Published by Australian Academy of Science
Measurement in sport – the long and the short of it Box 2 | Perfect timing: Timing devices and reaction time

Timing devices

Hand-held stopwatches have become more accurate, but they depend on human judgement and reactions. This places an absolute limit on accuracy – times will be uncertain by at least 0.2 of a second. Over a 100 metres foot-race this is equal to an error of 2 metres.

Such inaccuracy presents considerable difficulties. For example, in the 1960 Olympic Games in Rome, Australia's John Devitt and America's Lance Larson finished neck-and-neck in the final of the 100 metres freestyle swimming event. Two of the three first-place judges had Devitt as the winner, but two of the three second-place judges had Devitt second. Among the timekeepers there was no doubt: all three on Devitt's lane gave him 55.2 seconds, while the timekeepers on Larson's lane gave him 55.0, 55.1 and 55.1 seconds – all faster than Devitt.

But all six measurements were within 0.2 of a second of each other; thus, they did little to help decide the winner. On the basis of the decisions by the first-place judges, the race was awarded to Devitt and the official time for both was recorded as 55.2 seconds. John Devitt received the gold medal.

In 1964 an electronic quartz timing system was used for the first time in international events, thereby improving timing accuracy to 0.01 of a second. The computerised timing systems used in events today have increased the accuracy to less than 0.001 of a second, which is 10 times the accuracy required under the rules.

Judging very close running races remained a problem until photo-finish video cameras were used at the finish line. (Originally, film-based cameras were used, but this meant that athletes and spectators had to wait until the film was developed before they knew the result.) The introduction of the vertical line-scanning video system in 1991 totally removed human judgement and reactions from the timing and judging of world class running events. The starter's pistol is linked to a transducer, which detects the sound made when the starter pulls the trigger. The transducer is connected to a timing computer, which starts to count immediately it receives the signal.

Connected into this system is a high quality video camera located at the finish line. This produces the official time and a video image of the athletes as each one passes the finish line. The video camera scans a thin line aligned with the finish line up to 3000 times per second. The video image of each athlete as they actually cross the line is shown superimposed with a grid that records the time for each competitor. This system allows judges to declare the result more quickly and more accurately. (Two parallel infra-red beams also located at the finish line are directly linked to display boards within the stadium. They provide the audience with an instant but unofficial time for the race.)

Reaction time

Reaction time is the time that elapses between the moment a stimulus is detected by the brain and the moment a response starts. Tests have confirmed that nobody can react in less than 0.110 of a second. Sprinters need excellent reactions to ensure that they leave the blocks as quickly as possible after hearing the gun. Australia's Cathy Freeman, a world-class athlete, had a reaction time of 0.223 seconds in the 1995 World Championships women's 400 metres final.

A device within each starting block records the interval between the gun firing and the first athlete leaving the blocks. A false start is declared if this interval is less than 0.110 of a second, since the runner must have decided to go before hearing the gun.

Related site

• Timekeepers delve deep in search of Olympic accuracy (ABC News, 15 June 2008)

Other boxes

Box 1. Olympic track and pool facilities

Box 3. Physics in sport: Forces on an athlete

Box 4. Rackets and balls