Depends a lot on degree and nature of Flat Feet. Studies have shown people with flat feet are 50% more likely to have osteoarthritis. Also postural problems have been found linked to flat feet.
The chain reaction starts with a fallen arch causing inward twist to the ankle, shin and knee, in a corkscrew motion putting stress on the hip and pelvic joints. Then the spine will tilt and the back muscles will strive towards correcting an upright posture