Foot callus thickness does not trade off protection for tactile sensitivity during walking.

Until relatively recently, humans, similar to other animals, were habitually barefoot. Therefore, the soles of our feet were the only direct contact between the body and the ground when walking. There is indirect evidence that footwear such as sandals and moccasins were first invented within the past 40 thousand years1, the oldest recovered footwear dates to eight thousand years ago2 and inexpensive shoes with cushioned heels were not developed until the Industrial Revolution3. Because calluses-thickened and hardened areas of the epidermal layer of the skin-are the evolutionary solution to protecting the foot, we wondered whether they differ from shoes in maintaining tactile sensitivity during walking, especially at initial foot contact, to improve safety on surfaces that can be slippery, abrasive or otherwise injurious or uncomfortable. Here we show that, as expected, people from Kenya and the United States who frequently walk barefoot have thicker and harder calluses than those who typically use footwear. However, in contrast to shoes, callus thickness does not trade-off protection, measured as hardness and stiffness, for the ability to perceive tactile stimuli at frequencies experienced during walking. Additionally, unlike cushioned footwear, callus thickness does not affect how hard the feet strike the ground during walking, as indicated by impact forces. Along with providing protection and comfort at the cost of tactile sensitivity, cushioned footwear also lowers rates of loading at impact but increases force impulses, with unknown effects on the skeleton that merit future study.