Block starts are used in events that are no more than one lap: 100m, 200m and 400m flat; hurdles over 100m (women), 110m (men) and 400m distances.
World Athletics rules that if an athlete moves within 100 milliseconds (0.1 seconds) of the pistol being fired to start the race, then that constitutes a false start. An athlete is disqualified if, within that time, they exert more than 25kg of force on the starting blocks — athletes push back in the blocks to start, which launches them forward.
Upper body movements are not measured by the blocks, so athletes may ‘twitch’ (head or shoulder movements) without registering sufficient force, which is deemed to not have sufficient positive impact to require disqualification.
Athletes are immediately disqualified after one false start. If an athlete false-starts, they are permitted to ‘run under protest’, providing the track referee finds reason to believe there is doubt. If the ‘Start Information System’ ratifies the time as under 0.1 seconds, athletes cannot ‘run under protest’. If they do, a decision on their eligibility (the reason for their disqualification) is made afterwards.
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GO FURTHER
False starts on the track at the Olympic Games: What are the rules?
