Human Anatomy - Kotsan I. Ya. 2009

Muscles and Fasciae of the Lower Limb
Muscles of the Plantar Surface of the Foot
Middle Group of Plantar Foot Muscles

The Middle group of plantar Muscles is the most numerous. It comprises 13 short muscles: the flexor digitorum brevis, quadratus plantae, four lumbricals, three plantar interosseous muscles, and four dorsal interosseous muscles.

The flexor digitorum brevis (m. flexor digitorum brevis) lies superficially, immediately beneath the plantar aponeurosis. Laterally, it adjoins the abductor digiti minimi, and medially, the abductor hallucis. It originates from the anterior part of the calcaneal tuberosity on its plantar surface and from the plantar aponeurosis. Distally, the Muscle divides into four tendons, each of which splits into two slips at the level of the proximal phalanx. The tendon of the flexor digitorum longus passes through the cleft between them. The flexor digitorum brevis inserts into the middle Phalanges of digits II–V. Function: flexes digits II–V and assists in supporting the longitudinal arches of the FOOT.

Innervation: medial plantar nerve (L_V–S_I).

The quadratus plantae (m. quadratus plantae) lies deep to the flexor digitorum brevis and has two heads: lateral and medial. The lateral HEAD (caput laterale) originates from the Inferior surface of the calcaneus on the lateral side and from the lateral margin of the long plantar ligament, while the medial head (caput mediale) originates from the inferior surface of the calcaneus on the medial side and from the medial margin of the long plantar ligament. Both heads unite into a single flat muscle that inserts laterally into the tendon of the flexor digitorum longus at the mid-plantar level. Function: assists the flexor digitorum longus in flexing the digits and strengthens the longitudinal arches of the foot.

Innervation: lateral plantar nerve (S_I–S_II).

The lumbricals (mm. lumbricales) are four small, spindle-shaped muscles. They are located between the tendons of the flexor digitorum longus and are covered by the flexor digitorum brevis. The three laterally positioned muscles originate from the facing surfaces of the tendons of the flexor digitorum longus, whereas the medially placed muscle arises from the medial side of the corresponding tendon of the flexor digitorum longus. The lumbricals insert into the Medial surface of the bases of the proximal phalanges of digits II–V. Some of their tendinous fibers wrap around the medial side of digits II–V, pass onto the dorsal aspect of the foot, and blend into the dorsal aponeuroses of these digits. Function: flex the proximal phalanges of digits II–V while simultaneously extending the middle and distal phalanges of the same digits.

Innervation: lateral and medial plantar nerves (L_V–S_II).

The plantar interossei (mm. interossei plantares) are three narrow, short muscles located in the interosseous spaces between the second and third, third and fourth, and fourth and fifth Metatarsal Bones. They originate from the medial surfaces of the third to fifth metatarsal bones and insert into the bases of the proximal phalanges of digits III–V, partially extending into the dorsal aponeurosis. Function: flex the proximal phalanges and extend the middle and distal phalanges of digits III–V, as well as adduct these digits toward the second digit.

Innervation: lateral plantar nerve (S_I–S_II).

The four dorsal interosseous muscles (mm. interossei dorsales) completely fill the interosseous spaces from the dorsal side. They originate from the facing surfaces of adjacent metatarsal bones and insert into the bases of the proximal phalanges and the tendons of the extensor digitorum longus. The first dorsal interosseous muscle originates on the side of the great toe and inserts into the medial side of the second digit, while the remaining three insert respectively into the lateral sides of digits II–IV. Function: flex the proximal phalanges and extend the middle and distal phalanges of digits II–IV. Additionally, the first dorsal interosseous pulls the second digit medially, whereas the remaining interosseous muscles pull digits II, III, and IV laterally (abducting them away from the second digit toward the little toe).

Innervation: lateral plantar nerve (S_I–S_II).