Okay, this tutorial is on the intrinsic muscles of the foot. The intrinsic muscles are those muscles which originate and insert in the foot. these muscles act to produce the fine movements of the toes and they also support the arches of the foot.
The relationship between some of the intrinsic muscles with the tendons of the leg muscles which enter the foot means that these muscles can actually change the way that the forces are produced by some of these muscles and we’ll look at that in a bit more detail.
Just like the other groups of muscles that I’ve talked about, the intrinsic muscles of the foot can be separated into different layers. You’ve got a plantar group and a dorsal group. The dorsal group is really simple because there’s actually just one muscle in this group, but the plantar group has four different layers. I’ll talk you through those.
Just before I start, just a quick mention about the innervation of the foot muscles. Apart from the extensor digitorum brevis and the first two dorsal interosseous muscles, all the intrinsic muscles are innervated by branches of the tibial nerve. We’ve got two branches, the medial and lateral plantar branches. I’ll just bring in the nerves and we’ll take a look at those quickly.
We’re looking posteriorly at the distal leg and foot. You can see the tibial nerve running medially behind the middle malleolus and entering onto the flexor retinaculum to enter the tarsal tunnel. I’ll just fade away the muscles we can have a look at this nerve.
I’m just showing you the plantar surface of the foot. You can see the tibial nerve running medially in the distal foot and it enters the plantar surface of the foot and it splits into these two branches. Laterally, you’ve got the lateral plantar nerve and medially, we’ve got the medial plantar nerve. These two branches for the tibial nerve are responsible for innervation of the majority of the intrinsic muscles of the foot.
The extensor digitorum brevis muscle is innervated by the deep fibular nerve. this is this nerve here which runs anteriorly on the tibia. It also supports and innervates the anterior muscles of the leg. this branch, this nerve, the deep fibular nerve innervates the extensor digitorum brevis muscle and the first two dorsal interosseous muscles, but all the other intrinsic muscles are innervated by the medial and lateral plantar nerves, which are branches of the tibial nerve.
In the dorsal group, you only have one muscle and that’s the extensor digitorum brevis muscle. I’ll just quickly show you that.
I’ve just isolated the extensor digitorum brevis muscle and you may notice that there’s this other label here, which says extensor hallucis brevis. This is what the software has brought up, but this muscle is actually often considered part of the extensor digitorum brevis muscle. I’m just going to talk about the extensor digitorum brevis as it includes the extensor hallucis brevis.
I’ve removed all the other muscles, so we can take a closer look at this. Just looking at its origin, you can see it originates superiorly and laterally on the calcaneus. And if we look at the insertion, we can see the first tendon of the extensor digitorum brevis inserts at the base of the proximal phalanx.
And if I bring the rest of the muscles in, we can see the insertion point of the other three tendons. You can see that they insert laterally on these tendons. These are the tendons of the extensor digitorum longus muscle. The extensor digitorum brevis muscle inserts laterally on these tendons. These three toes here, the extensor digitorum brevis inserts laterally on the tendons of the extensor digitorum longus muscle.
You must notice that there’s no tendon coming from the extensor digitorum brevis muscle to the little toe. It’s just the medial four toes that there are tendons to.
This muscle extends the MPT joint, but it’s innervated by the deep fibular nerve.
I\'ve just rotated the model around, so we\'re now looking at a plantar view, so we\'re looking at the inferior surface of the foot.
I\'ll just remove away the plantar aponeurosis, and we\'ll take a look at some of the muscles in the plantar group of muscles.
In the plantar group you\'ve got four different layers.
You’ve got this first layer, which is the most superficial layer, and three other layers, which I\'ll come on to talk about. These muscles form a lot of the soft tissue bulk which you can feel in the foot.
I\'ll start with this central muscle. There are three muscles in this first group. You’ve got the flexor digitorum brevis muscle, you\'ve got the abductor hallucis, and the abductor digiti minimi. Right in the middle here, we\'ve got the flexor digitorum brevis muscle.
