There are two basic states of the foot that should be very familiar: arch up and arch down. The arch down state happens when, after the heel strikes the ground, the foot unwinds inside and the arch does a free fall due to gravity. Most people are able to do this so well they overdo it. More on this later.

The arch up state comes next, because that is what makes the foot a rigid lever for push off. The foot must wind back up inside to raise the arch and stiffen itself. If the arch stays down you have to propel yourself forward with a mushy foot. That is not only tiring because your muscles have to work harder, it leads to the common foot pains and deformities that afflict millions every day: bunion, plantar fasciitis, and metatarsalgia to name a few.

The arch down state is important to absorb shock and adjust to uneven floor surfaces. But when it is overdone, the arch can't get back up. Going down hill is easy, but how does it get back up, into the arch up state? About 90% of the population can't get back up enough to have normal foot function. Assisted by gravity, body weight, muscle weakness and hard floors, the foot learns to unwind and flatten the arch ever closer to the ground, getting flatter with age. You may have heard this called over-pronation, which is just a fancy name for flat, flexible feet.