"Fallen arches" is a common term used to describe a flatfoot condition that develops during adulthood. This should not be confused with other causes of flatfoot that may develop during childhood or adolescence.
Most cases of “fallen arches” develop when the main arch-supporting tendon (the posterior tibial tendon) becomes weakened or injured, causing the arch to gradually become lower. With time, the shape of the foot changes and secondary symptoms start to appear.
Common problems associated with fallen arches include plantar fasciitis, tendonitis, increased fatigue, and arthritis of the foot and ankle.
Additional information is available by reading Posterior Tibial Tendon Dysfunction (PTTD), Flexible Flatfoot.