The human frame is built to handle running speeds up to 40 miles per hour, scientists say. The only limiting factor is not how much brute force is required to push off the ground as previously thought, but how fast our muscle fibers can contract to ramp up that force.
"If you just find a way to rev up those contractile fibers for the muscle, then everything else from human biology and gait would allow us to be that fast," said physiologist Peter Weyand of Southern Methodist University, lead author of a study published Jan. 21 in the Journal of Applied Physiology.
For years, scientists have sought to find the physiological limits of human running speed, and to understand why even the world's fastest man, Jamaican sprinter Usain Bolt, can't outrun some animals. Bolt's top speed of 27.3 miles an hour can't match that of horses, dogs or the hopping kangaroo, which can travel at 35 miles per hour.
"The current best guess as to why we can't run any faster is it's something to do with the maximum force that our legs can impose or experience," said zoologist Jim Usherwood of the Royal Veterinary College in London.
Earlier studies found that elite sprinters and average sprinters outperformed their mediocre counterparts in two ways: They were able to push harder on the ground in relation to their body's weight with each step, and they were able to do that in a shorter period of time. For instance, Bolt generates almost a ton of force in his leg in the less than a tenth of a second his foot is on the ground, Weyand said. The force you can generate before leaving the ground is a function of the rate at which muscle fibers generate force multiplied by the time the foot touches the ground.
But the previous work didn't reveal whether it is the amount of time the foot is pushing on the ground or the maximum force the leg is capable of that limits human running speed.
To get to the bottom of the human speed limit, Weyand and colleagues studied seven athletes, from an all-American track star to a dancer, as they sprinted forward, hopped on one leg and ran backwards on a turbo treadmill outfitted with force sensors. The team measured speed, upward force as each foot struck the ground, and the time the foot spent on the ground between strides.
The one-legged hop generated much more force in the leg than sprinting did, mainly because a runner has to jump higher in order to land on the same foot. Because the athlete's legs were capable of generating more force than when they run, the amount of force can't be what's limiting the runners' forward speeds.
The backwards run showed that the contact time with the ground was almost identical during each athlete's fastest forward and backward runs, suggesting contact time was the limiting factor keeping them from going faster in any direction.
This suggests the only way to increase speed is to generate force more quickly during the limited time the foot is on the ground. The key to doing this is increasing how fast the muscle fibers can contract to produce force. If that were possible, Weyand's team calculated that humans could theoretically run as fast as 35 or 40 miles per hour, based on our gait and the maximum forces our muscles can generate.
