Quadratus plantae muscle

The Quadratus plantae muscle is a flat, quadrangular muscle located in the second layer of the plantar aspect of the foot. It consists of two heads: a medial head, which originates from the medial surface of the calcaneus, and a lateral head, which arises from the lateral border of the calcaneus and the long plantar ligament. These heads converge and insert primarily into the lateral margin of the tendon of the flexor digitorum longus (FDL), and in most cases, also into the tendinous slips of the flexor hallucis longus (FHL) that communicate with the FDL tendons to the lesser toes.

The muscle is innervated by the lateral plantar nerve, a branch of the tibial nerve. Its function is to assist in toe flexion by modifying the oblique pull of the FDL, thereby producing a more direct posterior pull on the digits.

Origin: Calcaneus

Insertion: Tendons of Flexor Digitorum Longus

Nerve: Lateral plantar nerve (S1, S2)

Action: Assists Flexor Digitorum Longus in flexion of DIP joints

Gray's description: The quadratus plantae muscle (Flexor accessorius muscle) is separated from the muscles of the first layer by the lateral plantar vessels and nerve. It arises by two heads, which are separated from each other by the long plantar ligament: the medial or larger head is muscular, and is attached to the medial concave surface of the calcaneus, below the groove which lodges the tendon of the Flexor hallucis longus; the lateral head, flat and tendinous, arises from the lateral border of the inferior surface of the calcaneus, in front of the lateral process of its tuberosity, and from the long plantar ligament. The two portions join at an acute angle, and end in a flattened band which is inserted into the lateral margin and upper and under surfaces of the tendon of the Flexor digitorum longus, forming a kind of groove, in which the tendon is lodged. It usually sends slips to those tendons of the Flexor digitorum longus which pass to the second, third, and fourth toes.

Variations.—Lateral head often wanting; entire muscle absent. Variation in the number of digital tendons to which fibers can be traced. Most frequent offsets are sent to the second, third and fourth toes; in many cases to the fifth as well; occasionally to two toes only.