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Origins, insertions, innervation and functions of the medial muscles of the sole of the foot.
Hello, everyone. This is Joao from Kenhub, and welcome to another anatomy tutorial where this time we’re going to be talking about the medial muscles of the sole of your foot. Now, this might be a long title but for a short topic where we’re going to be discussing the different muscles that you can now see on your screen.
We’re looking at the bottom of your foot or the plantar side of your foot. And this is what we’re... where we find clearly the medial muscles of the sole of the foot. And note here that the medial side of your foot is towards or on the same side of your big toe, while the lateral side is where you find your little toe right here.
Important thing to add here on this tutorial is that the plantar fascia which surrounds all the muscle of the sole of the foot consists of three chambers. The muscles lying within the medial groove form what is known to be a bulge referred to as the ball of the big toe. It contributes to the surface anatomy of the medial sole of your foot, of course, and it is quite easy to palpate.
Now, before we start discussing all these muscles that you see here, it is important to list them. And the first muscle that we’re going to be talking about is known as the abductor hallucis. We’re also going to talk about the adductor hallucis. And finally, we’re going to be talking about the flexor hallucis brevis.
Now, let’s start off with the very first muscle here on our list, seen highlighted in green. This is the abductor hallucis. And the abductor hallucis has three origin points that we need to remember and talk about briefly. The first one is a bony origin point known as the medial process of the calcaneal tuberosity. And if you notice here on this image, this is the calcaneous, the bone. And you also see here the calcaneal tuberosity and the medial process of the calcaneal tuberosity, which will serve as the origin point for this muscle.
And as I promised, there are two more structures that will serve as origin points for the abductor hallucis. And these include the plantar aponeurosis and also the superficial layer of the flexor retinaculum.
Now, let’s take a look at the insertion point for the abductor hallucis. Yes, there is only one that you need to remember. If you notice here, the tendon of this muscle runs next to the medial sesamoid bone, and then it inserts on this bone here. This is the proximal phalanx of the big toe or the first proximal phalanx. And notice here that on the base of this bone, the muscle is going to insert.
Now, if we talk about the origins, insertion, now it is time for us to move on and talk about the innervation of the abductor hallucis, now seen highlighted here in green. This is the nerve that is going to be innervating this muscle, and this is the medial plantar nerve.
Now, let’s take a brief note on the different functions or actions associated to the abductor hallucis. Now, the first thing that you need to know is that this muscle is going to move the metatarsal phalangeal joint of the big toe. So this joint happening between the metatarsal bone, the first metatarsal bone, and also the first proximal phalanx right here. So this is the first metatarsal phalangeal joint or the metatarsal phalangeal joint of the big toe, which will be moved here by the abductor hallucis and cause what is known to be as abduction of the big toe—hence the name of the muscle. It’s usually easy to find out what is the main action that the muscle is able to cause in your body by just looking at the name, and this is clearly the case. So it is going to abduct the hallucis or the big toe.
Now, also important to add here is that, together, with the other two muscles that I listed before, the abductor hallucis will support plantar flexion. And this muscle will also support the longitudinal foot arch.
Now, let’s move on to the second muscle on our list. And if you remember well, yes, this is the adductor hallucis. Now, the adductor hallucis has two heads, two distinct heads that you can clearly see here on this image; this one being the transverse head, while this one right here is known as the oblique head of the adductor hallucis.
Now let’s talk about the different origin points associated to these heads. The first one is the transverse head that has several origin points including the ligaments of the third to fifth metatarsal phalangeal joints.
So these ligaments as you can see here, from the third all the way to the fifth metatarsal phalangeal joints, will serve as origin point for the transverse head. And another origin point for the transverse head is going to be the deep transverse metatarsal ligament.
Now, for the oblique head, the different origin points that we need to remember include a bone known as the cuboid bone. The other one, the other bone, is going to be the lateral cuneiform bone which will serve as an origin bone for this head, and the bases of the second to fourth metatarsal bones. And to be precise, the oblique head does not lie within the medial but central group of the plantar fascia.
Now, in terms of insertion point for the adductor hallucis, now this is an easy one. Because these two heads, as you see here, they will have a common tendon. And then insert here, this part known as the base of the first proximal phalanx.
In terms of innervation, as you now see on the image highlighted in green, this is the nerve that will be responsible for the innervation of the adductor hallucis. This is the lateral plantar nerve.
The next step here on this tutorial is to cover the different functions associated to the adductor hallucis. Now, this muscle moves the metatarsal phalangeal joint of your big toe, leading to what the name says, adduction, and also flexion of the big toe. And you can see here adduction indicated by this arrow.
Now, together with the other two muscles that I listed before, this muscle, the adductor hallucis, supports plantar flexion and also supports the longitudinal foot arch.
In addition to all of that, the transverse head of the adductor hallucis muscle is the only muscle securing the transverse foot arch.
So we’re going, now, to move on to the last muscle on our list. If you remember correctly, yes, this is the flexor hallucis brevis. And in terms of origin points, there will be two bones that will serve as origin points for this muscle. And these are the lateral cuneiform bone. The other one is the cuboid bone. A ligament will serve as an origin point for the flexor hallucis brevis, and this is the plantar calcaneocuboid ligament.
Now, if you look at the insertion point, there is one area or one bone that will serve as insertion point, and specifically, the base of the first proximal phalanx will serve as insertion point. But there is an interesting point here to make on the insertion point of the flexor hallucis brevis, is that this muscle has two heads, and it will insert... both of these heads will insert on the base of the first proximal phalanx. But the medial head will do it via the medial sesamoid bone, while the lateral head is going to do so via the lateral sesamoid. So important thing to add to your notes is that, yes, these two heads are going to insert on the base of the first proximal phalanx but via two different bones, via two different sesamoid bones.
Now, let’s take a look at the innervation of the flexor hallucis brevis. While the medial head is innervated by this nerve that you see now highlighted in green, which is the medial plantar nerve, the lateral head of course is going to be, then, innervated by this one now highlighted. If you remember correctly, this is the lateral plantar nerve. So the flexor hallucis brevis has double innervations—keep that in mind—with the medial and lateral plantar nerves.
Now we’re going to move on and finalize this tutorial by talking about the different functions associated to the flexor hallucis brevis. Now, what you need to know is that this muscle moves the metatarsal phalangeal joint of the big toe which leads to flexion of the big toe indicated by this arrow.
And together with the other two muscles that we already discussed on this tutorial, the flexor hallucis brevis supports plantar flexion as well as provides support for the longitudinal foot arch.