Hello, everyone! This is Joao from Kenhub, and welcome to another anatomy tutorial where, this time, I’m going to be talking about a long title but easy one. This is the central muscles of the sole of your foot.
And today, we’re going to be talking about the origins, insertions, innervation, and also functions related to these muscles that you now see on the screen.
The central muscles of the foot, they lie within the central compartment between the muscles of your big toe and also your little toe. And the compartment comprises numerous short foot muscles in different layers.
Now, together, these muscles form the central surface of the sole of the foot.
And before I move on and talk about the different aspects, the different things that we need to know about these muscles, I would like to list them. And the first one that we’re going to be talking about on this tutorial is known as the flexor digitorum brevis muscle. The other one is the quadratus plantae muscles.
We’re going to be talking about the lumbrical muscles, also the plantar interossei muscles. And finally, we’re going to be covering the dorsal interossei muscles.
We’re going to move on to the very first one here on our list: the flexor digitorum brevis muscle, seen now highlighted in green.
The flexor digitorum brevis muscle lies relatively superficially under the plantar aponeurosis and marks the largest muscle in the central compartment. So this is an important information that you need to know – the largest muscle of the central compartment. So this is the largest muscle of the list that we just covered before, so keep this in mind and on your notes.
And as I promised, we’re going to be, now, covering the origin points for the flexor digitorum brevis. Now, you need to know two main points. The first one is a bony point known as the medial process of the calcaneal tuberosity. And you can clearly see here, this is the calcaneal tuberosity, and this is the origin point for the flexor digitorum brevis.
Now, another point, or another structure better said, that will serve as an origin point for the flexor digitorum brevis is the plantar aponeurosis.
Now that we talked about the origin points, let’s move on to, then, the insertion of the flexor digitorum brevis. And distally, this muscle will divide into four tendons moving towards the second to fifth toe, and you can clearly see here. So it’s moving from the second all the way to the fifth toe, which is your little toe as you can see here.
And what happens at the proximal phalanges, these tendons separate further into two smaller tendons which finally will insert medially and also laterally at the middle phalanges of the second to fifth toes.
And like every good muscle out there, the flexor digitorum brevis needs to have an innervation, and the nerve that is going to be supplying it is now seen on the screen on the right side, highlighted in green. And this is the medial plantar nerve.
Now, before we finish talking about the flexor digitorum brevis, let’s talk about the different functions or actions associated to this muscle. And on this illustration, you can clearly see the main action or the main function that this muscle is able to do, and this is known as flexion.
So, the flexor digitorum brevis muscle is responsible for flexing your second and also all the way to your fifth toe.
Now, this happens because it has to do with origins and insertions, and if you remember correctly, it’s originating here on the calcaneal tuberosity, and that it is going to insert from your second toe all the way to your fifth toe. Then when the muscle contracts, it’s going to, then, produce this movement here that you see, represented by this arrow. And we call it flexion.
Another function you could add associated to the flexor digitorum brevis would be supporting the longitudinal arch of your foot.
Now, we’re ready to move to the next muscle on our list, now seen here highlighted in green. If you remember correctly (you have one second to guess it), yes, this is the quadratus plantae muscle.
Now, in terms of origin point, you only need to remember one, and this will be the tuber calcanei. Now, this is a fancy name or a commonly used Latin term for the calcaneal tuberosity, for this part of the calcaneus, which will serve then as the origin point for the quadratus plantae.
Now, if you look at the insertion point, there is a very interesting thing here about the insertion point for this muscle – that you’re not going to see the quadratus plantae being inserted on a bony point. Here, this muscle is inserting on a tendon, as you can see here on this image.
And this tendon belongs to the flexor digitorum longus muscle.
So the tendon of the flexor digitorum longus muscle will serve as insertion point for the quadratus plantae.
Another point that we need to make here, and now you see it highlighted in green, this is the innervation here. This structure that you see highlighted is the innervation for this muscle, and it is the lateral plantar nerve.
And if you remember correctly from the previous muscle that we talked about, this is the medial plantar nerve that was responsible for innervating the flexor digitorum brevis.
Now, we’re going to move on and talk about the functions or actions associated to the quadratus plantae.
Now, a simple thing here to remember is that the quadratus plantae muscle does not move any joints but has a rather special function. By pulling the tendon of the flexor digitorum longus, it shifts the tendon’s force effect to the longitudinal direction, and by doing this, it will increase the effectiveness of the flexion, the plantar flexion that the flexor digitorum longus is able to perform.
So for that reason, I would say this… the lesson here to take is that the quadratus plantae helps another muscle perform a function rather than creating any movement on its own, at least any movement of a particular joint.
And we also have here an arrow indicating the movement here caused by the muscle.
Now, let’s move on to the next muscle on our list, the next muscles. This is a group of four muscles, the lumbrical muscles, seen here highlighted in green.
