Genetic influence on athletic performance

Abstract

Introduction

Both the scientific and sporting communities acknowledge that genetic factors undoubtedly contribute to athletic performance. As of 2009, more than 200 genetic variants had been associated with physical performance, with more than 20 variants being associated with elite athlete status (1). Although few studies have examined the link between genetic factors and athletic performance in children or adolescents, this area of research is highly relevant to a pediatric population; the idea of predicting future athletic success through genetic testing in children is becoming increasingly common. The present review will provide an overview of the genetics of athletic performance and will focus on the relevance to young athletes.

Components of Performance

A primary challenge when attempting to describe the influence of genetic factors on athletic performance is its multifactorial nature. Every sport has unique physical requirements and these requirements can be dramatically different between sports. Therefore, any study of the genetic influence on performance must consider the performance components most appropriate for the sport of interest.

Considering the number of body systems that must interact (musculoskeletal, cardiovascular, respiratory, nervous, etc.), athletic performance is one of the most complex human traits. Perhaps the first noticeable difference between athletes of different specialties is in body morphology (i.e., height and body composition), with specific body types naturally suited to specific sports. Beyond body morphology, endurance, strength, and power are primary factors underlying athletic performance.

Aerobic endurance is the ability to sustain an aerobic effort over time, such as distance running or cycling. At the most basic level, aerobic endurance requires the ability of the cardiovascular system to deliver oxygen to the working muscles and the ability of the muscles to utilize that oxygen. The most common quantification of endurance is the maximal rate of oxygen uptake (VO_2max). However, VO_2max does not perfectly correlate with endurance performance (e.g. marathon running), as other factors such as economy and ventilatory threshold also influence performance.

Muscular strength is the ability of the muscle to generate force. Muscular strength is generally quantified by the one repetition maximum. Muscle power is the interaction between the force and velocity of a muscle contraction (e.g. an explosive movement such as vertical jump). Muscle strength and power are critical in athletic events such as sprinting, jumping, and weightlifting.

Additional components of athletic performance include cognitive factors and injury susceptibility. It is critical to remember that the environment (e.g., training, nutrition) also influences many of these traits. An individual’s “trainability,” or response to exercise training, is also partially dependent on genetic factors, as recently reviewed by Bouchard (2). The relative importance of environment versus genetic factors on athletic success likely varies widely between sports as well (i.e., gymnastics vs. 100m sprint). Elite athletic status, therefore, results from the interaction of an optimal combination of genetically driven physical and mental traits with the ideal environment for athletic success (3).

Heritability of sub-traits

The heritability of a trait is generally considered an estimation of the importance of genetic factors to that trait. For example, the heritability of athletic status (regardless of sport) is estimated to be 66% (4). Height, which is critical for success in some sports, is highly heritable, with about 80% of the variation due to genetic factors (5). Body type (having mesomorphic or ectomorphic somatotype) is also highly heritable (6). These somatotypes are classically associated with power or endurance athlete status, respectively (7).

Costa et al. (8) recently reviewed the existing family and twin studies related to specific endurance and muscular strength phenotypes. Aerobic endurance, as reflected by VO_2max has a heritability of about 50% (9). Heritability estimates for muscular strength, and power range from 30 to 83%, depending on the specific muscle and type of contraction (8).

Key Performance Genes

Though many specific genes and sequence variants (polymorphisms) within genes have been associated with performance, many of the findings to date have not been adequately replicated. Two notable exceptions are the angiotensin-1 converting enzyme insertion/deletion (ACE I/D) polymorphism, and the α-actinin-3 (ACTN3) R577X polymorphism, both of which have been examined in several populations using a variety of experimental approaches.

Injury Risk

Resistance to and/or the ability to recover from injury is another critical factor for optimal performance. Two main injury types have been studied with regard to genetic risk: concussion and tendinopathies. These areas of research are particularly important to the skill development of the pediatric athlete, as injuries can dramatically decrease time spent in training. Additionally, some injuries can result in recurrent issues throughout life. For example, athletes who sustained concussions decades earlier exhibited more brain anomalies and greater cognitive decline with aging compared to athletes that had never been concussed (21). A better understanding of the genetic components to injury risk and recovery could improve our ability to protect at-risk young athletes from serious injury and to optimally treat the injuries that do occur.

