KINGSTON, Jamaica - EVEN as Jamaicans received word of the country's best-ever performance at the World Youth Athletics Championships, comes the staggering news that several Jamaican athletes, including Olympic medallists Mr Asafa Powell and Ms Sherone Simpson, have tested positive for banned substances. Details related to those adverse analytical results were only just emerging as this commentary was being written. What is clear, however, is that following on an adverse finding on Mrs Veronica Campbell Brown a month ago, the credibility of Jamaica's track and field programme is now open to question. Of course, it's not just Jamaican athletics that's in trouble. News which coincidentally broke yesterday that the great American sprinter Mr Tyson Gay has also tested positive means USA athletics, and indeed, global athletics have their backs to the wall.