The Flexor hallucis longus muscle (FHL) is one of the three deep muscles of the posterior compartment of the leg that distally attaches to the plantar surface of the hallux (great or big toe). The other deep muscles are the flexor digitorum longus and tibialis posterior; the tibialis posterior is the most powerful of these deep muscles. All three muscles are innervated by the tibial nerve which branches from the sciatic nerve.
The flexor digitorum longus is situated on the tibial side of the leg. At its origin it is thin and pointed, but it gradually increases in size as it descends. This muscle serves to curl the second, third, fourth, and fifth toes (flexion of phalanges II-V).
The popliteus muscle in the leg is used for unlocking the knees when walking, by medially rotating the tibia during the closed chain portion of the gait cycle (one with the foot in contact with the ground). It is also used when sitting down and standing up. It is the only muscle in the posterior (back) compartment of the lower leg that acts just on the knee and not on the ankle. The gastrocnemius muscle acts on both joints.
The tibialis posterior is the most central of all the leg muscles, and is located in the deep posterior compartment of the leg.