Midsole

The midsole is the layer in between the outsole and the insole, typically there for shock absorption. Some types of shoes, like running shoes, have additional material for shock absorption, usually beneath the heel of the foot, where one puts the most pressure down. Some shoes may not have a midsole at all.

Running shoes have a thick midsole. In contrast, racing flats, which are designed to be lightweight, have a thin midsole.

Midsoles are typically made of ethyl vinyl acetate (EVA), which is the same material often used for orthotic topcovers, only firmer. In general, gray midsole EVA is firm and white EVA is soft.

The firmer the midsole, the stiffer and heavier it will make the shoe. Soft midsoles, on the other hand, improve shock absorption but wear more quickly. Firm midsoles are generally favored for plantar fasciitis, Achilles tendonitis, posterior tibial tendonitis, hallux limitus/rigidus and most other conditions related to excessive pronation. Soft midsoles can be helpful for shin splints due to tibial varum and metatarsalgia.