is the main cause of heel and arch pain. The plantar fascia is a large ligamentous structure that runs from the heel bone to the bases of the toes. It provides support to the arch and middle of the foot and acts as a “spring” to provide additional power for propulsion or push off when walking or running.
The plantar fascia is very strong in one way but weaker in another. It has great tensile strength. Tensile strength means “pulling strength.” If I take metal wire and pull on it until it breaks, the amount of pulling force that it took to break it is it's tensile strength. The same wire, if twisted, breaks much easier. In other words the metal wire has less torsional (twisting strength) than tensile strength.
Ligaments and tendons of the body have good tensile strength but are susceptible to damage either with strong twisting forces or smaller twisting forces that are repetitive. A foot that rolls in too much (overpronation) or rolls out too much (oversupination) leads to chronic repetitive twisting or torsional strain of the plantar fascia. This is very old information that has been forgotten or neglected in the design of many modern shoes.
The shank is the portion of the shoe that resists twisting or torsional forces. Steel shanks have existed for centuries. Other materials such as fiberglass, graphite and rigid thermoplastics can be used in lieu of steel to ensure that shanks in shoes do not twist or bend. The part of the shoe that corresponds to the ball of the foot should be flexible but not the shank. Here is a brief primer on running shoe anatomy: http://www.aapsm.org/runshoe-running-anatomy.html