Scientists may have hit on the secret behind Usain Bolt's success on the track - his perfect knees.
A study of Jamaican youngsters found those whose legs, and particularly knees, match in size grow up to be the best sprinters.
The finding could help explain why athletes from the Caribbean island dominate international 100m and 200m races - the distances Bolt specialises in.
The theory may seem bizarre, but many scientists believe perfectly paired body parts are a sign of good genes and good health. Previous studies have linked symmetrical bodies and faces with everything from good looks to fertility.
The latest study, which started in 1996, involved almost 300 Jamaican schoolchildren whose bodies were measured from the age of eight.
When they reached 22, they were asked to undergo more tests, including two sprint races, with just over half taking part.
The results showed that those with the most symmetrical legs - and in particular knees - as children were more likely to agree to race. Significantly they clocked up faster times than the other young men and women.
Northumbria University researcher John Manning said his results could help explain the success of six-time Olympic gold medallist Bolt - whose 100m world record is 9.58 seconds - and his teammates, but added more research is needed to back up his symmetry theory.
He told the Independent: "We need to... look at international level sprinters and add some genetic tests. Also more data on Caucasians will help to further clarify the issue." Writing in the journal PLoS ONE, he added that symmetrical knees could also be a sign of general healthy development of the body.
Other theories behind Jamaicans' sprinting success include higher numbers of 'speed genes' and fast-twitch muscle fibres that provide the explosive power needed for sprinting.