From the June 2012 issue of Runner’s World
Just what goes on inside your body’s control room when you tackle 42.2 kilometres?
By Ruth Emmett
The repeated impact of your feet hitting the ground sends a force equal to twice your body weight reverberating through your body. Over a 42.2-kilometre course, this can actually cause a marathoner to (temporarily) lose 2cm of height.
Around 708 per cent of the energy produced in your muscles is lost as heat throughout the marathon, causing your body temperature to creep up by 2-5 degrees Celsius, depending on the weather.
In an effort to normalise your body temperature and cool you back down, your sweat glands pump out up to a litre of sweat per hour. You need about 500mL of sports drink per hour to replace the water and electrolytes lost through this process.
Your blood pressure will steadily creep up throughout the race, despite your blood vessels widening to try to limit the rise. Then, after a spike in adrenaline as you cross the finish line, blood pressure drop vertiginously, leaving you feeling dizzy if you don’t cool down with a walk.
A 67kg runner will burn 10,050-10,885 kilojoules during the race.
The cramp-causing byproduct of anaerobic respiration, lactic acid can build up in the final few kilometres of a marathon if your body fails to get enough oxygen to your muscles. Speed demons take note: in one study, sub-2:45 runners had almost double the lactate in their blood upon finishing than marathoners who clocked 3:45.
As muscles call out for blood-borne oxygen and nutrients, the heart speeds up to around 140 beats per minute (bpm) and can go as high as 180bpm in a final kick towards the finish. The average resting heart rate is 70-80bpm.
Blood Plasma Volume
Blood cells travel via a watery liquid called plasma, the volume of which drops by around 4.7 per cent during a marathon thanks to fluid loss through sweat. This means your heart has to work even harder to pump this thicker blood around.
Most studies show a post-race increase in blood sodium and potassium levels – but only because the salts have become more concentrated after the loss of blood plasma volume. In fact, runners need to top up electrolyte levels at regular intervals to avoid hyponatraemia (dangerously low levels of salts in the blood).
The body’s first choice for energy, glucose is stored in the body as glycogen. In a study, blood-glucose levels almost halved during a marathon, from 14.4 to 7.4mmol/l. The morning after the race, muscle glycogen content was still 56 per cent lower than pre-race levels.
In one study, marathoners experienced an average of 2.5kg reduction in body mass over 42.2 kilometres, the equivalent of dropping 0.69 points from their average body mass index (BMI).
In the same study marathoners lost an average 0.48kg of fat of their body fat. This is because after one to two hours on your feet, your body resorts to burning fat when liver glycogen stores run low.
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