The flexor hallucis longus is a long, strong muscle in your lower leg that helps you bend your big toe downward and supports the arch of your foot. It starts on the back of the lower part of your fibula (the smaller bone on the outside of your lower leg), then runs down behind the inside of your ankle, travels under your foot, and attaches to the tip of your big toe. Because it crosses the ankle and the big toe joint, it influences both foot stability and how you move through each step.
In daily life and training, it’s a key “push-off” muscle. When you walk, run, jump, sprint, or do anything explosive, it helps you press the big toe into the ground so you can drive forward. It also acts like a dynamic strap for your arch, helping your foot stay stable when your body weight shifts onto one leg, like during lunges, single-leg work, or changing direction.
You may notice it most during calf raises, especially when your heel is low and you’re rising up onto the ball of the foot. If your big toe can’t bend well, or if the muscle is irritated, your calf work, running mechanics, and even squat or deadlift stability can feel “off.” Tightness or overuse can show up as pain behind the inner ankle, under the foot, or near the big toe, and it can contribute to foot cramps or a feeling that your arch gets tired quickly.
Even if your main goal is visible abs, this muscle matters because good foot mechanics improve training quality. Stable feet help you brace better, transfer force efficiently, and keep your core engaged under load, which supports consistent, safe progress.