A YouTube video of Robert Gill, an undrafted rookie for the Arizona Cardinals, running on a treadmill set at 25 mph has garnered over two million hits in less than two weeks, but how does his speed stack up in the world of great sprinters?

Gill's gallop actually out paces Usain Bolt's sprint from last year's Olympic — average speed 23 mph — although the Jamaican gold medalist ran for about four seconds longer.

Bolt's supersonic speed has been attributed to his extremely long legs and stride — two traits that have gradually evolved over the decades in the field of sprinting.

However, Robert Gill lacks this trait; at 5'10", Gill is more than half a foot shorter than Usain Bolt.

Gill's pace probably lies in the muscle content of his giant thighs. Years of training have undoubtedly built the fast-twitch muscle fibers in his legs, which are required for swift movements. A genetic predisposition toward building fast-twitch muscles has probably given Gill the edge in terms of speed, but not necessarily endurance.

What's cool is, one day, scientists might be able to program endurance into muscle fibers. Two years ago, a study on mice found that turning off one gene, called the IL-15Ra, could switch fast-twitch muscles into slow-twitch muscles. These genetically modified mice could then run for hours without tiring. Drugs that block this gene could, in theory, help humans improve their endurance.