A sprinter judged to have started falsely may not have moved before the gun at all. The rule is built on a physiological threshold rather than on visible movement.
What the blocks actually measure
Starting blocks contain sensors recording the force applied to each footplate over time. The trace shows exactly when pressure begins to rise above the settled baseline.
That instant is compared with the moment the starting signal was issued, giving a reaction time for every athlete. The measurement is continuous rather than a single judgement.
Officials receive the traces immediately, so a decision can be made before the race is allowed to stand. Human observation supplements the data rather than replacing it.
Why a minimum reaction time exists
Hearing a sound, processing it and initiating a muscular response takes a measurable interval. Below roughly a tenth of a second, a response cannot be a reaction to the signal.
An athlete responding faster than that must have begun moving in anticipation, timing the gun rather than responding to it. The rule treats this as a start before the signal.
The threshold is a conservative approximation of human capability, chosen so that genuine reactions are never penalised. It is deliberately generous.
How the one-start rule changed behaviour
Sprinting moved from allowing a collective false start to disqualifying the responsible athlete immediately. The change was driven by scheduling and broadcast reliability.
Athletes responded by reacting more cautiously, since the cost of anticipating is elimination rather than a warning. Average reaction times lengthened slightly.
The trade is between competitive fairness and the risk that a major final is decided by a single lapse. Both consequences are real.
Why the set position matters so much
Athletes must come to a still position before the gun, and the starter holds them there for a variable interval. Unpredictability prevents timing the signal.
Holding a loaded position is physically demanding, and a long hold degrades the ability to react explosively. The starter's discretion therefore affects performance.
Movement during the set position can itself constitute an infringement, even without leaving the blocks. Stillness is part of the requirement.
Where the system reaches its limits
Sensors detect force, not intent, so an athlete who twitches without pushing may still register a change. Officials review the trace shape rather than a single number.
Crowd noise and stadium acoustics mean athletes in different lanes may perceive the signal marginally differently. Speakers behind each block address this.
The rule accepts a small residual unfairness in exchange for objectivity. Measurement replaced argument without eliminating every source of doubt.
