Myostatin is the growth factor that acts as a brake on muscle development; a person with a high level of myostatin will have less well developed muscles. A mutation in the gene for myostatin expression will remove this brake, and could lead to rapid muscle growth. A German toddler already has twice the muscle density of his peers – useful if he becomes a bodybuilder.
Insulin-like growth factor 1 (IGF-1) is the hormone most responsible for regulating cell growth and development. An athlete with abundant IGF-1 and other growth hormones and regulators will be tall; useful for basketball players.
Super-mice have been genetically engineered to run faster and further. They over-express a gene for the enzyme phosphoenolypyruvate carboxykinases (PEPCK-C), which is modified so that it is only active in skeletal muscles. The mice produce less lactic acid and burn more fat. They can run at 20 metres a minute on a treadmill for up to six hours.
Erythropoietin (EPO) is a hormone that regulates the numbers of red blood cells. Altering the EPO receptor enables blood cells to carry higher levels of oxygen, which can result in greater cardiovascular stamina. The effect is similar to that of blood doping, an illegal practice.
The gene ACTN3 – the "speed gene", for fast-twitch muscle – makes muscles contract quickly. It's crucial for sprinters and sports requiring short, powerful movements. A variant (R577X) is needed for the slow, efficient muscle contractions required by endurance athletes.
A variant of the bradykinin beta 2 receptor gene (BDKBR2) has recently been linked to the ability to run long distances in Olympic standard athletes. It is thought to work by causing skeletal muscles to contract efficiently.