This originates on the calcaneus, on the medial process of the calcaneus, and it forms four tendons which insert onto the middle phalanx of the lateral four digits. It inserts on the sides of the middle phalanges of the lateral four digits. Just like in the hand, you\'ve got the flexor digitorum superficialis and the flexor digitorum profundus muscle.
In the hand, the flexor digitorum superficialis splits and it inserts on the middle phalanges, and it allows the deeper tendon - the flexor digitorum profundus to pass through - so it\'s exactly the same in the foot. The flexor digitorum brevis muscle, which is this muscle here, is analogous to the flexor digitorum superficialis, so it, because it\'s the superficial muscle, and then it splits and inserts on the sides of the medial, the middle phalanx to allow the deeper tendon - so the flexor digitorum longus muscle - to pass through.
The flexor digitorum longus is analogous to the flexor digitorum profundus, because it\'s the deep tendon. What this muscle does is that it flexes the lateral four digits at the proximal interphalangeal joint. This muscle is innervated by the medial plantar nerve.
Just lateral, sorry medial to this muscle, we\'ve got this muscle. This is the abductor hallucis, so you can feel this in your foot, it makes up a lot of the bulk that you can feel in the medial side of your foot. This originates on the medial process of the calcaneal tuberosity and it inserts at the base, it inserts medially on, at the base of the proximal phalanx of the great toe.
This muscle can flex the great toe at the MTP joint, and it also obviously abducts at the MTP joint - as the name suggests.
The third muscle of the first layer is this muscle here, the abduct digiti minimi, so this muscle sits laterally and it abducts the little toe at the MTP joint, so this has got an origin on the lateral side of the calcaneal tuberosity, and it also stretches across to the medial process, so it originates on the medial and the lateral process, of the calcaneal tuberosity, so it\'s got quite a large origin. And then it runs forward and it inserts at the base of the proximal phalanx laterally, so it abducts the little toe at the MTP joint, so it brings the little toe away from the midline.
The abduct digiti minimi muscle is innervated by the lateral plantar nerve, and the abductor hallucis is innervated by the medial plantar nerve. Those are the three muscles of the first layer.
Just a quick note about abduction and adduction in the foot.
In the foot abduction and adduction are defined in relation to a longitudinal axis that runs through the second digit, so abduction is movement, any movement away from this axis, and adduction is any movement towards this axis. This is the same as in the hand, so abduction and adduction are defined relative to a midline that runs through the middle digit, so abduction is movement away from this line, and adduction is movement towards this line.
It’s just to clear up any confusion that might arise because when we talk about abduction and adduction in the rest of the body we talk about it in relation to a central midline that runs through the centre of the body, so abduction is movement away from this central midline and adduction is movement towards this.
In the foot and hand, the foot and hand have their own midlines. Next we\'ve got the second layer of muscle, so I\'ll just remove this superficial layer, and we\'ll take a look at those. Immediately deep to the first layer, you\'ve got the flexor digitorum longus tendon.
This is the tendon which passes through the flexor digitorum brevis and it inserts distally on the lateral four phalanges. The muscles of the second layer are related to this tendon, because they lie at the same level as this tendon.
We’ve only got two muscles in this layer, it\'s quite an easy layer. In layer two we\'ve got two muscles, in layer three we\'ve got three muscles. In this second layer we\'ve got the quadratus plantae, and we\'ve got the lumbricals. The quadratus plantae muscle is this muscle here, which lies posteriorly.
It’s got these two heads as you can see, it\'s got a medial and a lateral head. The lateral head originates on the lateral process of the calcaneus...calcaneal tuberosity, and the medial head originates medially on the calcaneus.
This, these two heads form this tendon which inserts onto the lateral side of the flexor digitorum longus tendon. You can see it attaching to the tendon here. Because of its attachment on this tendon, it actually just helps this tendon to flex the lateral toes, so toes two to five.
