A new study published in the Journal of Applied Physiology theorizes that Usain Bolt's astounding speed might not begin to push the limits of human running. Bolt has been clocked at 28 mph but researchers foresee humans reaching speeds of 35 or even 40 mph without mechanical assistance.

It has been believed for quite some time that the physical forces being exerted by today's top sprinters had reached the limits of human physiology. Sprinters exert peak forces of 800-1000 pounds on a single limb with each sprinting step and it was thought that that was the force limit for muscles and limbs. A new paper contradicts that assessment after studying the forces applied by athlete's doing one-legged hops.

The study showed that one-legged hops exerted more than 30% more peak forces on the limbs and suggests that athletes might be able to push running speeds even higher. It seems that time, specifically the brief moment that a sprinter's foot touches the ground and exerts force, is the biggest limiting factor to how fast a person can run.

A world-class sprinter's foot contacts the ground for less than one-tenth of a second, with peak ground forces occurring within the first one-twentieth of a second of foot to ground contact. Based on experiments conducted on state of the art treadmills researchers are changing their way of thinking about peak human speed. Their work suggests that speeds are more limited by the speed at which the muscles themselves work.

Muscle fiber contractile speeds limit how effectively muscles work, suggesting that proper training and fitness techniques could continue to raise top running speeds.

"Our simple projections indicate that muscle contractile speeds that would allow for maximal or near-maximal forces would permit running speeds of 35 to 40 miles per hour and conceivably faster," said Matthew Bundle, an assistant professor of biomechanics at the University of Wyoming. Bundle says the data shows that it is the biological limits of the muscles that restrict running speeds, and not a physiological one, as had been the traditional thinking on the issue.

As for Bolt it seems there's no need to worry just yet. The team has only been able to test this theory by building a special treadmill and ‘runner' for the tests and the mechanical runner isn't eligible for the Olympic trials.