Many of the intrinsic muscles of the foot insert into the free margin of the hood on each side. The attachment of these muscles into the extensor hoods allows the forces from these muscles to be distributed over the toes to cause flexion of the metatarsophalangeal joints while at the same time extending the interphalangeal joints (Fig. 6.117). The function of these movements in the foot is uncertain, but they may prevent overextension of the metatarsophalangeal joints and flexion of the interphalangeal joints when the heel is elevated off the ground and the toes grip the ground during walking.
Intrinsic muscles of the foot originate and insert in the foot: the extensor digitorum brevis and extensor hallucis brevis on the dorsal aspect of the foot; all other intrinsic muscles—the dorsal and plantar interossei, flexor digiti minimi brevis, flexor hallucis brevis, flexor digitorum brevis, quadratus plantae (flexor accessorius), abductor digiti minimi, abductor hallucis, and lumbricals—are on the plantar side of the foot in the sole where they are organized into four layers.
Intrinsic muscles mainly modify the actions of the long tendons and generate fine movements of the toes.
All intrinsic muscles of the foot are innervated by the medial and lateral plantar branches of the tibial nerve except for the extensor digitorum brevis, which is innervated by the deep fibular nerve. The first two dorsal interossei also may receive part of their innervation from the deep fibular nerve.
On the dorsal aspect
The extensor digitorum brevis is attached to a roughened area on the superolateral surface of the calcaneus lateral to the tarsal sinus (Fig. 6.118 and Table 6.10).
The flat muscle belly passes anteromedially over the foot, deep to the tendons of the extensor digitorum longus, and forms three tendons, which enter digits II, III, and IV. The tendons join the lateral sides of the tendons of the extensor digitorum longus. The extensor digitorum brevis extends the middle three toes through attachments to the long extensor tendons and extensor hoods. It is innervated by the deep fibular nerve.
The extensor hallucis brevis originates in conjunction with the extensor digitorum brevis. Its tendon attaches to the base of the proximal phalanx of the great toes. The muscle extends the metatarsophalangeal joint of the great toe and is innervated by the deep fibular nerve.
In the sole
The muscles in the sole of the foot are organized into four layers. From superficial to deep, or plantar to dorsal, these layers are the first, second, third, and fourth layers.
There are three components in the first layer of muscles, which is the most superficial of the four layers and is immediately deep to the plantar aponeurosis (Fig. 6.119 and Table 6.11). From medial to lateral, these muscles are the abductor hallucis, flexor digitorum brevis, and abductor digiti minimi.
The abductor hallucis muscle forms the medial margin of the foot and contributes to a soft tissue bulge on the medial side of the sole (Fig. 6.119). It originates from the medial process of the calcaneal tuberosity and adjacent margins of the flexor retinaculum and plantar aponeurosis. It forms a tendon that inserts on the medial side of the base of the proximal phalanx of the great toe and on the medial sesamoid bone associated with the tendon of the flexor hallucis brevis muscle.
The abductor hallucis abducts and flexes the great toe at the metatarsophalangeal joint and is innervated by the medial plantar branch of the tibial nerve.