What is a false start, and why is the reaction time limit set where it is?

Leaving the blocks before the gun, or reacting faster than a human physiologically could — and the second definition is the one that surprises people, because a runner can be disqualified while still in contact with the blocks.

How it is detected. Starting blocks contain force sensors. From the moment the gun fires, the system measures when force applied against the footplates exceeds a threshold, indicating the athlete has begun to push.

The 0.100 second rule. A reaction measured at less than 100 milliseconds after the gun is ruled a false start, on the basis that the sound cannot be heard, processed and converted into muscular force any faster. It is treated as evidence that the athlete anticipated rather than reacted.

The rule is contested. Some research suggests that trained sprinters can react faster than 100ms, and that the threshold is therefore conservative and occasionally penalises a genuine reaction. World Athletics has reviewed the figure, and the underlying physiology is genuinely uncertain rather than settled.

The disqualification rule and its history. The rules changed substantially:

Originally, each athlete was allowed one false start, with disqualification on the second.

Then one false start was permitted for the field, with anyone subsequently false-starting disqualified — which encouraged deliberate early starts as a tactic to unsettle rivals.

Since 2010, in most events, any false start means immediate disqualification. The change was driven largely by broadcast scheduling, which needed predictable race timings, and it has produced high-profile disqualifications including a world champion in a global final.

Combined events retain a more lenient rule.

Why a good reaction matters less than people think. The difference between the fastest and slowest legal reactions in an elite final is typically a few hundredths of a second — meaningful over 100m but far less important than acceleration and top speed. Athletes with the best reaction times rarely win.

Speakers behind each block are used because the speed of sound would otherwise disadvantage runners in outside lanes.