Athletic performance is a complex trait that is influenced by both genetic and environmental factors. Many physical traits help determine an individual’s athletic ability, primarily the strength of muscles used for movement (skeletal muscles) and the predominant type of fibers that compose them. Skeletal muscles are made up of two types of muscle fibers: slow-twitch fibers and fast-twitch fibers. Slow-twitch muscle fibers contract slowly but can work for a long time without tiring; these fibers enable endurance activities like long-distance running. Fast-twitch muscle fibers contract quickly but tire rapidly; these fibers are good for sprinting and other activities that require power or strength. Other traits related to athleticism include the maximum amount of oxygen the body can deliver to its tissues (aerobic capacity), muscle mass, height, flexibility, coordination, intellectual ability, and personality.
Studies focused on similarities and differences in athletic performance within families, including between twins, suggest that genetic factors underlie 30 to 80 percent of the differences among individuals in traits related to athletic performance. Many studies have investigated variations in specific genes thought to be involved in these traits, comparing athletes with nonathletes.
The best-studied genes associated with athletic performance are ACTN3 and ACE. These genes influence the fiber type that makes up muscles, and they have been linked to strength and endurance. The ACTN3 gene provides instructions for making a protein called alpha (α)-actinin-3, which is predominantly found in fast-twitch muscle fibers. A variant in this gene, called R577X, leads to production of an abnormally short α-actinin-3 protein that is quickly broken down. Some people have this variant in both copies of the gene; this genetic pattern (genotype) is referred to as 577XX. These individuals have a complete absence of α-actinin-3, which appears to reduce the proportion of fast-twitch muscle fibers and increase the proportion of slow-twitch fibers in the body. Some studies have found that the 577XX genotype is more common among high-performing endurance athletes (for example, cyclists and long-distance runners) than in the general population, while other studies have not supported these findings. The 577RR genotype is associated with a high proportion of fast-twitch fibers and is seen more commonly in athletes who rely on strength or speed, such as short-distance runners.
The ACE gene provides instructions for making a protein called angiotensin-converting enzyme, which converts a hormone called angiotensin I to another form called angiotensin II. Angiotensin II helps control blood pressure and may also influence skeletal muscle function, although this role is not completely understood. A variation in the ACE gene, called the ACE I/D polymorphism, alters activity of the gene. Individuals can have two copies of a version called the D allele, which is known as the DD pattern, two copies of a version called the I allele, known as the II pattern, or one copy of each version, called the ID pattern. Of the three patterns, DD is associated with the highest levels of angiotensin-converting enzyme. The DD pattern is thought to be related to a higher proportion of fast-twitch muscle fibers and greater speed.
Many other genes with diverse functions have been associated with athletic performance. Some are involved in the function of skeletal muscles, while others play roles in the production of energy for cells, communication between nerve cells, or other cellular processes.
Other studies have examined variations across the entire genomes (an approach called genome-wide association studies or GWAS) of elite athletes to determine whether specific areas of the genome are associated with athleticism. More than 150 different variations linked to athletic performance have been identified in these studies; however, most have been found in only one or a few studies, and the significance of most of these genetic changes have not been identified. It is likely that a large number of genes are involved, each of which makes only a small contribution to athletic performance.
Athletic performance is also strongly influenced by the environment. Factors such as the amount of support a person receives from family and coaches, economic and other circumstances that allow one to pursue the activity, availability of resources, and a person’s relative age compared to their peers all seem to play a role in athletic excellence. A person’s environment and genes influence each other, so it can be challenging to tease apart the effects of the environment from those of genetics. For example, if a child and his or her parent excel at a sport, is that similarity due to genetic factors passed down from parent to child, to similar environmental factors, or (most likely) to a combination of the two? It is clear that both environmental and genetic factors play a part in determining athletic ability.