## Is it all about practice or does biology help?

Elite sports performance is the result of many hours of practice, and it is thought that we need to train 10,000 hours to be successful in sports. It also assumes that just that much time is needed for the natural differences between individuals to disappear and the difference in training to manifest itself. Constant repetition is sufficient to improve an individual's performance, but the question remains as to why some athletes achieve better results than others. So does the 10,000-hour rule apply equally to everyone? The answer may lie in the structure of our muscles and what affects their performance. Muscles are composed of slow type I muscle fibres and fast type II muscle fibres. Type I slow-twitch muscle fibres have a smaller diameter, contain many mitochondria, and are more resistant to fatigue. In these fibres, the aerobic (oxidative) energy system dominates. Fast-twitch (type II) muscle fibres contract about twice as fast as slow-twitch fibred during explosive movements, but they tire very quickly. In these muscle fibres, the anaerobic (glycolytic) energy system dominates. When exposed to strength training, the fast-twitch fibred grow more than slow-twitch ones, thus the more fast-twitch fibres a muscle contains, the greater its ability to grow. Most people have muscles that are composed of slightly more than half slow-twitch muscle fibres. However, for athletes, the ratio of these fibres is different and corresponds to the sports discipline they are engaged in. Sprinters need to quickly produce explosive power, so their calf muscles contain up to 75% fast-twitch muscle fibres. Athletes who run 800 m have 50% slow and 50% fast fibres in these muscles. Endurance runners have more slow-twitch fibres, which fatigue very slowly - in marathon runners they make up to 80% of the muscle fibres.

Muscle composition affects endurance, strength, and speed. Endurance ability means performing a certain physical activity with a given intensity for a longer period, without reducing performance. From the point of view of endurance abilities, the dominant energy system is the aerobic energy system. Aerobic endurance requires the ability of the cardiovascular system to deliver oxygen to the working muscles and the ability of the muscles to use this oxygen. The most common quantification of endurance is the maximum rate of oxygen uptake (VO2 max), which represents the amount of oxygen that the lungs can extract from inhaled air and deliver, via the bloodstream, to the working muscles per unit of time. The higher the VO2 max, the more oxygen that gets to the muscles. Aerobic capacity represents the total amount of energy released in an oxidative way, without disturbing the metabolic homeostasis associated with an increase in the level of lactate in the blood.

Muscle strength is the interaction between the force and speed of muscle contraction. The amount of force developed depends on the structure of the muscle (type, proportion, and diameter of muscle fibres), and the number of contracting muscle fibres and is also influenced by other factors, such as energy status, integrity of muscles, and their innervation. Muscular strength is crucial in athletics such as sprinting, jumping, and weightlifting. But do athletes acquire their unique combination of muscle fibres through training, or is it genetically determined? Much evidence suggests the latter possibility. No training study has shown that it is possible to change slow-twitch fibers into fast-twitch fibers. Aerobic training can thus make fast fibres more persistent and strength training slow fibres stronger, but they never completely "switch".

Trainability, or an athlete's response to exercise training, also depends on genetic factors. Already in the 1970s, scientists were dealing with the influence of genes on sports performance, with the first studies mainly focused on identical twins who lived separately for a long time in different environments. This made it possible to investigate the influence of heredity and distinguish external environmental factors. In 1998, the British doctor Hugh Montgomery began to deal with sports predispositions at the genetic level. He mainly focused on gene polymorphism mutations that could affect sports performance. We define DNA polymorphisms as the existence of DNA sequence variants that are not associated with any observable phenotypic change and can be found anywhere in the human genome. Thus, polymorphism means two alternative forms of a chromosomal region (allele) that differ either in the nucleotide sequence or in the number of repeats of a short DNA sequence (see Chapter 15 - DNA as evidence).

