What is Flexor Hallucis Longus Tendinopathy? The Flexor Hallucis Longus muscle is located in the lower leg, and attaches to the Flexor Hallucis Longus tendon. This tendon runs from the outside of the ankle, down the side of the foot. When the Flexor Hallucis Longus muscle becomes damaged from overuse, this also impacts the Flexor ...