Flexor accessorius emerges by two heads via the medial and lateral tubercles of the calcaneus and the adjacent tubercles of the long plantar ligament and lies deep to flexor digitorum brevis.
Proximal to the origin of the lumbricals, a flattened muscular band is formed by the combining the two heads which enters into the tendon of flexor digitorum longus in the midpoint of the sole.
The quadratus plantae muscle is a flat quadrangular muscle with two heads of origin:
- One of the heads originates from the medial surface of the calcaneus inferior to the sustentaculum tali.
- The other head originates anterior to the lateral process of the calcaneal tuberosity and the attachment of the long plantar ligament, from the inferior surface of the calcaneus.
Lateral plantar nerve supplies Flexor accessorius, root value S2, S3. The skin above the area is supplied by root S1.
It helps the long flexor tendons contract all the joints of the lateral four toes. It changes the direction of pull to ensure that the toes flex in the direction of the heel and not in the direction of the medial malleolus, by pulling on the lateral side of the tendon of flexor digitorum longus.
Whenever flexor digitorum longus is already contracted because of plantar flexion of the ankle joint flexor accessorius has an important role to play in gait.
For facilitating the flexing of toes to grasp the ground providing support and thrust during the propulsive stage, this muscle applies its action on the long flexor tendons. This action basically means that flexor digitorum longus can be considered to act strongly across two joints at the same time