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.[4] 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."[5] The rule had a dramatic impact at the 2011 World Championships, when current world record holder Usain Bolt was disqualified.[6][7]
Runners typically reach their top speed just past the halfway point of the race and they progressively decelerate in the later stages of the race. Maintaining that top speed for as long as possible is a primary focus of training for the 100 m.[8] 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.
The winner, by IAAF Competition Rules, is determined by the first athlete with his or her torso (not including limbs, head, or neck) over the nearer edge of the finish line.[9] 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, 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 m/s is allowed for a 100 m performance to be considered eligible for records, or "wind legal."
Furthermore, sprint athletes perform better 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 (explaining why many athletes choose not to breathe for the duration of the race)[citation needed]. While there are no limitations on altitude, performances made at altitudes greater than 1000 m above sea level are marked with an "A."Only male sprinters have beaten the 100 m 10-second barrier, nearly all of them being of West African descent. Namibian (formerly South-West Africa) Frankie Fredericksbecame the first man of non-West African heritage to achieve the feat in 1991 and in 2003 Australia's Patrick Johnson (who has Irish and Indigenous Australian heritage) became the first sub-10-second runner without an African background.
In the Prefontaine Classic 2015 Diamond League meet at Eugene, Su Bingtian ran a time of 9.99 seconds, becoming the first Asian athlete to officially break the 10-second barrier. In the 2015 Birmingham Grand Prix Diamond League meet, British athlete Adam Gemili, who is of mixed Iranian and Moroccan descent, ran a time of 9.97 seconds on home soil, becoming the first athlete with either North African or Middle Eastern heritage to break the ten-second barrier.[15] Of the six men's continental record holders, currently three of them were born in Nigeria.
It is believed that biological factors may be largely responsible for the notable success in sprinting events enjoyed by athletes of West African descent. This includes:[16]
Relatively less subcutaneous fat on arms and legs and proportionately more lean body and muscle mass, broader shoulders, larger quadriceps, and bigger, more developed musculature in general;
Denser, shallower chests;
Higher center of gravity, generally shorter sitting height, narrower hips, and lighter calves;
Longer arm span and "distal elongation of segments" – the hand is relatively longer than the forearm, which in turn is relatively longer than the upper arm; the foot is relatively longer than the tibia (leg), which is relatively longer than the thigh;
Greater body density, which is likely due to higher bone mineral density and heavier bone mass at all stages in life, including infancy (despite evidence of lower calcium intake and a higher prevalence of lactose intolerance, which prevents consumption of dairy products);
Modestly, but significantly, higher levels of plasma testosterone (3–19 per cent), which is anabolic, theoretically contributing to greater muscle mass, lower fat, and the ability to perform at a higher level of intensity with quicker recovery;
The ACTN3 protein, a "speed gene" most common among persons of West African descent that renders fast twitch muscle fibers fast. African American 200 m and 400 m world champion Michael Johnson has suggested that the presence of ACTN3 is at the root of the success of these athletes in sprinting events;[17][18]
And finally, a higher percentage of fast-twitch muscle fibers (Type II) and more anaerobic enzymes, which can translate into more explosive energy. The enzyme creatine kinaseis abundantly expressed in these fibers. The enzyme rapidly regenerates the biological fuel molecule ATP needed for the sprint. The enzyme has been reported to be twice as high in subjects of sub-Saharan African descent.[19] Creatine kinase is the final common pathway of muscle activity. It is tightly bound to the muscle fibers and directly fuels fast muscle contraction. Therefore, the creatine kinase system is considered to be the major factor, downstream of other factors, that modulates the biological capacity to sprint.[20]
Top sprinters of differing ancestry, such as Christophe Lemaitre, are believed to be exceptions in that they too likely have the genes favourable for sprinting.[18] Colin Jackson, an athlete with mixed ethnic background and former world record holder in the 110 metre hurdles,[21] noted that both his parents were talented athletes and suggested that biological inheritance was the greatest influence, rather than any perceived racial factor. Furthermore, successful black role models in track events may reinforce the racial disparity.[22]
Major 100 m races, such as at the Olympic Games, attract much attention, particularly when the world record is thought to be within reach.
The men's world record has been improved upon twelve times since electronic timing became mandatory in 1977.[23] The current men's world record of 9.58 s is held by Usain Bolt of Jamaica, set at the 2009 World Athletics Championships final on 16 August 2009, breaking his own previous world record by 0.11 s.[24] The current women's world record of 10.49 s was set by Florence Griffith-Joyner of the US, in Indianapolis, Indiana, on 16 July 1988.[25]
Some records have been marred by prohibited drug use – in particular, the scandal at the 1988 Summer Olympics when the winner, Canadian Ben Johnson was stripped of his medal and world record.
Jim Hines, Ronnie Ray Smith and Charles Green were the first to break the 10-second barrier in the 100 m, all on 20 June 1968, the Night of Speed. Hines also recorded the first legal electronically timed sub-10 second 100 m in winning the 100 metres at the 1968 Olympics. Bob Hayes ran a wind-assisted 9.91 seconds at the 1964 Olympics.
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