Running without practice could prove fatal
Last weekend, four male competitors died during the Great North Run, the world's largest half-marathon event. Stifling temperatures and high humidity were blamed. Here, leading sports physician Dr Tom Crisp describes what happens to the average runner's body during a marathon - and why a few unlucky people never make it to the finish line
0-30 mins (3.25 miles) Body heat rises
Even before the race has started, a runner will have a raised heart rate due to adrenaline and the excitement. This is perfectly normal and helps to get the body prepared for the challenge.
An average male runner, aged about 40, should have a resting pulse rate of close to 80 beats per minute. Women tend to have slightly faster resting rates.
In the first half-hour of the race, the heart rate of both men and women should climb to about 140 beats per minute. A healthy person who has done plenty of training should find this no problem.
Body temperature, which starts at around 37C, will also climb to around 40C. The hotter the weather, the hotter the runner will be. The body increases the output of sweat to try to cool the body.
Temperatures in the North East last Sunday reached 68F (20C), four degrees higher than the average, which meant runners were at greater risk of over-heating.
30mins to 1 hr (6.5 miles) Mild dehydration
Now, the runner should be in his stride and the going should be relatively easy. The heart rate will be at a steady 140 beats per minute. The body will be relying on carbohydrate fuel, in the form of glycogen manufactured by the liver.
Mild dehydration could become a problem, particularly if someone has failed to drink plenty before a race. Drinking alcohol the night before, or even too much coffee on the morning of the race, can make dehydration more likely.
1 to 2 hrs (13 miles) Burning fat
The body now starts to burn fat to power the muscles. The efficiency of this depends on the fitness of the runner. Stores of glycogen are running low.
The heart rate may have crept up and the body temperature could be hovering at 41C, which is as high as you can safely go before the risk of heat stroke, which can be fatal.
A fit runner should only be running at around 70 per cent maximum effort and keeping some in reserve.
A runner must make sure he gets regular drinks to stop himself becoming dangerously dehydrated. In some cases, he may suffer stomach cramps due to the fact that oxygen-rich blood has been diverted away from the digestive system to the muscles.
Some runners also suffer from diarrhoea, caused by the interruption of the normal bowel motion.
The bladder should not need to be emptied because the kidneys slow down urine production during strenuous exercise due to the fact the blood is diverted away from the kidneys.
2 to 3 hrs (19.5 miles) Lactic acid build-up
This is the point when many runners realise they can go no further. The body's glycogen stores are now exhausted and the muscles must rely on the breakdown of fat.
People who have not trained properly can start to go into anaerobic respiration, when there is too little oxygen reaching the muscles.
If you are out of shape, your body is not very efficient at taking in oxygen, and you hit your anaerobic threshold while exercising at very low levels of intensity.
One of the by-products of anaerobic respiration is lactic acid, which causes pain and muscular cramps. Lactic acid also makes it more difficult for the body to break down fat, so a runner slows right down.
At the end of this period, many runners hit the so-called Wall.
This is a psychological and physiological barrier when you feel that you cannot go another step.
Glycogen levels have bottomed out so blood sugar levels are very low. Blood sugar is needed as fuel for the brain, so you can feel faint and woozy. Some competitors are mentally exhausted, or even just bored.
Lactic acid levels may be high and the levels of important salts in the body, known as electrolytes, which include sodium and potassium chloride, may be very low.
The joints, particularly the knee caps, will be sore because they are under an enormous amount of stress as the legs pound the hard road surface.
3 to 4 hrs (25 miles) Exhaustion point
The runner's body is now reaching exhaustion point and his pace has usually slowed right down.
This is the point where people at risk may suffer from heart attacks, because the heart is under maximum stress. For an unfit runner in a half-marathon such as the Great North Run, that point will come much earlier.
Dehydration is also more likely, which thickens the blood and slows down the circulatory system. This means that the heart is working even harder to push the sluggish blood around the body.
There are two possible types of heart attack victim. Younger runners may collapse for no apparent reason if they have an undiagnosed genetic condition called cardiomyopathy, when the heart muscle is abnormal.
Older runners, in their 40s and 50s, are more at risk of collapse due to pre-existing coronary heart disease, when arteries in the body become narrowed by fatty deposits made of cholesterol.
4+ hours (the last mile) Dangerous heart rate
Even exhausted runners will often push themselves to the limit at the last to try to make a target or beat the runner ahead.
The heart rate can soar to near the maximum of 180 beats a minute and the blood pressure goes through the roof. The core body temperature can go dangerously high. The heart can suddenly fail within sight of the finish line.
After the race is over. Beware hypothermia.
When a runner stops after completing a marathon, the blood pressure plummets and can even go too low. This can lead to fainting and giddiness. It is important to keep walking around so the blood pressure has a chance to go back to normal more slowly.
The heart rate should go back to normal fairly quickly, as will the body's core temperature.
In cold weather, a runner should make sure they quickly put on something warm, or they could suffer from hypothermia, when the body cools down too much.
A fit runner will have a normal resting heart rate just 10-16 minutes after the race is over. Muscles and joints will be very sore for several days.Read more:
http://www.dailymail.co.uk/health/article-362942/Lethal-threats-marathon-running.html#ixzz1oup8vB7F
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