The 100 metres, or 100-meter dash, is a sprint race in track and field competitions. The shortest common outdoor running distance, the dash is one of the most popular and prestigious events in the sport of athletics. It has been contested at the Summer Olympics since 1896 for men and since 1928 for women. The inaugural World Championships were in 1983. The reigning 100 m Olympic or world champion is often named "the fastest man or woman in the world". Fred Kerley and Shelly-Ann Fraser-Pryce are the reigning world champions; Marcell Jacobs and Elaine Thompson-Herah are the men's and women's Olympic champions. On an outdoor 400-metre running track, the 100 m is held on the home straight, with the start usually being set on an extension to make it a straight-line race. There are three instructions given to the runners immediately before and at the beginning of the race: "on your marks," "set," and the firing of the starter's pistol. The runners move to the starting blocks when they hear the 'on your marks' instruction. The following instruction, to adopt the 'set' position, allows them to adopt a more efficient starting posture and isometrically preload their muscles: this will help them to start faster. A race-official then fires the starter's pistol to signal the race beginning and the sprinters stride forwards from the blocks. Sprinters typically reach top speed after somewhere between 50 and 60 m. Their speed then slows towards the finish line. The 10-second barrier has historically been a barometer of fast men's performances, while the best female sprinters take eleven seconds or less to complete the race. The current men's world record is 9.58 seconds, set by Jamaica's Usain Bolt in 2009, while the women's world record of 10.49 seconds set by American Florence Griffith-Joyner in 1988 remains unbroken. Race dynamics Start 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