervation.
So, the two nerves of interest in our case without which there wouldn’t be any movements are the medial and lateral plantar nerves which are the terminal branches of the tibial nerve. Out of the central plantar muscles, the medial plantar nerve innervates the flexor digitorum brevis and the first lumbrical while the lateral plantar nerve innervates the quadratus plantae, the lateral three lumbricals, the plantae interossei, and the dorsal interossei.
Okay, so finally, we finished recapping the anatomy of the central plantar muscles and we’re ready to move on with the functions. Let’s take them one by one and see what they can do.
So, firstly, we’re going to take a look at the two movements of the flexor digitorum brevis muscle. So, the main action that this muscle is capable of doing is flexion of the lateral four digits, just as our 3D model is demonstrating for us, which occurs at the metatarsophalangeal and proximal interphalangeal joints.
Remember when flexing your toes, you’re bending or decreasing the angle between them and the sole of the foot. So, if we kindly ask our friend on the screen to show us that movement one more time – thank you there – we can see how flexion brings the lateral four toes closer to the ground and the sole of your foot.
The second function of the flexor digitorum brevis is to support the longitudinal arch of the foot, and the easiest way to understand this movement is to simply see it in action. So, every time you plant your foot on the ground and put your weight on it like our 3D model is doing right now, the longitudinal arch of your foot is flattened and its length is increased which affects your stability. And to prevent this problem, the flexor digitorum brevis contracts pulling the bones together and returning the arch back to its original shape, thereby tensing it and this results in better weight distribution. And this movement is extremely important when walking and running in order to stabilize your foot.
Now let’s continue with the functions of the second central plantar muscle, and that’s our quadratus plantae. So, it’s important to keep in mind that this muscle is not responsible for any direct movement on its own, but rather it plays the role of an axillary for the flexor digitorum longus – a muscle responsible for flexing your toes.
On its own, the tendency of the flexor digitorum longus would be to pull the toes and foot in a posteromedial direction. However, by inserting into the tendon of the flexor digitorum longus, the quadratus plantae aligns the pull in such a way that it takes place in a straight line rather than posteromedially, and this provides a better mechanical advantage to the flexor digitorum longus muscle in its function of flexing the toes.
Okay, so we’ve covered the flexor digitorum brevis and the quadratus plantae, so let’s have a look at the functions of the next group of central plantar muscles – the lumbrical muscles.
And the first action that they’re responsible for is flexion of the lateral four toes of your foot, as you can see right now happening on your screen, and this movement happens at the metatarsophalangeal joints. The second action the lumbricals are capable of is to provide assistance in extending the interphalangeal joints of the same four lateral toes. Now this one is a little bit tricky, so just bear with me.
So remember we said that the lumbrical muscles attach to the dorsal or the extensor expansion of each digit? Well, when the lumbrical muscles contract, they pull on the extensor expansion causing extension of the digits at the interphalangeal joints, just like our trusty friend is currently demonstrating.
Another movement which the lumbricals are involved in is adduction of the lateral four toes which occurs at the metatarsophalangeal joints, and as you can see, this movement is quite insignificant, but if you try and do it yourself, you’ll see that it’s not as simple as it looks.
Before moving any further, let’s clarify what adduction and abduction mean when talking about the foot.
So, generally speaking, adduction means to move towards the midline in a medial direction while abduction means to move a body part away from the midline in a lateral direction. However, the foot is an exception to this rule because those two terms are used relative to an axis running through the second digit, and this means that adduction is a movement towards the second digit and abduction is the opposite bringing something away from the second digit.
Therefore, if we take a second look at the adduction of the lateral four toes, we can clearly see that the third, fourth and fifth toes are indeed moving towards the second digit.
Okay, so, we’ve managed to successfully get through three muscles, so hang in there just a bit more. We’re more than halfway through the functions. And we’re going to now take a look at the plantar interossei muscles.
So, if you remember the attachments and I hope you do, it is very easy for you to name the three movements that those muscles are responsible for. So, the first movement is flexion of the third to fifth digits, a movement that happens at the metatarsophalangeal joints. Easy peasy and straightforward, right?
The second function the plantar interossei are involved in is assisting with extending the same lateral three toes which occurs at the interphalangeal joints via the attachment of the muscles to the extensor hood or expansion of the foot just like we saw with the lumbrical muscles.
And last but not least, the third action of the plantar interossei is adduction, the third through fifth digits are movement taking place at the metatarsophalangeal joints, and the movement follows the exact same logic that you have seen for the lumbricals so it should be a piece of cake for you by now.
So now we’re going to look at the fifth and last group of muscles of the central plantars which are the dorsal interossei. So, we’re almost done here, and they’re responsible for three functions so let’s examine each one individually.
First one is flexion of the digits of the second, third and fourth toes at the metatarsophalangeal joints which is happening right now for you on the screen. The second function of the dorsal interossei are involved in is assisting with extending the same three toes – second, third and fourth – which, of course, occurs at the interphalangeal joints once again via the attachment of the plantar interossei to the extensor hood or expansion of the foot. And finally, the dorsal interossei are also responsible for the abduction of the second, third and fourth toes.
Remember that what we talked about earlier about abduction and adduction when it comes to the foot and they’re both referenced to the long axis of the second toe. We saw earlier that the lumbricals and the plantar interossei adducts the digits causing them to move towards the second toe and the dorsal interossei counteracts that movement and instead abduct the three toes moving them away from the second digit.
Now if you’ve paid very close attention, you might have realized that the plantar and dorsal interossei are not only closely related in terms of names but also in terms of their movements. But there’s one main difference in their movements and I’m going to let you in on a little secret to help you remember them a little bit more easily – the mnemonic PAD DAB. So, this stands for plantar interossei adduct and dorsal interossei abduct. Hopefully, that’s not too difficult to remember. So, you should be able to keep this in mind for your next exam about the anatomy of the foot.
And that’s it! We finally reached the end. We’ve successfully covered the functions of the five central plantar muscles of the foot and that was quite a ride, wasn’t it?
So, let’s quickly recap what we’ve gone through today very, very quickly.
So we looked at the flexor digitorum brevis and its two functions – the flexion of the lateral four digits which brings the toes down a plantar direction and support of the longitudinal arch of the foot. Then we looked at the quadratus plantae which has only one axillary role – to align the toe flexion caused by the flexor digitorum longus so that it happens in a straight line backwards.
Our third muscle group were the lumbricals which have the three functions on the lateral four toes – flexion, extension causing them to move up in a dorsal direction, and adduction which is a movement which brings them closer towards the second digit. Then we moved on to the plantar interossei which flex, extend, and adduct the lateral three toes before finally concluding with the dorsal interossei which act on the second, third and fourth toes as follows – flexion, extension, and abduction – a movement which brings them away from the second digit.
So there you have it, all functions of the central plantar muscles of the foot. I hope you enjoyed our video and we’ll see you next time.