The tibialis posterior is a deep muscle in your lower leg that plays a big role in how your foot and ankle handle force when you walk, run, jump, and lift. It starts along the back of the tibia and fibula and the strong membrane between them, then becomes a tendon that runs behind the inside ankle bone. From there it attaches to several bones on the inside of the foot, including the area around the arch.
Its main job is to support the arch and keep the foot from collapsing inward under load. When it contracts, it helps turn the sole of the foot slightly inward and helps point the toes down a bit. That inward control matters because most people naturally pronate a little as they land, but too much inward roll can stress the ankle, knee, and even the hip over time. The tibialis posterior acts like a stabilizing cable that keeps the arch strong and the heel and midfoot tracking well, especially during the push off phase of walking and running.
For clients chasing visible abs, this still matters. If your feet and ankles are unstable, your whole system tends to leak force. Squats, deadlifts, lunges, loaded carries, sprint work, and even planks and hanging leg raises feel less solid because you cannot create as strong a base. Good tibialis posterior function helps you maintain better alignment from the ground up, which can improve training quality and reduce overuse issues that interrupt consistency.
When this muscle or tendon gets irritated or weak, people often notice soreness along the inside of the ankle or arch, fatigue in the foot during long sessions, or a gradual flattening of the arch. Strengthening the foot and calf complex, improving ankle control, and managing training volume usually restores it and keeps your foundation strong for everything else.