[TA] muscle of second layer of plantar muscles; origin, by two heads from the lateral and medial borders of the inferior surface of the calcaneus; insertion, tendons of flexor digitorum longus; action, assists long flexor; nerve supply, lateral plantar. SYN: musculus quadratus plantae [TA], flexor accessorius (muscle)*, musculus flexor accessorius*, accessory flexor muscle of foot, caro quadrata sylvii, musculus pronator pedis, plantar quadrate muscle, quadrate muscle of sole.