Flexor digitorum brevis
Flexor digitorum brevis is the most central muscle in the first layer of the plantar foot. Its action may be tested by placing two fingers over the central aspect of the plantar fascia and asking your subject to repeatedly flex their toes against resistance.
Flexor digitorum brevis is the most central muscle in the first layer of the plantar foot. It lies immediately deep to the central part of the plantar fascia and is situated between abductor hallucis medially and abductor digiti minimi laterally. Flexor digitorum brevis primarily arises from the medial process of the calcaneal tuberosity, although it also has attachments to the central aspect of the plantar fascia. The insertion of flexor digitorum brevis is complex. It begins at the level of the proximal phalanges where each of the four tendinous slips of insertion divides in two to form medial and lateral slips. These slips pass distally and then reunite to insert into the shafts of the middle phalanges of toes 2-5. As its name suggests, the function of flexor digitorum brevis is to flex the lateral 4 toes at the proximal interphalangeal joints.