Over pronation

Feet that flatten out upon standing are referred to as over-pronated feet or 'flat feet'. Over-pronation means that the ankles roll in towards each other and the arches flatten.

Some people have a fairly high arch when they are sitting, but as soon as they stand up or when they walk, their foot arch collapses. If your feet collapse or pronate, then the lower legs  internally rotate contributing to knee joint pain, particularly at the anterior or front and medial or inside of the knee.  The knee-cap can move side-to-side which creates a problem called "runner's knee" or "patellofemoral syndrome".

Furthermore, if the feet collapse, then the muscles and tendons extending from the lower leg down to the foot stretch or strain excessively (often referred to as shin splints).