This muscle is actually innervated by the lateral plantar nerve. That’s the quadratus plantae muscle. Next we\'ve got the lumbrical muscles, so we\'ve got four lumbricals.
Just like in the hand, the lumbrical muscles originate from the sides of this tendon, so this is the flexor digitorum longus, and in the hand, they originate from the sides of the flexor digitorum profundus muscle.
Just like I mentioned before, the profundus muscle is analogous to the flexor digitorum longus muscle. We’ve got these four lumbricals.
The first lumbrical originates from the medial side of this tendon to the second toe, so the, so this only has one origin, whereas the other three originate from the adjacent sides.
These are bipennate muscles, so they have two, two sort of, they have two sets of fibres which converge onto this central tendon - so they originate on the adjacent sides of the flexor digitorum longus tendons, and they then insert onto the extensor hood of these lateral four digits.
Because of their insertion onto the extensor expansions, they actually can flex the MTP joints, while extending the interphalangeal joints.
The lumbrical muscles are innervated by the medial and lateral plantar nerves, so the first lumbrical is innervated by the medial plantar nerve and the other three lumbricals are innervated by the lateral plantar nerve.
That kind of makes sense. Those are the two muscles you have in the second layer.
Remember second layer - two muscles; third layer - three muscles, so I\'ll come on to talk about the next layer now. I\'ll just remove away the second layer and we\'ll take a look at the third layer.
The three muscles you have in the third layer are associated with the little toe and the big toe. With the little toe you\'ve got the flexor digiti minimi brevis, and related to the big toe, you\'ve got the adductor hallucis, and you\'ve got the flexor hallucis brevis.
First we\'ll just take a look at this medial muscle, so the flexor hallucis brevis muscle. You can see it here lying underneath this tendon. This tendon runs inferiorly to this muscle, and it\'s the tendon of the flexor hallucis longus muscle.
The flexor hallucis brevis muscle is quite interesting because it\'s got two origins and two sort of heads where it inserts. You can see one origin on this bone, so this is the lateral, one of the lateral tarsal bones, so it\'s the cuboid bone. And this tendon here is the tendon of the tibialis posterior muscle.
The medial side of the flexor hallucis brevis muscle actually originates on the tendon of the tibialis posterior muscle, and laterally you\'ve got this origin on the cuboid bone. Then anteriorly where the, or distally where this muscle inserts, it actually has two heads, so this is shown a little bit short of where it actually inserts, because this is still the end of the metatarsal, so the head of the metatarsal, and this muscle actually inserts medially and laterally on the base of the proximal phalanx of the great toe.
It’s shown with its two heads here, so splitting medially and laterally, but it actually inserts a bit further up on the base of the proximal phalanx of the great toe. This muscle is innervated by the medial plantar nerve and its function is to flex the big toe at the MTP joint.
Next we\'ve got the adductor hallucis muscle, so we\'ve got this in the hand as well, so it\'s exactly the same - it\'s got a transverse head and an oblique head.
The transverse head runs horizontally across the foot, and it originates on the deep transverse metatarsal ligament, so you can see this ligament here, running across the MTP joint, so it originates on the deep transverse metatarsal ligament and the plantar ligaments of these lateral three toes, and then it inserts on the base of the proximal phalanx.
Again, for some reason it\'s shown a little bit out of place here, it\'s shown inserting on the metatarsal, whereas it obviously inserts at the base of the proximal phalanx, so a bit further up here. And then you\'ve got the oblique head, so it\'s called the oblique head because the fibres are angled obliquely, so this muscle originates on the bases of metatarsals two to four, so these, metatarsals two three and four, and it also has an origin on this tendon here.
If you remember from my other tutorials this tendon is the tendon of the peroneus longus, or the fibularis longus, so it enters the foot laterally and crosses over underneath the cuboid bone to insert medially on the foot.
