Why is it a false start to react in under a tenth of a second?

Reacting in under a tenth of a second counts as a false start because the rules assume no human can hear the gun and respond that fast. World Athletics says that a sprinter who moves within 0.1 seconds after the gun has false-started. The figure is based on tests showing that the brain cannot hear the start sound and process it in less than 0.10 seconds. So a runner who leaves earlier must have been guessing, launching on anticipation rather than on the sound.

Psychologists who study reaction times agree on the principle. For young adults, the average reaction to a sound is about 160 milliseconds, and many researchers treat anything faster than 100 to 200 milliseconds as an anticipatory response, a movement already programmed before the signal arrived.

At top meetings, sensors in the starting blocks are wired to the starter's gun. If one detects a push less than 0.100 seconds after the shot, the timing system flags a false start and the race is stopped. Since 2009, the offender risks immediate disqualification. Before 2003, everyone got a second chance.

The rule has claimed a famous victim. At the 2011 World Championships in Daegu, the first global final under the no-false-start rule, world record holder Usain Bolt left his blocks too early in the 100 m final and was disqualified on the spot. Yohan Blake won. The limit isn't perfect, though: a 2009 study found that top sprinters can sometimes react in 0.08 seconds.