Quadratus plantae
Quadratus plantae is a difficult muscle to test in isolation, however, you can gain an appreciation of its function by placing two fingers over the central aspect of the foot, just anterior to the calcaneal tuberosity and asking your patient to repeatedly flex their toes.
The second layer of plantar muscles contains quadratus plantae and the lumbricals. Quadratus plantae has two heads at is origin, a medial and a lateral. The medial head originates from the medial plantar surface of the calcaneus and the lateral head arises from the lateral plantar surface of the calcaneus, just anterior to the calcaneal tuberosity. The two heads are separated at their origin by the long plantar ligament. Both heads of quadratus plantae insert into the deep surface of the tendon of flexor digitorum longus at the point where it divides into is four tendinous slips of insertion. The function of quadratus plantae is to help straighten the pull of the obliquely oriented flexor digitorum longus and aid in flexion of the lateral four toes.