The tibialis posterior muscle is a large muscle that runs along the back of the tibia bone and sends a tendon around the inside of ankle and foot. It then attaches to bones across the underside of the middle of the foot. Its job is to stiffen the foot and prevent the foot and ankle rolling inwards and limit how much the foot flattens. Degeneration of the tendon can cause pain and weakness on the inside of the foot and ankle. For more on the importance of tibialis posterior, click here.