Fallen arches are when the natural shape of your arch is limited and results in an increase of contact between the foot and the surface of the footwear. Feet like these tend to yield when cycling, so they need support to sustain the arch, without massively changing how the cyclists normally rides.
Medium arches are the most common type of foot and represents what many defined as a "normal" foot. It has a correct distribution between the arch\'s dimension and the foothold\'s surface. During the riding action it needs an appropriate support to work with the transversal arch, but it won\'t need additional material to sustain the foot.
A hollow arch is a foot with a pronounced arch that is higher and slightly shorter than other arches. It\'s a foot that is functionally more reactive but under load tends to become stiff. During the riding action, the arch\'s height and length changes significantly between the pushing phase and the recovery phase. Hollow arches require a footbed with an appropriate support that takes into account the different shapes of the foot during the riding action.