What are you doing to your body?

Marathon Countdown:It is not just your legs that carry you through the race. Each organ has to work hard to get you to the finishing-line. Ian O'Riordanfinds out how the marathon affects your body

As you would expect, it's the legs that take most of the pounding when running 26.2 miles, but the marathon is also an endurance test for practically every working part of the body. Few people know exactly what goes on inside the heart, lungs, liver, brain, kidneys, etc, when running this great distance, and in some cases may not want to know.

Yet each of these organs play a key role in helping you get that finisher's T-shirt. Essentially, they are all working together to keep the energy process in motion. But when it comes to the marathon, this presents something of a metabolic dilemma. While energy stored as fat is plentiful, energy stored as carbohydrate is not. It means carbohydrate exhaustion in working muscles with subsequent slowing of race pace is inevitable.

While fat is, therefore, the logical energy source, it also relies on a plentiful oxygen supply through the bloodstream.

The battle to maintain the ideal energy process is one of the many struggles the body endures over the course of a marathon.

Someone who knows all about this is Dr Giles Warrington, a sport and exercise physiologist and lecturer in the School of Health and Human Performance at Dublin City University.

He is also head sports physiologist to the Olympic Council of Ireland, and was a member of the medical team at the 2004 Athens Olympics. Over the past 18 years he has worked with and advised elite athletes from over 20 different sports, and the accompanying diagram is his insight in the physiology of marathon running. (Click on the image to see the diagram in full.)