The human foot and its mechanics are complex. When this dynamic system malfunctions it sets off a chain of adverse effects that impact on the leg and even on the rest of the body. The best visual example of an overpronated foot is what is normally called a flat foot. This may in itself not be painful.
An overpronated foot places more pressure on the inside of the foot, causing incorrect joint positioning in the knee, hip and opposite shoulder. Overpronation also causes an imbalance of the muscles in both the upper and lower leg. The inner leg muscles are underdeveloped and used while the outer leg muscles are overused and abused. This disturbs the core balance of the body.
South African Podiatry Association (SAPA)