Single nucleotide polymorphisms (SNP) correspond to a difference in one nucleotide pair. From the point of view of the position in a certain part of the genetic information, we can divide SNPs into coding and non-coding. Coding point mutations are found in the coding regions of genes. At the same time, they can be synonymous (silent), which does not change the amino acid, or non-synonymous, which changes the codon, which changes the amino acid in the protein or shortens the protein. Non-coding point mutations are found in non-coding regions of genes or outside genes (you can read more about mutations in Chapter 5 - Mutations: how they arise and what to do with them). Since these regions contain various regulatory elements necessary for the correct transcription and translation, they can affect the function of the gene.

An insertion-deletion polymorphism (indel) is a type of genetic variation in which a specific nucleotide sequence is present (insertion) or absent (deletion). If they are not a multiple of three nucleotides, they lead to frameshift mutations in the coding region of the gene, resulting in the formation of a completely different set of amino acids, or the occurrence of a premature stop codon. These mutations can fundamentally affect the structure and function of the given protein. The indel variants, which are multiples of three, do not affect the other amino acids, but the resulting protein sequence is altered, with either amino acid changes occurring or some missing.

Genetic factors can affect some components of athletic performance, such as strength, endurance, flexibility, or neuromuscular coordination. Indeed, there are specific areas of DNA that can differ between individuals, and it is through these polymorphisms that it is possible to explain why some individuals have a different reaction to sports training than others. In certain sports, the presence of specific polymorphisms can also contribute to a higher level of performance. Currently, more than 200 candidate sports genes are known, of which more than 20 are directly related to the elite level of the athlete. These genes affect physiological factors such as blood circulation, blood pressure control, lung and heart capacity, muscle fibre composition and hypertrophy, muscle metabolism, mitochondrial protein synthesis, or adaptation to training load. The most studied genes include ACE, ACTN3, and PPARA.

The ACE gene polymorphism was described in 1998 as the first-ever genetic marker of sports predisposition. This gene encodes angiotensin-converting enzyme (ACE), which is part of the renin-angiotensin system (RAS). The RAS is a hormonal system that regulates blood pressure, fluid and salt homeostasis in the human body. The ACE enzyme changes angiotensin I to angiotensin II, which increases blood pressure due to its vasoconstrictive effect. The ACE gene is located on chromosome 17 and the polymorphism of this gene consists in the insertion (allele I) or deletion (allele D) of a 287 bp DNA segment in the non-coding part of the gene. Thus, there are three genotypes in the population: II, ID, and DD. The presence of allele I leads to a lower level of the ACE enzyme in serum and tissue, which is associated with a higher proportion of type I slow-twitch muscle fibres, higher efficiency of aerobic performance, better resistance to fatigue, higher oxygenation of peripheral fibres during activity and a more pronounced aerobic response to training. This allele represents an advantage for endurance athletes. Among the given disciplines are endurance running, triathlon, cross-country skiing, swimming over medium (200-400 m) and long distances (over 400 m), and mountain climbing. The D allele, on the other hand, is responsible for a higher level of the serum enzyme and is associated with speed and strength performance. The DD genotype is associated with a higher percentage of type II fast-twitch muscle fibres, which are essential for maximal strength output over a short period. A high representation of the D allele was recorded in athletes of speed-power disciplines with a predominance of anaerobic performance. The given disciplines include sprinting, long and high jump, weightlifting, swimming, and short distance running (up to 200 m).

The structural protein α-actinin 3 (ACTN3) functions as part of the Z-line of muscle cells and has an important role in the anchoring of actin filaments (Figure 16.1). It is encoded by the ACTN3 gene, which is located on chromosome 11 in the human genome. The polymorphism of this gene consists of the substitution of cytosine for thymine at position 577, which leads to the exchange of the codon for the amino acid arginine (R) with a stop codon (X). In this case, the R allele represents the normal functional version of the gene, while the X allele contains the sequence change that prevents the production of functional α-actinin 3. A quarter of people of East Asian descent have the XX genotype, compared to only 18% in the European population and only 1% in the African population.

