A helpful way to learn about the many muscles in the plantar foot 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.
Second Layer of the Plantar Foot
The second layer contains two muscles and two tendons. Of the two tendons, one is the tendon of flexor hallucis longus, and the other consists of the group of tendons from the flexor digitorum longus. While the tendon of flexor hallucis longus is just passing through this layer, the tendons of flexor digitorum longus serve as attachments for both muscles in this second layer.
Like in the palm of the hand, a set of four lumbrical muscles weaves between and takes origin off the tendons of flexor digitorum longus. The lumbricals insert on the dorsal sides of the proximal phalanges of the four smaller toes—really into their tiny extensor hoods from the dorsal side. By this pathway, as the lumbricals do in the hand, the muscles cross the plantar side of the metatarsophalangeal joints of the four smaller toes, so flex the MTPs, but then pass to the dorsum, where they weakly act to extend the interphalangeal joints.
As in the hand, these lumbricals have dual innervation: The one to the second toe is supplied by the medial plantar nerve, while the others are supplied by the lateral plantar nerve.
The other muscle in the second layer of the plantar foot is quadratus plantae, a quadrangular-shaped muscle of the sole of the foot. It’s really a two-headed muscle that takes part of its origin off the medial side of the calcaneus and originates partly off the inferolateral calcaneus. Both heads converge and insert on the lateral edge of the tendon of flexor digitorum longus.
Quadratus plantae functions to adjust the pull of the flexor digitorum longus tendons to help them flex the toes. Quadratus plantae is lateral to its tendon of insertion, and that helps us remember it is innervated by the lateral plantar nerve.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
The Third Layer
The third layer of muscles contains three muscles, and two of those three mirror the pattern seen in the first layer. In the first layer, there is an abductor of the big toe and an abductor of the little toe; in this third layer, there’s a flexor of the big toe and a flexor of the little toe.
Flexor hallucis brevis is a two-bellied muscle that originates off the cuboid and lateral cuneiform. Its medial and lateral heads insert, respectively, on the medial and lateral sides of the base of the proximal phalanx of the big toe. In each of those muscle bellies, a sesamoid bone typically grows at the head of the first metatarsal. Remember, sesamoid bones are those that are seeded within a tendon. So, when we’re standing, because of the arches present in the foot, we’re really balancing our weight on the calcaneus distally, the two sesamoid bones of the great toe medially, and the heads of the other four metatarsals, passing laterally. This flexor of the big toe is supplied by the medial plantar nerve.
Muscles in the Third Layer
Laterally, in this third layer, flexor digiti minimi—to which some sources add the adjective brevis—originates off of the base of the fifth metatarsal and from the fibularis longus tendon that crosses the plantar foot. Flexor digiti minimi inserts on the lateral side of the base of the little toe’s proximal phalanx. As its name implies, it flexes the little toe, and does so at the metatarsophalangeal joint. Flexor digiti minimi is supplied by the lateral plantar nerve.
Between the two flexors in this third layer is an interesting, two-headed muscle named adductor hallucis. It has an oblique head that originates from the bases of the second through fourth metatarsals and the tendon of fibularis longus, but its transverse head comes off the metatarsophalangeal ligaments of some of the smaller toes. Together, both heads insert on the lateral side of the base of the proximal phalanx of the big toe. The net result is a two-headed muscle that roughly has the shape of the number seven, at least when looking at the plantar surface of a right foot.
In terms of innervation, the adductor hallucis is supplied by the lateral plantar nerve. As the name implies, the adductor hallucis draws the metatarsals together, which helps maintain the transverse arch of the foot and helps support our feet while standing so the bones don’t splay out.
The Fourth Layer
In the fourth and final layer, there are two muscles—really two groups of muscles—and two tendons. The tendons are from the fibularis longus and tibialis posterior muscles.
Remember that fibularis longus is a muscle of the lateral compartment of the leg that sends its tendon across the sole of the foot from lateral to medial. The fibularis longus tendon is not only part of the origin for two muscles of the third layer but also helps support the arches of the foot by its insertion on the medial cuneiform bone and base of the first metatarsal. The tendon of tibialis posterior fans out on the medial plantar surface and links together several of the tarsals, including the navicular, cuboid, all three cuneiforms, the sustentaculum tali on the calcaneus, and even several metatarsals.
Muscles in the Fourth Layer
The muscles of the fourth layer are the plantar and dorsal interossei. There are usually three plantar interossei; they originate from the third through fifth metatarsals and insert on the medial sides of the proximal phalanges. There are typically four dorsal interossei that originate between adjacent metatarsals and insert on the proximal phalanges.
As in the hand, the mnemonic devices PAD and DAB apply here. Palmar interossei ADduct the metatarsals and Dorsal interossei ABduct the metatarsals. In the foot, the axis of adduction and abduction is the second toe, not the middle toe.
Both the palmar and dorsal interossei also aid flexion of the metatarsophalangeal joints, and both are supplied by the lateral plantar nerve. Though the dorsal interossei can be best seen from the dorsum of the foot, deep to extensor hallucis brevis and extensor digitorum brevis, they’re still considered the deepest layer of the plantar foot so as not to break innervation rules.
Common Questions about Layers of Muscles in the Plantar Foot
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.
Quadratus plantae functions to adjust the pull of the flexor digitorum longus tendons to help them flex the toes.
The muscles of the fourth layer are the plantar and dorsal interossei.