The flexor digitorum longus is a long muscle in your lower leg that helps you grip the ground with your toes and control your foot when you walk, run, jump, and lift. It sits deep on the back and inside part of the lower leg, starting near the shin bone and traveling down behind the ankle. Its tendon then splits to attach to the ends of your four smaller toes. When it contracts, it bends those toes downward, which is the same action you use when you “claw” the floor for balance.
Even though it is not an ab muscle, it matters a lot for athletic movement and for building a body that looks tight and defined. Strong, well coordinated feet and ankles let you squat, hinge, sprint, and change direction with better stability. That stability helps you keep good alignment up the chain through the knees, hips, and pelvis, which makes your core work more efficiently. If your toes cannot help stabilize, your body often compensates by shifting pressure to the outside of the foot or collapsing the arch, and that can reduce power output and increase stress on the ankles, shins, knees, and even the lower back.
In training, this muscle works hard during loaded carries, deadlifts, lunges, calf raises, and anything that challenges balance. It also contributes to maintaining the arch of the foot, especially when you are barefoot or in flat shoes. Common signs it is weak or irritated include cramping under the foot, soreness along the inner shin, or discomfort behind the inside ankle, especially after running or high volume jumping.
For your six pack goal, think of it as part of the foundation. Better foot function improves training quality, and better training quality makes it easier to build muscle and drop body fat to reveal your abs.