Flat feet or “fallen arches” are caused chiefly by weak flexor muscles. The major flexor muscles of the foot are meant to propel you forward by pushing against the earth behind you as you walk or run barefoot. However, lifelong use of shoes allows these muscles to atrophy and become weak. As these muscles weaken, they stretch out, allowing the ball of the foot to creep forward, resulting in the eventual collapse of the underside of the foot. Custom orthotics or dynamic rehabilitative orthotics are designed to recruit and strengthen the flexor muscles while you stand, walk, or run.