Draft:100 metres

Race dynamics

Start

Male sprinters await the starter's instructions

At the start, some athletes play psychological games such as trying to be last to the starting blocks.

At high level meets, the time between the gun and first kick against the starting block is measured electronically, via sensors built in the gun and the blocks. A reaction time less than 0.1 s is considered a false start. The 0.2-second interval accounts for the sum of the time it takes for the sound of the starter's pistol to reach the runners' ears, and the time they take to react to it.

For many years a sprinter was disqualified if responsible for two false starts individually. However, this rule allowed some major races to be restarted so many times that the sprinters started to lose focus. The next iteration of the rule, introduced in February 2003, meant that one false start was allowed among the field, but anyone responsible for a subsequent false start was disqualified.

This rule led to some sprinters deliberately false-starting to gain a psychological advantage: an individual with a slower reaction time might false-start, forcing the faster starters to wait and be sure of hearing the gun for the subsequent start, thereby losing some of their advantage. To avoid such abuse and to improve spectator enjoyment, the IAAF implemented a further change in the 2010 season – a false starting athlete now receives immediate disqualification. This proposal was met with objections when first raised in 2005, on the grounds that it would not leave any room for innocent mistakes. Justin Gatlin commented, "Just a flinch or a leg cramp could cost you a year's worth of work." The rule had a dramatic impact at the 2011 World Championships, when current world record holder Usain Bolt was disqualified.

Mid-race

Runners usually reach their top speed just past the halfway point of the race and progressively decelerate to the finish. Maintaining that top speed for as long as possible is a primary focus of training for the 100 m. Pacing and running tactics do not play a significant role in the 100 m, as success in the event depends more on pure athletic qualities and technique.

Finish

The winner, by IAAF Competition Rules, is determined by the first athlete with their torso (not including limbs, head, or neck) over the nearer edge of the finish line. There is therefore no requirement for the entire body to cross the finish line. When the placing of the athletes is not obvious, a photo finish is used to distinguish which runner was first to cross the line.

Climatic conditions

See also: Wind assistance

Climatic conditions, in particular air resistance, can affect performances in the 100 m. A strong head wind is very detrimental to performance, while a tail wind can improve performances significantly. For this reason, a maximum tail wind of 2.0 metres per second (4.5 mph) is allowed for a 100 m performance to be considered eligible for records, or "wind legal".

Furthermore, sprint athletes perform a better run at high altitudes because of the thinner air, which provides less air resistance. In theory, the thinner air would also make breathing slightly more difficult (due to the partial pressure of oxygen being lower), but this difference is negligible for sprint distances where all the oxygen needed for the short dash is already in the muscles and bloodstream when the race starts. While there are no limitations on altitude, performances made at altitudes greater than 1000 m above sea level are marked with an "A".