The world's fastest man Usain Bolt says he is sad that questions about doping in athletics are taking centre stage before the World Championships in Beijing.
The Jamaican world record holder says he is running for himself and that the responsibility for clean sport is "not only on me".
Bolt is expected to face AmericanJustin Gatlin, the world's fastest man this season - in Sunday's 100m final.
Gatlin have served two drugs bans.