More dramatically, the British cyclist David Millar has just confessed to its use. Endurance is partly dependent on the amount of oxygen reaching the muscles, and is carried there in the red blood cells. Another factor, erythropoietin, promotes the production of red cells. It was found that a Finnish cross-country skier and gold medallist at the 1996 Winter Olympics had a mutation that caused an excessive response to erythropoietin.

The technique was based on introducing the gene for the growth factor which should then result in it being produced for the life of the cell into which it was introduced. This was done by putting the gene into a virus that infects muscle cells, and then injecting the virus into the muscles of the young mice.It is not just muscle mass or the rate of contraction that are important for athletic performance, as endurance is essential for many competitive sports. A factor that stimulates the division of these satellite cells is insulin-like growth factor. When this substance was introduced into the muscles of the limbs of young mice, the rate of muscle growth was greatly increased.This was not done by simply injecting the factor since it would disappear within a few hours. Muscle mass normally declines with age and lack of use has a similar effect, as was so clearly demonstrated with astronauts working in the absence of normal gravity. A special feature of our muscles that are used for common movements is the presence of satellite cells that lie on the outside of the muscle membrane.These are muscle-specific stem cells, and when they divide their offspring fuse with the muscle and contribute to its growth. His mother, who carries a similar gene, was a professional athlete and other members of the family also possess great physical strength.

More generally, second-and third-generation professional athletes are not that rare and could owe their abilities in part to variants in genes like myostatin. One such family is rumoured to include a European weightlifting champion.Another example of a gene involved in muscle development that could offer an advantage to sprinters is the gene that codes for fast muscle fibres. An unusually high number of sprinters have copies of this gene, and this is particularly true of female sprinters who have two copies of the gene. Sporting ability offers a fascinating insight into the complex relationship between nature and nurture – between the characteristics one’s body has because of its genes, and those it has due to training. There are many elements involved, but muscle and blood are, for athletes, fundamental. And it raises the question: could gene technology be used, illegally, to increase an athlete’s performance?
A medical journal has just described the case of a child with greatly increased muscle mass because of a mutation in a gene called myostatin.