LONDON, July 31 The dominance of Jamaicans and Americans of west African and Caribbean descent in world class sprinting has sparked intense debate about whether running at speeds that push the limits of what is humanly possible is all in the genes.
It is an idea that has its attractions. After all, it does seem baffling that the tiny island nation of Jamaica with a population reaching barely 2.8 million can consistently produce world-beating sprinters, while the whole of Europe can hardly register more than a handful of athletes in the top 100.
Yet sports scientists and geneticists say pinning sprinting success purely on nature rather than nurture is overly simplistic and ignores a wealth of cultural and societal factors that are equally important to beating the clock.
"What we know about genes in sport is that genetic make-up accounts for about 50 percent of variability in baseline performance," said Ken van Someren, director of sports science at the English Institute of Sport.
"What that basically tells is that sports performance is a combination of both nature and nurture."
Bengt Saltin, a professor of human physiology at the University of Copenhagen's Muscle Research Centre in Denmark, says the balance of fast twitch to slow twitch muscles is key.
Fast twitch fibres produce the same amount of force for each contraction as slow muscles, but they get their name because they can fire far more rapidly - making them better for explosive, fast and forceful sports such as the 100m final.
And while training and practice can obviously improve muscle performance, evidence suggests slow twitch fibres cannot be converted into fast twitch, meaning that what athletes have is what their genes gave them.
"If you don't have at least 70 to 80 percent fast twitch muscle fibres, I'd say it's unlikely you could be among them (the world's top sprinters)," Saltin told Reuters.
"But if you have that kind of level you could probably do well - and if you have 80 to 90 percent that's even better."
A flurry of excitement about the idea of genes for athletics prowess took off in 2003 when Australian scientists found that a gene called ACTN3 has certain variants which may give the muscles of elite athletes a performance advantage.
Their study, published in the American Journal of Human Genetics, found that ACTN3 could give sprinters a boost because it gave extra power to fast twitch muscles.
Studies show this version of ACTN3 - dubbed the "sprint gene" - is more common in Jamaicans and other people of West African descent than in people of European ancestry.
Scientists are keen to point out, however, while the "right" kind of genotype is likely to be more prevalent among successful sprinters, for example, than among the general population, there is also likely to be wide variation between genetic profiles of those at the top of the sport.
"The closer towards elite you get, and the closer towards the limits of performance, so genetic make-up may well put some sort of glass ceiling there," said van Someren.
"But there is no single gene that accounts for speed and power, or for sprinting. From what we know so far it appears to be a really complex interaction of lots of genes.
So it's impossible to say there's a west African genotype for sprinting, or an east African genotype for endurance running. Genes only play a part."
BEYOND THE GENOME
Scientist say any gene-centred explanation also dismisses the importance of a whole host of psycho-social and cultural factors that are likely to be major contributors to the success of Jamaican sprinters.
Track and field holds a position of high respect in Jamaica. The annual school athletics championships, known as Champs, is a major national event whose significance ranks with the Super Bowl for Americans or the FA Cup final for the English.
Experts also note Jamaica's investment in an infrastructure and training system to pick out and nurture potential elite track athletes, a culture that idolises sprint heroes, and a powerful desire among young Jamaicans to use sport to lift themselves and their families out of poverty.
"They have role models and opportunities, it's a fun, sociable and competitive event from a very early age, and it has great rewards, both financial and social," said van Someren.
Daniel MacArthur, one of the researchers who published the 2003 paper linking ACTN3 and sprinting performance, says he regrets the study has led to far too much emphasis being put on what some like to see as an evolutionary advantage.
"It is almost certainly true that Usain Bolt carries at least one of the 'sprint' variants of the ACTN3 gene," he wrote in a science blog about the issue. "But then so do I - along with around 5 billion other humans worldwide.
"That doesn't mean you'll see me in the 100 metre final in London in 2012. Unfortunately for me, it takes a lot more than one lucky gene to create an Olympian." (Editing by Alastair Himmer)