FYI: Why do so many marathon runners wobble across the finish line then faint?

It's been a week since the Standard Chartered marathon, yet some runners, especially those new to the sport, may still be feeling the effects every time they walk, sit or visit the bathroom. According to Sam Murphy, author of Marathon: From Start to Finish, the runners have small tears in their muscles, are 2cm shorter and could still be very dehydrated.

The body begins to deteriorate after about 30 minutes, or 5.2km, of running, sports physician Tom Crisp told a British newspaper after four male competitors died in last year's Great North Run half marathon. A runner's heartbeat will be on the rise even before the race begins as excitement and anticipation send adrenalin coursing through the veins, says Crisp.

For an average male runner in his 40s, the resting heart should beat 80 times a minute, while for women it's slightly faster. The heart rate for both sexes climbs to about 140 beats a minute during the first half-hour of a race, while body temperature rises from 37 degrees Celsius to 40 degrees. Last Sunday's 'very high' pollution rating and temperatures close to 20 degrees would have compounded the stress on the body. In these conditions, it's easy for runners to overheat.

At 10.5km, mild dehydration sets in as experienced runners settle into their groove. The heart should be pumping at a steady 140 beats a minute, while the body relies on carbohydrate fuel manufactured in the liver as glycogen. Dehydration could become a problem.

Two hours into a race, at about the 20km mark, the body is burning fat to power the muscles as stores of glycogen begin to run low. The risk of heatstroke at this point is high. Some may experience cramp as oxygen-rich blood is diverted to the muscles from the digestive system. With less blood reaching the kidneys, urine production slows. However, diarrhoea can be a problem as normal bowel motion is interrupted.

After three hours, or about 30km, many runners hit a wall. Glycogen levels are extremely low and muscles are relying on fat stores. Those without adequate training can start to go into anaerobic respiration as too little oxygen reaches the muscles. A build up of lactic acid causes pain and muscular cramps, while the acid makes it difficult for the body to break down fat, causing the runner to slow. Levels of important electrolytes are also low. Depleted glycogen stores drag down blood-sugar levels leaving runners faint, woozy, mentally exhausted and even bored. Joints, particularly the kneecaps, will be sore and under stress.