For most runners though, speed is largely determined by how much force they can apply when their foot is on the ground. They have two simple options for running faster: hit the ground harder, or exert the same force over a longer period.
The second option partly explains why greyhounds and cheetahs are so fast. They maximise their time on the ground using their bendy backbones. As their front feet land, their spines bend and collapse, so their back halves spend more time in the air before they have to come down. Then, their spines decompress, giving their front halves more time in the air and their back legs more time on the ground.
Such tricks aren’t available to us two-legged humans, but technology provides alternatives. In the 1990s, speed skaters started using a new breed of “clap skates” where the blade is hinged to the front of the boot, rather than firmly fixed. As the skaters pushed back, the new design kept their blades in longer contact with the ice, allowing them to exert the same force over more time. Speed records suddenly fell.
People have tried to duplicate the same effect with running shoes, but with little success. That’s because a running leg behaves a bit like a pogo stick. As it hits the ground, it compresses. As it steps off, it gets a bit of elastic rebound. Technologies that try to alter a runner’s gait tend to interfere with this rebound, and diminish the leg’s overall performance. “It’s hard to intervene in a similar manner to the clap-skates without buggering up the other mechanics of the limb,” says Weyand. (Again, Pistorius bucks the trend because his artificial legs are springier than natural ones, and give him around 10% longer on the ground than other runners.)
For those with intact limbs, one option remains: exert more force on the ground. Put simply, fast people hit the ground more forcefully than slow people, relative to their body weight. But we know very little about what contributes to that force, and we are terrible at predicting it based on a runner’s physique or movements.
We know that champion male sprinters can hit the ground with a force that’s around 2.5 times their body weight (most people manage around two times). When Usain Bolt’s foot lands, it applies around 900 pounds (400kg) of force for a few milliseconds, and continues pushing for around 90 more.
Weyand likes to imagine a weightlifter trying to apply the same force in a one-legged squat – they would come nowhere close. “What we know about force under static conditions under-predicts how hard sprinters hit by a factor of two,” he says. “We just don’t have the ability to go from the movements of the body to the force on the ground.” Even if a sprinter’s muscles were eventually boosted by gene doping techniques, we have no way of calculating how much faster their owners would run.
Studies are underway to fill in those gaps, and Weyand is hoping that we’ll be able to make better predictions in five or 10 years. Just a few months ago, Marcus Pandy and Tim Dorn used computer simulations of sprinters to show that the calf muscles, more than any others, determine the amount of force that runners apply to the ground. At top speeds, the hip muscles become increasingly important too. “Maybe if you train a sprinter, you could potentially train them to have really strong calves,” says Hutchinson.
For the moment, however, any predictions about the ceilings of human speed are still ill-informed ones. The only way to work out if Bolt or some other sprinter will smash the existing record is to watch them.