Usain Bolt of Jamaica appeared on a video screen in a white singlet and black tights, sprinting in slow motion through the final half of a 100-metre race. Each stride covered nine feet, his upper body moving up and down almost imperceptibly, his feet striking the track and rising so rapidly that his heels did not touch the ground.

Bolt is the fastest sprinter in history, the world-record holder at 100 and 200 metres and the only person to win both events at three Olympics. Yet as he approaches his 31st birthday and retirement this summer, scientists are still trying to fully understand how Bolt achieved his unprecedented speed.