You’re ready to initiate the start of your first or 10th marathon, as your heels go up and down on the pavement, and your heart is racing as you have physically and mentally prepared for this event for weeks, or even months. The adrenaline, endurance, and speed to run long distances is countlessly done by humans every day, but just exactly how are our bodies able to run a marathon? According to the endurance running hypothesis, it all has to do with how we’re built.

“Long distance running built us,” Joe Hanson, host and writer of PBS' It’s Okay To Be Smart, said in the video. This stems from the belief human characteristics could be explained as adaptions to long distance running. Our large ear canals help us balance while we run, our eye reflexes keep our head steady as we move up and down, while our short arms and thin ankles take us less effort to swing. We are able to counter the rotation of our moving legs with our wide shoulders, thin waist, and narrow pelvis, while our sweat glands, body hair, and stature let us disperse more heat. Our brain, gluteus maximus muscles, and joints help to stabilize the upper body and give us shock absorption.

In marathon running, many of our systems are working simultaneously, and soon muscles throughout the body are demanding more oxygen than the body may supply, known as anaerobic threshold (AT). At this stage, the brain may force the body to slow down, leading the runner to “hit the wall,” signaling there are low blood sugar levels. Runners often feel fatigue before reaching the halfway mark of the race, so it is important to consume carbs throughout the course of the marathon.

“Joy, fatigue, and pain only exists in your mind,” Hanson said, because the “mind is connected to muscles.” Running a marathon is a competition against yourself and consists of having pure willpower. The pacing, hydration, and fueling of a marathon makes humans contenders in nature’s distance events.