The way I learned it is that there are two types flat feet (as you
mentioned): 'Rigid' and 'flexible'...aka 'pronated'
As I understand it, and correct me if i'm wrong, a Rigid flat foot has
no arch at all, congenital condition where arch was never formed. This is even without weight on the feet, the foot is completely flat.
Now, a Flexible Flat foot or pronated flat foot, actually has an
arch, but it collapses or flattens when a person stands on it. this is the most common flat foot. A rigid flat foot
is actually very rare.
As I said, I thought I had a rigid flat foot, because it looks very
flat when no weight applied, plus when I do a wet test (wet footprint
on paper) as someone else suggested here, it is almost complete
flat footprint...notice 'almost' an arch does show in the print even if very little. A Rigid flat foot will show no curve for arch. But when I flex my toes, a slight arch does appear on the foot. My pt told me I have a very flat foot, but it actually
flattens due to overpronation...and it wasnt' that i was born archless...this is what i was told..anybody more educated in the matter is welcome to set me straight!