There are sports such as weightlifting, marathon and speed 100 meters, in which performance depends almost exclusively on these variables (strength, endurance and speed). Other mixed sports depend on the optimal use of combinations between different abilities and even tactics, strategy and synergy with other athletes (football, tennis …).
To stand out in a sport, especially in those that require purer capacities, is conditioned by the ability that our muscles have to contract with more or less intensity at greater or lesser speed, using for it different metabolic substrates (‘gasoline vs. Diesel’). The muscle specialises in adapting to the stimulus in order to carry out this stimulus in the future with greater efficiency (less metabolic expenditure for the same effort) and effectiveness (better performance). Another conditioner of performance is anthropometry -study of the proportions and measures of the human body-, which can be genetically conditioned.
A question of mitochondria
The ability to maintain muscle contraction for longer – muscle endurance – is conditioned by the ability to use different metabolic substrates in the cellular motors, the mitochondria. Our muscles develop more strength if they are able to generate extra protein structure (hypertrophy) that allows them to maintain greater tension, and they will be faster if the structures that allow muscle contraction, the sarcomeres, are capable of being shortened with sufficient efficiency. The biological mechanisms responsible for these adaptations are well known by the physiology of sport. However, relatively little is known about the genes that condition a person’s biology so that he or she is more likely to have strong, resistant or fast muscles.
Countless studies have been carried out to associate a gene (or a modification of a gene, known as polymorphism) with a sport or physical ability. However, the conclusions are controversial, and in few cases has a consensus been reached with sufficient scientific rigour. For example, the genes responsible for a person having maximum oxygen consumption before starting training are not the same genes that determine the responsiveness of that maximum oxygen consumption to training.
On the other hand, our anthropometry conditions the ability to stand out in a sport. In order to be a good marathon runner, one must have light legs and little weight, while the best basketball players tend to be tall. Studies of the complete genome (GWAS) have shown that the genes that make an African taller do not necessarily have to be the same genes that make a European taller.
One exception to the lack of correlation between a gene and a physical capacity, in this case potency, is represented by the alpha-actinin 3 gene (ACTN3). The protein encoding this gene is required for explosive muscle contractions. A polymorphism of the ACTN3 gene, called R577X, has been identified, which causes muscles to contract with less explosiveness, making us slower but more resistant to fatigue and metabolically more efficient.
This is a natural mutation that is estimated to have emerged about 50,000 years ago when our species migrated from Africa to Europe and Asia favoring greater resilience over explosiveness. It is now estimated that approximately one billion people worldwide have this genetic variation. In sport, the homozygous genotype ‘null’ (XX) is infrequent among Jamaicans and African-Americans, the main protagonists of the speed tests; as well as in elite footballers, a predominantly explosive sport. However, the existence of association does not imply causality. There are sportsmen of reputed level that being homozygous XX for the gene of the ACTN3, they have become elite in sport specialties where the power and muscular explosiveness prevails, as the football.
Another example, somewhat more controversial, is the angiotensin converting enzyme (ACE) gene. Its allele I has been associated with better endurance, while the allele D of this gene is common in sports of muscular strength and explosiveness. A recent study has analyzed 346 elite sprinters of which only one athlete with homozygous genotype for ACE II -and none for variant XX of ACTN3- recorded a better time in the test of 200 meters than those classified for the final of this modality in the Olympic Games in London. In addition, sprinters with ACE DD genotype made better records in the 400-meter race than those with ACE II. Given the solid evidence found in animal research models, numerous studies have been carried out that have attempted to link the gene for myostatin, whose protein limits muscle hypertrophy, with muscle strength. However, these studies are not conclusive enough.
Of the 20,000-25,000 genes that the Human Genome Project estimates a person has, only 2 have been associated with basic physical capabilities, with sufficient scientific rigor. This fact evidences the complexity in the interrelation of the different genes to generate a specific muscular ability that translates into performance for a sport modality.
So, is the athlete born or is it done?
To solve the classic dilemma of whether an athlete is ‘born or made’ (or ‘genetics vs. environment’), we have to resort to the branch of genetics known as epigenetics, which studies how certain genes are expressed and inhibited as a function of external factors such as training, diet, climate, exposure to pollutants and even a mother’s treatment of her child in the early years of life. Genes may determine our athletic performance, but we don’t yet know which genes they are.