In sports genetics, the ACTN3 gene is also known as the “speed gene” since speed-power athletes need fast type II muscle fibres rich in glycolytic enzymes for their performance. Several studies have confirmed that elite sprinters had a high frequency of the RR genotype and an absence of the XX genotype. Allele R thus represents an advantage for speed-power sports such as weightlifting, sprints, and short-distance swimming. Loss of α-actinin 3 (genotype XX) leads to a decrease in the amount of muscle mass, muscle strength, and diameter of fast muscle fibres and increases the ratio of slow muscle fibres associated with endurance.

PPARα (peroxisome proliferator-activated receptor alpha) is a transcription factor that regulates lipid, glucose, and energy balance in the cell. It is encoded by the PPARα gene located on chromosome 22. In tissues involved in the use of fatty acids, such as the liver, skeletal muscles, and heart, the production of PPARα is increased, and the level of this receptor is also higher in slow muscle fibres. As endurance training stresses fatty acid metabolism, PPARα is an important factor in the adaptive response to training load. In this case, the SNP polymorphism replaces guanine with cytosine in the non-coding region of the gene. A high frequency of the C allele has been observed mainly in strength-oriented athletes, while both the G allele and the GG genotype represent an advantage for endurance athletes. In addition, the analysis of muscle fibre composition revealed a higher proportion of slow muscle fibers in the GG genotype compared to the CC genotype. The G allele is therefore considered an advantage in endurance performance.

Resistance to injury and the ability to recover from injury is another critical factor for optimal sports performance, as some injuries can lead to recurring problems. Collagens are the basic structural component of connective and supporting tissues, making up to 35% of all proteins in the body. They occur in connective tissue such as ligaments and tendons, but also in skin, bones, cartilage, or blood vessels. The task of collagens is to ensure the strength of the traction tissue, the elasticity of the muscles, the transfer of muscle forces to the bones, and much more. Currently, 28 different types of collagens have been described, which differ in shape and function, although about 90% of collagens belong to types I, II, and III. Polymorphisms of genes that code for collagen chains are currently being investigated concerning the risk of injury in sports. An example of a polymorphism that can affect resistance to injury is the COL1A1 gene polymorphism.

Type I collagen is found in connective tissues such as cartilage, ligaments, and tendons. It is composed of two α1 polypeptide chains encoded by the COL1A1 gene and one α2 polypeptide chain encoded by the COL2A1 gene (Figure 16.2). This gene is located on chromosome 17, has a length of 17,554 bp, and several different polymorphisms have been identified within the gene, which are related to the development of human diseases. For example, the disease osteogenesis imperfecta, known as brittle bones disease, is caused by a mutation in the genes responsible to produce type I collagen (COL1A1 and COL2A1). The mutation leads to a total change in the structure and quantity of collagen.

Osteoporosis is an age-related disease characterised by low bone mineral density, which leads to increased fragility and a high risk of fractures. Currently, the association between the development of osteoporosis and an SNP polymorphism located at position 1245 is being studied. In this polymorphism, a substitution of guanine for thymine occurs, with the presence of the T allele representing a higher risk of both osteoporotic and non-osteoporotic fractures and reduced bone mineral density. The TT genotype is associated with a higher likelihood of progressive degeneration of intervertebral discs and an increased likelihood of anterior cruciate ligament (ACL) tears in skiers. In other studies, the TT genotype was associated with a reduced incidence of ACL rupture, Achilles tendon rupture, and shoulder dislocation. It is known that tendon and ligament injuries that are related to sports performance are accompanied by various risk factors that are different for each sport. Since the individual studies were focused on different sports disciplines, the mechanism of injury was different, which could lead to conflicting observations. However, larger studies have shown that this polymorphism may be associated with the risk of ACL injury and the TT genotype may have a protective role.

Therefore, if someone wants to become an elite athlete, it is an advantage to know their sporting genetic predispositions. Accordingly, a person can more easily decide whether to choose speed-power sports or endurance disciplines, whether they will be the best in individual sports, or whether they have a better chance of success in collective disciplines. However, genes are only a small part of success. Without daily training, no one becomes a winner.

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