The flexor digitorum brevis is the foot equivalent of the flexor digitorum superficialis in the hand, arising from the plantar heel and splitting around the FDL tendons at the PIP joints of the lesser toes. It is the most superficial plantar muscle and forms the central muscle layer of the sole. Its attachment from the plantar fascia means that plantar fascia tension loads it directly, and its dysfunction contributes to the intrinsic muscle weakness seen in chronic plantar fasciitis.
| Origin | Medial process of the calcaneal tuberosity and the plantar fascia |
|---|---|
| Insertion | Sides of the middle phalanges of the lateral four toes, after splitting around the FDL tendon analogous to the FDS in the hand |
| Nerve Supply | Medial plantar nerve (L4, L5) |
| Blood Supply | Medial plantar artery |
| Actions | Flexion of the PIP joints of toes 2 through 5; Assists in MTP joint flexion; Supports the medial and lateral longitudinal arches |
|---|
Its PIP flexion action curls the tips of the lesser toes into the ground during push-off, providing the toe-grip traction that stabilises the forefoot during the propulsive phase of gait.
Flexor digitorum brevis weakness is increasingly recognised as a contributor to lesser toe deformities including hammertoe and claw toe, where intrinsic muscle imbalance allows the extensor tendons to hyperextend the MTP joints. Intrinsic foot muscle strengthening programmes that target the FDB and interossei are now incorporated into rehabilitation for plantar fasciitis and chronic foot pain.
The FDB is palpable in the mid-sole between the medial and lateral plantar pads during toe flexion against resistance, although its depth makes individual palpation difficult.
FDB and intrinsic foot muscle atrophy from disuse or neuropathy contributing to lesser toe deformity and forefoot pain, managed with intrinsic foot strengthening exercises and footwear modification.
