Your shoes hurt.
Modern footwear is causing us to change the way we move and, as a result, hurts our feet.
[New research from Harvard evolutionary biologist Daniel E. Lieberman ’86] examines how runners coped with the force of collision on their feet before the introduction of the modern running shoe in the 1970s. Comparing habitually barefoot runners with shoe-wearing runners, Lieberman and his fellow researchers found that runners who run with footwear tended to land on their heels, while runners who run barefoot tended to land on the front or middle parts of their feet.
Because humans do not have rigid bodies, only a certain percentage of human body mass—called effective mass—can feel the force of an impact.
Abstract, via Nature:
Humans have engaged in endurance running for millions of years, but the modern running shoe was not invented until the 1970s. For most of human evolutionary history, runners were either barefoot or wore minimal footwear such as sandals or moccasins with smaller heels and little cushioning relative to modern running shoes. We wondered how runners coped with the impact caused by the foot colliding with the ground before the invention of the modern shoe. Here we show that habitually barefoot endurance runners often land on the fore-foot (fore-foot strike) before bringing down the heel, but they sometimes land with a flat foot (mid-foot strike) or, less often, on the heel (rear-foot strike). In contrast, habitually shod runners mostly rear-foot strike, facilitated by the elevated and cushioned heel of the modern running shoe. Kinematic and kinetic analyses show that even on hard surfaces, barefoot runners who fore-foot strike generate smaller collision forces than shod rear-foot strikers. This difference results primarily from a more plantarflexed foot at landing and more ankle compliance during impact, decreasing the effective mass of the body that collides with the ground. Fore-foot- and mid-foot-strike gaits were probably more common when humans ran barefoot or in minimal shoes, and may protect the feet and lower limbs from some of the impact-related injuries now experienced by a high percentage of runners.