In my last post I highlighted some of the incredible things that distance runners are able to do, including very long runs at altitude (lower oxygen) and in extreme conditions. But what allows these people to do these kinds of things? The short answer is training. With enough training almost anyone (for the most part excluding the very elderly) could finish an ultra marathon. But why is this? The answer lies in the fact that humans are better adapted to run for long distances than any other animal on the planet. First of all, humans are bipedal meaning that we move around on two feet, and while other primates are able to walk with two limbs humans are the only primates who walk exclusively with only two legs. Bipedalism in itself isn't incredibly unique as other mammals such as macropods (kangaroos, wallabies...) and large birds like ostriches and emus rely on bipedal movement as well, however humans have other adaptations to make bipedalism more efficient. You may not realize it but the human foot is a very intricate mechanical structure containing 26 bones, 33 joints and over 100 muscles and tendons. While running the foot, specifically the arch, acts as a spring which absorbs and returns force to the ground which is done as follows: the foot lands on the outside of the forefoot and pronates inward, stretching muscles which absorb and store force. The foot rocks forward while it pronates so that by the time the front pad of the foot is flat on the ground the toes are pushing off the ground with the energy stored in the foot's muscles. In addition to the feet the rest of the muscles act as springs which store energy from the foot strike to be used as propulsion for that step. As a result, running is basically a process of converting kinetic energy (foot strike) into potential energy (stretched muscles) and back into kinetic energy (push off). Of course as in any system, energy is lost as heat thus cells must break down glucose during anaerobic and aerobic respiration to create ATP for your muscles to use to create additional energy to put into the ground.