Flexor hallucis brevis
As its name suggests, flexor hallucis brevis acts to flex the hallux. Its function may be tested by placing two fingers on the plantar surface of the first metatarsal in a position proximal to the sesamoids and asking your subject to flex their hallux against resistance applied to its plantar aspect.
The third layer of plantar muscles contains the flexor hallucis brevis, adductor hallucis and flexor digiti minimi brevis muscles. Flexor hallucis brevis is an interesting muscle in that it has a y-shaped tendon of origin, two muscle bellies and two tendons of insertion. At its origin, the lateral limb of flexor hallucis brevis arises from the plantar surface of the cuboid just anteriorly to the peroneal groove. The lateral limb also has an attachment to the lateral cuneiform. The medial limb arises from the metatarsal extension of tibialis posterior. The medial and lateral heads of flexor hallucis brevis remain separated and insert into the medial and lateral aspects of the base of the proximal phalanx of the hallux respectively. Note that each head contains an anatomical sesamoid bone.