Usain Bolt

Pro: At this year's World Championships, Bolt somehow topped his awe-inspiring performance at the Beijing Olympics. He ran the 100-m in 9.58 seconds and the 200-m in 19.19 seconds, breaking his own world records in each sprint and keeping track and field in the sports spotlight. In the wake of his latest achievements, scientists wondered if he'd pushed the limit of human athletic performance.

Con: Track and field is still essentially a fringe sport. Plus, given track's sordid steroid history, it's impossible for many observers not to wonder if performance-enhancing drugs had a role in his record-breaking dashes.

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