How Fast Can a Human Run? The Physics and Physiology of the 100 Metres
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Usain Bolt covered 100 metres in 9.58 seconds in Berlin in 2009, reached 44.7 kilometres an hour between 60 and 80 metres, and has not been within a tenth of a second of that time since, nor has anyone else. A cheetah does 100 kilometres an hour, a greyhound 70 and a racehorse 70 with a rider; a human is a slow animal that runs upright on two legs, and the question of what limits it has an answer that surprised the people who measured it. The limit is not how fast the legs can move but how hard the foot can hit the ground.
The race
A 100-metre race has three phases. The start, from blocks, in which the runner drives out at an angle and reaches about 60 percent of top speed in the first 10 metres; reaction time is measured, and a start under a tenth of a second is ruled a false start because the nervous system cannot respond faster. The acceleration, over the next 40 to 60 metres, in which the body rises to upright and speed builds with every stride. And the maintenance, from about 60 metres, in which the best sprinters hold their top speed and everyone else slows, the race being won by whoever slows least. Bolt's stride at top speed was 2.77 metres, taken 4.3 times a second; a good club sprinter takes shorter strides at about the same rate, and the difference is entirely in the length.
The ground, not the legs
Sprinters of every standard swing their legs at about the same speed, and the time their feet spend in the air is nearly the same for a champion and a novice. What differs is what happens in the fraction of a second the foot is on the ground. Measurements on treadmills with force plates, by Peter Weyand's laboratory from 2000 onward, showed that faster runners hit the ground harder, with a peak force of up to five times body weight for an elite sprinter against three to four for the rest, and for a shorter time, about 80 thousandths of a second against 100 or more. The greater force in the shorter contact throws the body further before the next foot lands, which is the longer stride. Top speed is set by how much force the legs can deliver in the time available, and the time available is fixed by the biomechanics of running, so the limit is the muscle's ability to produce force very fast. The elements:
- •Contact time: 80 to 90 milliseconds at the top level, and it cannot go much lower because the leg must still reverse direction
- •Ground force: up to five times body weight, delivered mostly in the first half of contact
- •Fast-twitch fibres: sprinters' muscles are 70 to 80 percent fast-twitch, a proportion largely set by genes
- •Stiffness: tendons and joints that act as springs, returning energy stored on landing
- •Technique: keeping the body tall and the foot striking under the hips, so that force goes into the ground rather than braking
What the sprinter is made of
The genetics are real and limited. The variant of the gene ACTN3 that makes a protein in fast-twitch fibres is carried by almost every elite sprinter, but it is carried by most of the population too, and the West African ancestry of nearly every sub-ten-second sprinter reflects a distribution of fibre types and limb proportions, longer legs and narrower hips, that is a statistical tendency and not a rule. Training changes what can be changed: the strength of the hip, hamstring and calf muscles, the stiffness of the tendons, the reaction to the gun, and the coordination that lets a runner apply force in the right direction. Bolt, at 1.95 metres, was thought too tall for the event until he ran it, since a tall sprinter starts slowly but takes fewer strides; his height was his advantage once he was upright.
The track, the shoes and the wind
The environment gives back what the body puts in. Modern tracks are a rubber surface over a stiff base tuned to return energy; the track in Berlin in 2009 was among the fastest laid. A tailwind up to two metres a second is allowed for records and is worth about a tenth of a second at the limit; altitude thins the air and is worth a little more, which is why the records of the 1968 Mexico City Olympics stood so long. Starting blocks, in use since the 1930s, and the spiked shoe, now with a carbon plate, each shave hundredths. The electronic timing and the false-start rule tightened the sport in the other direction, removing the human timekeeper's generosity and the anticipated gun.
How much faster
The record has fallen from 10.6 in 1912 to 9.58 in a century, with most of the gain in the first fifty years, and models of the trend and of the physiology put the eventual limit somewhere between 9.4 and 9.5 seconds, a few tenths from where it stands. The limit is the force the leg can produce in 80 milliseconds, and no training or genetics found so far pushes it much past what Bolt had; the one lever that remains is technology, and the shoe rules exist to keep it small. A human running at 45 kilometres an hour is a slow animal at the top of what its design allows, and the fact that fifty thousand people will fill a stadium to watch ten seconds of it is a measure of how much the design still impresses its owners.
The takeaway
The fastest humans run 100 metres in under 9.6 seconds and reach about 45 kilometres an hour, and what limits them is not how fast the legs swing, which is similar for everyone, but how much force the foot can drive into the ground in the 80 milliseconds it is there, which sets stride length. That force depends on fast-twitch muscle, tendon stiffness and technique, the track, shoes and wind add or subtract tenths, and the physiology suggests a limit only a few tenths below the current record.