Now, in terms of origin points, there is something very interesting going on here. If you notice, the muscle is originating from a tendon as well, one of the tendons that we just talked about, about the quadratus plantae, the insertion of the quadratus plantae.
So the tendon of the flexor digitorum longus, as you can see here on this image, is serving as an origin point for all lumbrical muscles, all four lumbrical muscles.
Now, in terms of insertion points, there is a different story going on right here. And you can notice here that the muscle is going to insert medially at the bases of the phalanges of your second all the way to your fifth toes, as you can see here on this image.
Also important to add here that another insertion point or another structure where these muscles are going to be inserting is the dorsal aponeurosis.
When it comes to the innervation of the lumbricals, there is something that you need to remember – that the first lumbrical muscle will be innervated by this nerve that you now see highlighted in green on the image: the medial plantar nerve, if you remember correctly when we talked about the flexor digitorum brevis.
Now, the other three lumbricals are going to be innervated by the lateral plantar nerve that is now seen, also highlighted in green. So as you can see, this collection of muscles have a… what is called a double innervation by the medial and lateral plantar nerves.
Now, we’re going to move on and talk about the different actions or functions associated to the lumbricals, and you can clearly see here two images, and one of them (on the far right) is clearly indicating some action going on.
Now, in terms of the first image, we’re going to show that the lumbrical muscles are going to be able to perform adduction of your toes. So when you have your toes spread, and you want to close or bring them together, this is what we call adduction, and this… these muscles are definitely contributing to this action.
The other one is seen on the far right, and this is, if you remember correctly, this is flexion, and the lumbrical muscles are also able to cause flexion of the second to fifth toes.
We’re going to move on and talk about another set of muscles, now three muscles known as the plantar interossei.
Now, the plantar interossei are known to originate from the medial side of the third to fifth metatarsal bones, and you can clearly see here from the third all the way to the fifth metatarsal bones, where these bones are serving as origin points for these three muscles.
Now, they go all the way to, then, insert on the medial side of the bases of the proximal phalanges of the… also the third all the way to the fifth toes.
And one important thing to mention here is that when we number these muscles, we’re not numbering according to the toe, the respective toe. We’re numbering simply from the medial side all the way to the lateral side. So we start here with the first one, all the way to the third one. This is how we number them.
Let’s talk about the innervation of the plantar interossei muscles, now seen highlighted in green. Yes, if you guess this correctly, this is the lateral plantar nerve, which is responsible for innervating these muscles, these three muscles.
Now, we’re going to move on to the functions associated to the plantar interossei muscles.
Now, what you need to know is that these muscles, when they contract, they were go… they’re going to be causing what is known to be adduction of your toes. So when you spread your toes, and then you want to bring them closer together, as you can see here, represented by these images, these arrows. This is what we call adduction of the toes.
Now, another function that will be a result of the contraction of the plantar interossei is going to be the flexion or flexion of your toes.
We’re going to move on and talk about the last group of muscles here on our tutorial. If you remember the first list that I talked about, yes, this is a dorsal or these are the dorsal interossei. And I can tell you that you have some good memory if you remember the last one on the list.
Now, let’s talk about the origin points for the dorsal interossei. The dorsal interossei are different from the other groups of muscles that you’ve seen throughout this tutorial because these muscles are two-headed muscles, as you can also notice here on this image. So these muscles have two small heads here, as you can see.
And these heads are arising from the adjacent sides of two neighboring… two neighboring metatarsal bones, as you can see.
And I can clearly notice that, here on this image, that two neighboring metatarsal bones are serving as origin points for one dorsal interosseous muscle.
And also important to add here is that you have four dorsal interossei muscles. So one, two, three, and four.
When it comes to, then, their insertion points, there is something that is important to highlight here, is that the tendons will insert laterally (remember this) on the bases of the proximal phalanges of the second to fourth toes.
And you can clearly see here that they’re inserting laterally on the bases of these bones here, the proximal phalanges on the lateral side. So this is the lateral side of the toe, and they’re inserting right about here.
But there is an exception. So there is one dorsal interosseous muscle that is going to insert on the medial side of the second toe… or at the second toe. So it’s on the medial side of the base of the proximal phalanx of the second toe, and this is an exception.
We’re going to move on and talk about the different innervation… or the innervation of the dorsal interossei muscles and clearly highlighted here. And if you remember well, this is the lateral plantar nerve.
Now, for the last topic of this tutorial, we’re going to be talking briefly about the different functions or actions associated to the dorsal interossei.
And what you need to know is that, when these muscles contract, they’re able to cause flexion on your second toe, all the way to your fourth toe. So second, third, and fourth toes are able to flex when dorsal… the dorsal interossei contract.
Now, another function here associated is abduction, represented by these arrows. So abduction of your toes happens—this is just a fancy term—but happens when you spread your toes to the sides. This is what we call abduction of your toes.