Inside the body of a runner

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How endurance runners push their bodies to the limit, changing their physiology forever

A marathon is one of the toughest endurance running events on the planet, but race day is just a tiny part of the challenge. On average it takes a runner four-and-a-half hours to complete the 42.2-kilometre (26.2 mile) course, but before that they must endure months of training. During this period, runners average 80 kilometres a week or more, fundamentally changing the fabric of their bodies.

A marathon run drains thousands of calories, and the first major shift in runner physiology is in energy consumption. The body has three energy systems that supply fuel to working muscles, and they all depend on a small molecule called ATP. ATP stands for adenosine triphosphate – a molecule made from an adenosine attached to three phosphate groups. When muscles contract, they snap off one of ATP’s phosphate groups, turning the triphosphate into a diphosphate, called ADP. To carry on exercising, the body needs to reverse the process.

This article was originally published in How It Works issue 137, written by Laura Mears 

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