This provides a new twist on the old school of thought that speed limits depended on how much force a runner could exert against the ground. Past studies showed that sprinters can apply up to 1,000 pounds of force with a single limb during each sprinting step, and so researchers thought that humans simply could not push beyond that point.[...]
One-legged hopping produced ground forces greater than those applied during normal running by 30 percent or more, and active leg muscles also generated about 1.5 to 2 times greater force during one-legged hopping. That shows how humans don't exert the maximum possible force during the act of forward sprinting, the researchers say.
Going one step farther, the researchers also found that the "critical biological limit" depends upon how quickly runners can exert ground force while sprinting. Elite runners have foot-ground contact times of less than one-tenth of a second, and max out ground forces within one-twentieth of one second when their foot first hits the ground.
http://www.popsci.com/science/article/2010-01/scientists-investigate-how-break-biological-speed-limits-humans | Photo: US Department of State