Image caption Rowers can have a large build-up of acid in their muscles after a race

Sportsmen and women pushing their bodies to the limit. That's what the Olympic Games are all about.

We have become familiar with the sight of rowers collapsing after giving everything in pursuit of gold, of athletes staggering over the finish line and cyclists with nothing left in their legs.

So what is happening when these elite athletes collapse with effort and exhaustion? Is a build-up of lactic acid in the muscles to blame, as many people assume?

Lactic acid is often used to describe the pain felt during strenuous exercise and the cause of sore muscles, cramps and fatigue afterwards.

But Dr Steve Ingham, head of physiology at the English Institute of Sport, says lactic acid has had a bad press.

"It's a natural consequence of training hard. But it's not the thing that hurts. That's your nerves picking up that your muscles are working hard."

Athletes can work harder for longer to produce the same amount of lactate in the bloodstream - and cope with it better. Dr Tom Saw, Sports and exercise physician

Lactic acid is a by-product when the muscles use carbohydrates to create energy for exercise. Lactic acid then breaks down into hydrogen ions and lactate, and it's the hydrogen ions that cause the pain.

Dr Ingham adds: "They are making it uncomfortable for you and they're saying 'stop it'."

Work harder

Where the average person in the street might stop running in response to this pain, a top athlete will show much more resilience.

Dr Tom Saw, a sports and exercise physician in Milton Keynes who has worked closely with GB Rowing, says this is to do with their training.

"Athletes can go for longer before the breakdown products of metabolism start causing problems. They can work harder for longer to produce the same amount of lactate in the bloodstream - and cope with it better."

In elite athletes, lactic acid starts to build up when they are operating at somewhere between 80% and 90% of their maximum heart rate.

Yet people like Sir Chris Hoy and Mo Farah need to train seriously at least five times a week to tolerate it and produce high volumes of it, whether it hurts or not.

"You can overcome it by pushing yourself very, very hard. As soon as you are finished exercising the body will use it as fuel," says Dr Ingham.

Image caption Sally Pearson of Australia after winning the gold medal in the Women's 100m hurdles

Many years of research in this area by scientists at the University of California, Berkeley, has found that endurance training makes lactic acid a friend - not a foe.

They say that intense training teaches the body to use lactic acid as a source of fuel on a par with the carbohydrates stored in muscle tissue and the sugar in blood.

Lactate can then be burned with oxygen to create even more energy.

This efficient use of lactic acid not only prevents lactate build-up, it ekes out more energy from the body's fuel.

If you're going to go chasing after an animal to eat it and expend lots of energy, then it needs to be worth it. Dr Steve Ingham, English Institute of Sport

Jelly legs

Dealing with the muscle pain differs from sport to sport, but the key is to keep the muscles moving, says Dr Saw.

"You see cyclists going straight on to stationary bikes after their races to recover.

"Rowers have to stop rowing quickly after a long race. Because of that sudden stopping their muscles are full of hydrogen ions and there's an imbalance of biochemistry in their legs, which causes pain and weakness."

Hence the pictures of rowers being helped from their boats by Sir Steve Redgrave.

The key to getting rid of pain-inducing hydrogen ions is simply to train harder, although some people, like Redgrave and Sir Matthew Pinsent, are just genetically predisposed to being 'big engines'.

These are athletes who can get rid of hydrogen ions, by breathing in oxygen and creating more energy.

The GB rowing squads are tested on their lactate thresholds - how much effort they can put in before the lactic acid builds up.

"The slower it builds up the better," says Dr Saw.

In the end, your body will know when you've had enough.

Dr Ingham says lactic acid is simply evolutionary feedback, a useful tool for ancient man on the hunt for food.

"It's your body telling you that energy is in poor supply.

"If you're going to go chasing after an animal to eat it, and expend lots of energy, then it needs to be worth it."