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The world's fastest man, Usain Bolt, will run his last ever professional race at the World Athletics Championships in London, which begin on 4 August.

He's been an unstoppable force since winning his first Olympic gold medal in Beijing, China, nine years ago.

Between 2008 and last summer's Rio Olympics in Brazil, Bolt has won an incredible 19 Olympic and World Championship gold medals out of the 20 events he took part in.

It's a medal collection that makes him 'the greatest sprinter of all time', according to the International Olympic Committee.

But why is it not 20 gold medals from 20 races? What happened when world record holder Bolt did not win?

Let's go back to the World Athletics Championships in Daegu, South Korea, in 2011. It's the 100m final.

Usain has looked good so far during the competition. He's been among the fastest sprinters in the qualifying rounds.

As he get ready for the final, the Jamaican does his famous lightning bolt pose and the crowd cheers.

On your marks, set, go!

Usain springs out of the blocks but then the starter's gun goes off. That's not what's supposed to happen.

Disaster, it's a false start!

A false start happens when an athlete begins a race before the official signal has been given.

They can't start running until after the starter's gun has been fired.

The Jamaican sprinter is well known for starting his races slowly. Some scientists think this is because Bolt is so tall, making it hard for him to get up to full speed quickly.

He makes up for the slower starts with his long strides catching other racers further down the track.

Normally, judging by all the gold medals he's won, his slow start clearly doesn't cause him any problems.

Usain said: "I have worked hard on my start all season. In the first round of the 100m it was perfect, in the second it was OK. But in the final I think I was suffering from anxiety, I was ready to go and get on the track and run. I think anxiety got the best of me."

In the past, athletes were allowed one false start before being disqualified from a race if they did it a second time.

But in 2010, new rules were introduced by the people in charge of athletics, the International Association of Athletic Federations, which disqualified athletes after just one false start.

Many people did not agree with the new rules as they believed that everyone should allowed a second chance.

After Bolt was disqualified from the 2011 final, his training partner Yohan Blake took gold in 9.92 seconds - 0.4 seconds slower than Bolt's world record.

It wasn't the end of the Bolt's World Championships though.

He went on to win the 200m and 4x100m relay with his Jamaica teammates.

Usain then continued to dominate athletics for the next six years.