Paul Hackett / Reuters

Usain Bolt rocked his huge frame back into his too-small chair, wrapped his arms behind his head and prepared to deliver his well-worn defense after another bombardment of doping-related questions at a news conference on Thursday.

Ahead of Bolt's return to London for this weekend's Diamond League meet, the script was supposed to be all about his happy memories from 12 months ago when the Jamaican lit up the Olympic stadium with another superlative triple-gold medal performance in front of 80,000 dazzled fans.

Yet the No 1 topic was the recent positive dope tests on fellow Jamaicans Asafa Powell and Sherone Simpson as well American Tyson Gay, leading one journalist to ask Bolt if the public could trust the fastest man in history.

"How long have you been following Usain Bolt - maybe since 2008?" the runner responded.

"If you'd been following me from 2002 you would know I've been doing phenomenal things since I was 15.

"I was the youngest person to win the world juniors at 15. I set a world youth junior record at 17.

"I've broken records in every race I've ever done, so I've proven myself. You could say I'm underperforming right now.

"I was made to inspire people and to run. I was given a gift and that's what I do. I'm going to continue running and using my talent to help the sport."

Bolt is accustomed to his role as his sport's beacon of hope, and with every failed test his position becomes more important to the credibility of athletics.

Next month he will race in the world championships in Moscow, where his main rival is likely to be American Justin Gatlin, a twice-convicted doper now back in the big time.

Powell and Gay, the fastest man in the world this year, will be watching from afar and working on their defense that their failed tests were the result of inadvertently taking contaminated supplements.

Bolt was cagey when asked about the latest batch of positive tests.

While he stopped short of condemning his rivals, he did issue a reminder of the principle that every athlete is responsible for what they put into their body.

"I think there are a lot of details that are still to be discussed, a lot of things that haven't been said so I'm just waiting to see what happens," he said.

"In life things happen and people make mistakes. But as an athlete you have to be very careful and aware. It's hard, but that's why you have a team to help you out with these things."

Powell was said to be on an incredible regime of 19 supplements, including some injectable, but Bolt, who said he had spoken to his compatriot and told him to "stay strong." said he trod a far safer path.

"I have vitamins that I take, every athlete does, but I don't really take supplements," he said.

"I work hard every day, go out with one focus and don't worry about other athletes.

"I'm not going to stress about it (doping). I know I am clean and I just want to improve the sport.

"Definitely it's going to set us back a little bit but I can't focus on this. I still have the worlds ahead of me and everyone is stepping up their game."

Bolt, who ran the 100 meters on Friday night, is also part of Jamaica's 4x100m relay squad.

His best time of the year so far is 9.94 sec - pedestrian alongside his 2009 world record of 9.58 - but he has looked sharper over 200m, where his 19.73 leads the world.

"I could have run faster at the national trials; the main aim was to go there and qualify," he said.

"I've had three weeks of solid training, technique work, speed endurance, working on my core. I'm in great shape.

"This year I want to break a world record, but it has been an up and down season. Hopefully this will put me on the right path to run fast at the worlds."