The flexor hallucis longus muscle is one of the three major deep muscles found in the lower back region of the leg. Specifically, the muscle spans part of the calf. It is the largest and strongest deep muscle of the leg's posterior section.
The muscle originates from just below the middle of the fibula, or calf bone, nearest the back of the leg. It then extends down the calf, to the side of the ankle, and into the foot. The muscle reaches all the way to the bone at the tip of the large toe known as the distal phalanx. The tibial nerve provides the muscle with nerve endings.
With the flexor hallucis longus muscle, an individual can flex his or her large toe. It also aids the ankle in pointing the foot to a downward position, an action known as plantar flexion. The flexor hallucis longus muscle aids the foot in turning to one side so that the sole is facing inward.