The flexor digitorum brevis muscle lies immediately superior to the plantar aponeurosis and inferior to the tendons of the flexor digitorum longus in the sole of the foot (Fig. 6.119). The flat spindle-shaped muscle belly originates as a tendon from the medial process of the calcaneal tuberosity and from the adjacent plantar aponeurosis.
The muscle fibers of the flexor digitorum brevis converge anteriorly to form four tendons, which each enter one of the lateral four toes. Near the base of the proximal phalanx of the toe, each tendon splits to pass dorsally around each side of the tendon of the flexor digitorum longus and attach to the margins of the middle phalanx.
The flexor digitorum brevis flexes the lateral four toes at the proximal interphalangeal joints and is innervated by the medial plantar branch of the tibial nerve.
The abductor digiti minimi muscle is on the lateral side of the foot and contributes to the large lateral plantar eminence on the sole (Fig. 6.119). It has a broad base of origin, mainly from the lateral and medial processes of the calcaneal tuberosity and from a fibrous band of connective tissue, which connects the calcaneus with the base of metatarsal V.
The abductor digiti minimi forms a tendon, which travels in a shallow groove on the plantar surface of the base of metatarsal V and continues forward to attach to the lateral side of the base of the proximal phalanx of the little toe.
The abductor digiti minimi abducts the little toe at the metatarsophalangeal joint and is innervated by the lateral plantar branch of the tibial nerve.
The second muscle layer in the sole of the foot is associated with the tendons of the flexor digitorum longus muscle, which pass through this layer, and consists of the quadratus plantae and four lumbrical muscles (Fig. 6.120 and Table 6.12).
The quadratus plantae muscle is a flat quadrangular muscle with two heads of origin (Fig. 6.120):
One of the heads originates from the medial surface of the calcaneus inferior to the sustentaculum tali.
The other head originates from the inferior surface of the calcaneus anterior to the lateral process of the calcaneal tuberosity and the attachment of the long plantar ligament.
The quadratus plantae muscle inserts into the lateral side of the tendon of the flexor digitorum longus in the proximal half of the sole of the foot near where the tendon divides.
The quadratus plantae assists the flexor digitorum longus tendon in flexing the toes and may also adjust the “line of pull” of this tendon as it enters the sole of the foot from the medial side. The muscle is innervated by the lateral plantar nerve.
The lumbrical muscles are four worm-like muscles that originate from the tendons of the flexor digitorum longus and pass dorsally to insert into the free medial margins of the extensor hoods of the four lateral toes (Fig. 6.120).
The first lumbrical originates from the medial side of the tendon of the flexor digitorum longus that is associated with the second toe. The remaining three muscles are bipennate and originate from the sides of adjacent tendons.
The lumbrical muscles act through the extensor hoods to resist excessive extension of the metatarsophalangeal joints and flexion of the interphalangeal joints when the heel leaves the ground during walking.
The first lumbrical is innervated by the medial plantar nerve, while the other three are innervated by the lateral plantar nerve.
There are three muscles in the third layer in the sole of the foot (Fig. 6.122 and Table 6.13):
Two (the flexor hallucis brevis and adductor hallucis) are associated with the great toe.
The third (the flexor digiti minimi brevis) is associated with the little toe.