Last year, Usain Bolt stunned athletics fans when he hacked 0.11 seconds off his previous world record for the 100-metre sprint. But what's the ultimate human speed limit?

Intrigued by this question, Mark Denny at Stanford University, California, decided to work out how fast a human could possibly sprint 100 metres. He examined previous records for various athletics competitions - and greyhound and horse races for good measure - since the 1920s, and found that performances in many events followed a similar pattern, improving steadily until they reached a plateau. Horses in the Kentucky Derby, for example, appeared to approach their speed limit in 1949. Since then any improvements have become minimal and increasingly rare.

Human athletes, too, seem to follow this pattern. Timings for the women's 100 metres approached a plateau in 1977. Male sprinters are still improving, but having constructed a model based on other events, Denny says they too seem on the verge of topping out. He predicts an absolute limit of 9.48 seconds for the 100 metres, just 0.1 seconds under Bolt's current record. "If he keeps on course he will soon come close to the absolute limit," says Denny.

What gives rise to this human speed limit? Denny reckons it's as simple as the athlete's power-to-weight ratio. Beyond a certain point, the benefits of stronger muscles and longer limbs will be offset by the increased energy required to shift the greater load.

Read more: Maxed out: Testing humans to destruction

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