The only man on the planet who could have stopped Maurice Greene winning the 100m title was not Tim Montgomery, Ato Boldon or Dwain Chambers.

It was a 61-year-old retired primary school teacher from Calgary, Alberta, named Barrie Callaway.

Callaway is the chief starter at the World Championships. It was his job to get the men's 100m final underway and his responsibility to recall the athletes in the event of a false start.

Sprinters spend months working on their starts, for the very obvious reason that the first out of the blocks is often the first to the finishing line.

In the final of the women's 100m, Zhanna Pintusevich-Block beat Marion Jones to gold by three hundredths of a second, a gap that can be almost entirely put down to her better reaction at the start.

The Ukrainian was out of the blocks on 0.123secs, compared to Jones' 0.145secs.

"The primary reason for us being out there is to produce a fair start, " he says. "If you can't do that you shouldn't be out there."

"I very rarely see the end of the race, because I'm generally getting ready for the next one."

"Somebody told me they showed the men's 100m final five times on the giant screens in the stadium, but I didn't see it once."

"I wish I had. I wish I'd just looked up for that moment at the screen - but I was occupied."

Callaway's role is pivotal because it is him, and him alone, who decides how long to hold the sprinters between the 'set' and the 'bang'.

To prevent the runners anticipating the gun, there is no official rule on how long the hold should be

"You watch the athletes when you say 'set'," says Callaway.

"You watch them rise up to their final set position, and when they're all ready you can fire the gun.

"That means that no start will ever be the same, because all athletes are different. In the earlier rounds, where you have some runners who are not internationally experienced, that has an effect on the start.

"The experienced athletes make it easier. When you say, 'set', they all rise up together, hold, you look along the line and then you can fire the gun.

"The ideal 'set hold' time is between 1.4secs and 2.2secs."

In the men's 100m final on Sunday, when the sprinters' nerves were screaming on the blocks, there were three false starts.

"Experienced starters do not give fast guns after a false start," he says.

"An inexperienced starter would be likely to get quicker with the gun, to not even give them the chance to false starts."

Just as the sprinters themselves have risen through the ranks to be in the world final, so starters have to prove themselves over the years.