The plantar fascia is a ligament that supports the foot. It spans from the base of the heel to the bases of the toes. It is a strong ligament. It is strong in tensile strength. Tensile strength means pulling strength. If one stretches, for example, a wire, the point at which the wire breaks is called it’s tensile strength. It is hard to break a wire by stretching it but it can easily break if one twists the wire. The plantar fascia is similar – strong in tensile strength but weak in torsional or twisting strength. Excess twisting of the fascia causes heel pain due to plantar fasciitis.
How does twisting of the fascia occur. The answer is side to side motion. When the heel strikes the ground, the foot lowers to ground, rolls inward in a motion called pronation. It is the joint below the ankle joint called the subtalar joint that allows pronation. Pronation is a twisting motion. Some pronation is normal but too much or overpronation causes excess torsional stress on the fascia. Chronic repetitive twisting of the fascia causes damage.
If one steps onto a pillow, the foot pronates more, the arch lowers more. Many shoes sold nowadays are like pillows – very soft. Softness sells shoes. The area of the shoe from the heel to the ball of the foot should have some rigidity but flexibility should exist at the base of the toes. Shoes, historically, have 3 parts to the sole: the insole, the midsole and the outsole. The insole and outsole are visible but the midsole, when present, maintains the integrity of the shoe and support for the foot. The midsole contains the shank. The shank is a still plate that prevents twisting of the shoe in the middle and is what protects the heel, arch and plantar fascia. Steel shanks were invented over 800 years ago. So shoemakers were aware that the shank was a necessary part of shoe construction for centuries. It is only more recently that shanks were left out of the shoemaking process. Shanks do not need to be made out of steel. Newer shank materials include fiberglass, carbon graphite, hard plastics or even kevlar.
Steel shanks may still be found in certain work boots and other types of boots such as cowboy boots.
How can one tell if a shoe has a shank? Try bending the shoe in the middle. It should bend at the ball of the foot only.