By Elizabeth A. Murray, Mount St. Joseph University
Most people are surprised to learn that there are a dozen intrinsic muscles within each foot; more than there are in each hand. There aren’t any intrinsic muscles on the dorsum of the hand, but we do have a pair on the top of the foot, and each has a longus buddy in the anterior leg.
Muscles on the Dorsum of the Foot
Let’s start with the easier set of muscles, the two on the dorsum of the foot, which are the extensor hallucis brevis and extensor digitorum brevis.
Both extensor hallucis brevis and extensor digitorum brevis take origin off the anterior and lateral surfaces of the calcaneus, as well as the inferior extensor retinaculum on the dorsum of the foot. Due to this common origin, some references don’t even separate out the two muscles, and simply collectively call them extensor digitorum brevis.
But the difference is clear; extensor hallucis brevis inserts on the base of the big toe’s proximal phalanx, while extensor digitorum brevis inserts on the second, third, and fourth toes—actually on the lateral sides of their extensor digitorum longus tendons, which have small extensor hoods like those in the dorsal hand. So, extensor hallucis brevis extends the big toe at its metatarsophalangeal joint, while extensor digitorum brevis extends other toes, except for the little toe, at both their metatarsophalangeal and interphalangeal joints.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
Plantar Surface
These brevis muscle bellies are deep to the tendons of extensor hallucis longus and extensor digitorum longus that run distally along the dorsum of the foot.
What about their innervations? Do you know that the deep fibular nerve supplies muscles of the anterior leg? Well, that nerve continues distally to supply the extensor hallucis brevis and extensor digitorum brevis, and the deep fibular nerve then becomes cutaneous as the flip-flop nerve between the first and second toes.
Moving to the plantar surface, I should first mention that the skin on the sole of the foot is thick and protective, like the skin of the palmar hand. In addition, this plantar skin is firmly attached to deeper structures, unlike on the dorsum of the foot, which has loose skin like the back of the hand.
If this plantar skin wasn’t anchored to the deeper structures, the skin would slide around like a loose sock. Deep to this thick skin is a triangle of connective tissue, the plantar aponeurosis, more commonly known as the plantar fascia. Its apex attaches to the calcaneal tuberosity, and its wide end attaches to the heads of the metatarsals.
Muscles in the Plantar Foot
There are some tricks to learning the many muscles in the plantar foot. One helpful thing is to consider them in four layers. There are three muscles in the first layer, two muscles and two tendons in the second layer, three muscles in the third layer, and again, two muscles and two tendons in the fourth layer. In other words, the first and third layers—the odd numbered layers—each have three muscles in them; three is also an odd number. The second and fourth layers, the even-numbered layers, have an even number of muscles in them—two muscles, and an equal number of two tendons.
As for the innervations of the plantar muscles, the tibial nerve, one of the two terminal branches of the sciatic nerve, supplies the muscles of the posterior leg. Then, at the medial ankle, really at the instep of the foot, the tibial nerve divides into a medial plantar nerve and a lateral plantar nerve.
The First Layer
In the first layer, the three muscles from medial to lateral are the abductor hallucis, flexor digitorum brevis, and abductor digiti minimi for the little toe. There is no abductor digiti maximi—the counterpart for the little toe is hallux, the big toe. When we think of abduction, we should picture fanning the foot out, like stretching to spread our toes apart.
Abductor hallucis takes origin from the calcaneal tuberosity and plantar aponeurosis and inserts on the medial side of the proximal phalanx of the big toe. Not only does it abduct the great toe, abductor hallucis also slightly flexes the big toe at its metatarsophalangeal joint and helps maintain the medial side of the longitudinal arch of the foot.
Abductor digiti minimi also takes origin from the calcaneal tuberosity and plantar aponeurosis and inserts on the lateral side of the proximal phalanx of the little toe. In addition to abduction, abductor digiti minimi helps flex the little toe and maintains the lateral side of the longitudinal arch.
The third muscle, flexor digitorum brevis, also originates off the calcaneal tuberosity and from the plantar aponeurosis directly superficial to it. In lab, it can be difficult to remove the plantar aponeurosis from the surface of flexor digitorum brevis without damaging the muscle. The insertion is on the two sides of each middle phalanx of the second through fifth toes.
Innervations in the First Layer
These brevis tendons split like a fork, to allow the tendons of flexor digitorum longus—which are deep to the belly of flexor digitorum brevis—to pass through that fork and insert on the distal phalanges of the smaller toes. So, when we curl our toes, the flexor digitorum brevis muscle is flexing the proximal interphalangeal joints, while the flexor digitorum longus is flexing the distal interphalangeal joints as well as the proximal IPJs, since it crosses both. Both muscles also help flex the metatarsophalangeal joints, but all this applies to only the second through fifth toes.
Regarding the innervations in this first layer of the plantar foot, it’s no surprise that the medial plantar nerve supplies abductor hallucis on the big toe side, while the lateral plantar nerve supplies abductor digiti minimi on the little toe side. Flexor digitorum brevis is in the middle, between the two abductors, so let the m in middle remind you that flexor digitorum brevis is supplied by the medial plantar nerve. That’s not why, of course, but it does help remember the innervation.
The two abductors are really placed on the edges of the sole of the foot; they each insert on the outside aspect of their respective toes. They form the fleshy mass on either edge of the foot—felt especially more towards the heel end of our foot.
Common Questions about Muscles in the Human Foot
Extensor hallucis brevis inserts on the base of the big toe’s proximal phalanx, while extensor digitorum brevis inserts on the second, third, and fourth toes.
At the medial ankle, the tibial nerve divides into a medial plantar nerve and a lateral plantar nerve.
In the first layer, the three muscles from medial to lateral are the abductor hallucis, flexor digitorum brevis, and abductor digiti minimi.
