The |tibialis posterior| is a deep muscle that lies between the flexor longus digitorum and the flexor longus hallucis. Its arises from the shaft of the |tibia| and the interosseous membrane and tapers into a long ~tendon~. The ~tendon~ of this muscle inserts into the bottom arch of the foot. The |tibialis posterior| is an extensor of the foot and ankle joint. It works with the |tibialis anterior| to turn the sole of the foot inward. It also helps support the arch of the foot when walking.