The oblique head of the adductor hallucis muscle originates on the tendon, this tendon and also on the bases of metatarsals two to four, and it inserts at the base of the proximal phalanx of the great toe. As the name suggests, this muscle adducts the great toe at the MTP joint and both these muscles are innervated by the lateral plantar nerve.
Finally we\'ve got this muscle here, this little muscle the flexor digiti minimi brevis. This just actually means the flexor of the...the small flexor of the smallest toe.
As you can see it originates on the base of the fifth metatarsal, and it inserts laterally on the base of the proximal phalanx of the little toe. It flexes the little toe, and this is innervated by the lateral plantar nerve.
This muscle also originates, like the oblique head of the adductor hallucis, this muscle actually also has an origin on the tibialis, sorry the fibularis longus tendon, so this tendon is shown slightly proximal to where it should be.
You’ve got this groove here on the cuboid bone, so this tendon actually runs more like this, in this plane. This flexor digiti minimi brevis muscle also originates on the peroneus, or fibularis longus tendon. Those are the three muscles of the third layer of the plantar group.
Hopefully you\'re seeing some similarities to the muscles of the hands now, so remember my analogy to the flexor digitorum superficialis and profundus with the flexor digitorum brevis and flexor digitorum longus. And some of the muscles I\'ve talked about just now are similar to the muscles of the thenar and hypothenar eminence in the hand, but you don\'t have an opponens muscle, so you\'ve got the flexors and the abductors of the great toe, but there\'s no opponens muscle in the foot. And you\'ve also got adductor of the great toe, which is similar to the muscles of the hand, and you\'ve got lumbricals which are similar because they originate on the tendon of the flexor digitorum longus muscle, which is analogous to the flexor digitorum profundus muscle.
By looking at these kinds of similarities, it makes learning about these muscles a little bit easier.
I\'ll just remove these muscles of the third layer and we can look at the fourth and final layer, and this is an easy layer because it just includes the interosseus muscle, so you\'ve got the plantar and dorsal interosseus muscles.
Just like in the hand, the function of the interossei muscles can be remembered with the mnemonic "PAD" and "DAB".
Plantar ADduct - A D duct, and Dorsal abduct.
I\'ve just isolated the interossei muscles, and you\'ve got plantar and dorsal interossei muscles, so the plantar interossei muscles lie inferior to the dorsal interossei muscles, so you\'ve got three plantar interossei muscles, so there\'s an additional one shown here which doesn\'t exist.
You\'ve got three interossei muscles, which originate on one side of metatarsals three to five, so this is three four and five, and they originate medially on the sides of these metatarsals.
These plantar interossei muscles are unipennate, so they only originate on one side of the metatarsals. And then they insert at the base of the proximal phalanges of digits three to five and also on the extensor expansions of these digits.
You can see because of their insertion medially, they will adduct these three, these three digits so they\'ll bring it closer towards the midline of the foot.
The plantar interossei muscles are innervated by the lateral plantar nerve.
I\'ll just get rid of the plantar interossei muscles and we can see the final set of muscles - the dorsal interossei muscles.
There are four dorsal interossei muscles and these are a little bit different because they are bipennate muscles, so they originate on both sides of the metatarsals.
You can see here you\'ve got dorsal interosseus muscle two three and four attaching laterally on the bases of proximal phalanges two to, two to four.
This will cause the, these bones to be abducted at the MTP joint. And then you\'ve got this fourth one - so you don\'t have four plantar interossei muscles, so this first dorsal interosseus muscle inserts medially.
The second toe can be abducted either side.
The dorsal interosseus muscles abduct the digits two to four and they\'re innervated by the lateral plantar nerve, but you\'ve also got the first two dorsal interosseus muscles receiving innervation from the deep fibular nerve.
those are the, those are the muscles, the intrinsic muscles of the foot, there\'s quite a lot to learn there, but the anatomy is quite similar in some respects to the anatomy of the hand, and a lot of anatomy is logically named and if you think about it logically it becomes easier to remember.
I hope that has made things a bit easier for you to learn.