Relevance for Young Athletes

Few studies have examined the association between genetic variation and athletic performance phenotypes in child or adolescent athletes. This is unsurprising given the potential ethical considerations of genetic testing in children. The ACE II polymorphism was reported to be associated with significantly greater handgrip strength and vertical jump performance in female, but not male Greek adolescents (31), which contrasts the typical association of the D allele with better strength and power in adults. Recently, the ACE D allele has been associated with greater handgrip strength in adolescent girls (32) and standing long jump performance in middle-school age children (33).

The association of the ACTN3 genotype with performance has also been studied in children. Boys with ACTN3 RR genotype tended to swim faster (25m and 100m) (34) regardless of training status. The R allele was also associated with better 40m sprint performance in adolescent Greek boys (35). Late adolescent girls with the RR genotype performed better on sit up tests than girls with the RX genotype (32). However, the ACTN3 genotype was not associated with several other power or endurance phenotypes in adolescents of either sex (35). Additional correlations between PPARA, PPARD, and PPARGC1A genotypes with standard fitness tests in children were also reported (32,33). Overall, many of these studies associating genetic variation with performance in children have been underpowered and have failed to correct for multiple comparisons, so it is premature to draw firm conclusions. Further, the drive behind the prediction of athletic performance with genetics is primarily aimed at the early identification of individuals who will become exceptional athletes as adults.

Genetic Testing

Talent selection, the identification of promising athletes at a young age allows for an earlier adoption of specialized/dedicated training. Historically, talent identification has been based on physical and psychological characteristics and sport-specific performance. Genetic testing may provide an additional way to predict adult performance traits prior to their full development in untrained children by profiling combinations of gene variants associated with a particular trait. As DNA sequence is constant throughout life, genetic testing can be performed as soon as a DNA sample is available (in infancy or even prior to birth).

Despite the lack of performance predictability offered by single genetic variants, several companies are marketing genetic tests claiming to do just that. Tests are available for frequently studied genes (e.g., ACTN3 and ACE) as well as several genes with relatively little scientific evidence (36). In fact, a genetic test for ACTN3 was developed in 2004, only one year after Yang et al. (16) first described its potential connection to sport performance. Now this test is being marketed directly to coaches and parents with no prescription required (37). Based on this single test, this company will interpret an individual’s “genetic advantage” as “predisposed to endurance events,” “predisposed to sprint/power events,” or “equally suited for both endurance and sprint/power events.” This is by no means the only available direct-to-consumer genetic test, but is a representative example of the existing commercial options.

Beyond the oft-insufficient rationale for testing these variants, most coaches, parents, and athletes lack the scientific background required to understand the limitations of these tests or the implications of the results. However, some professional sports teams are already using the results of these tests to partially direct training prescriptions (38,39).

It remains to be seen whether the complex contribution of genetic factors to athletic performance can be used to improve talent selection. It is important to remember that genetic association studies reveal factors that are associated with athletic performance traits at a population level and the relative importance of any given variant for an individual is undoubtedly more variable, thus genetic screening cannot be used to conclusively predict or rule out athletic success (40). There is neither currently nor is there likely to be a gene variant that is either required or sufficient for superior athletic performance.

The potential for genetic testing to predict injury susceptibility, such as APOE genotype with response to concussion, may provide a unique and important avenue to improve safety for athletes. Though consensus remains elusive, the existing evidence points to the idea that genetic factors will be identifiable as important for injury susceptibility and could potentially be used to the advantage of young athletes as they consider sport participation. But, the ethical challenges related to genetic testing in relation to sport performance, especially in children, are especially difficult, as reviewed recently by Wackerhage et al. (41), and need to be considered carefully.

Conclusion

Current evidence suggests that a favorable genetic profile, when combined with the appropriate training, is advantageous, if not critical for the achievement of elite athletic status. However, though a few genes have now been repeatedly associated with elite athletic performance, these associations are not strong enough to be predictive and the use of genetic testing of these variants in talent selection is premature.

Acknowledgments

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