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Muscle composition is a key factor in athletic performance, with skeletal muscles composed of two types of muscle fibres: slow-twitch and fast-twitch. Slow-twitch fibres contract slowly but have high endurance, while fast-twitch fibres contract quickly and are suited for strength and power. Genetic factors play a significant role in determining an individual's dominant muscle fibre type, with studies suggesting that 30 to 80% of the differences in muscle fibre composition are due to genetics. Genes such as ACTN3 and ACE have been linked to muscle composition, influencing fibre type and impacting strength and endurance abilities. However, muscle fibres can adapt and shift in response to changes in exercise patterns, showcasing the interplay between genetic predispositions and environmental factors in shaping muscle composition.
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What You'll Learn
Genes influencing muscle fibre type
Muscle composition is influenced by both genetic and environmental factors. Skeletal muscles are made up of two types of muscle fibres: slow-twitch fibres and fast-twitch fibres. Slow-twitch muscle fibres contract slowly but can work for a long time without tiring and are thus suited to endurance activities like long-distance running, cycling, and swimming. Fast-twitch muscle fibres contract quickly but tire rapidly and are thus better for sprinting and other activities that require power or strength.
Genes play a role in determining whether an individual has a higher proportion of slow-twitch or fast-twitch muscle fibres. One study of twins found that about 45% of the differences in muscle fibre composition are due to genetic factors. The ACTN3 and ACE genes are the best-studied genes associated with athletic performance and they influence the fibre type that makes up muscles. The ACTN3 gene provides instructions for making a protein called alpha (α)-actinin-3, which is predominantly found in fast-twitch muscle fibres. A variation in the ACE gene, called the ACE I/D polymorphism, alters the activity of the gene and is associated with a higher proportion of fast-twitch muscle fibres and greater speed.
Other genes that have been associated with muscle fibre type include those involved in the calcineurin-NFAT pathway, mitochondrial biogenesis, glucose and lipid metabolism, cytoskeletal function, hypoxia and angiogenesis, and circulatory homeostasis. In addition, nuclear hormone receptor transcription factors have been shown to influence the type of fibres expressed in skeletal muscle by providing a direct link between intracellular metabolites and genomic expression.
While genetic factors play a role in muscle fibre type, it is important to note that muscle fibres can shift from one type to another in response to changes in exercise patterns. For example, endurance-based training can increase the proportion of slow-twitch muscle fibres, while resistance training can increase the proportion of fast-twitch muscle fibres.
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Genetic markers for athletic performance
Athletic performance is influenced by a combination of genetic and environmental factors. The former include muscle fibre typology, haemoglobin mass, mitochondrial biogenesis, maximal cardiac output, and maximal rate of oxygen consumption (VO2max), while the latter include the amount of support a person receives from family and coaches, and other economic circumstances.
Genes associated with athletic performance include ACTN3 and ACE, which influence the fibre type that makes up muscles and 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 fibres. A variant in this gene, called R577X, leads to the production of an abnormally short α-actinin-3 protein that is quickly broken down.
As of 2023, a total of 251 DNA polymorphisms have been associated with athlete status, of which 128 genetic markers were positively associated with athlete status in at least two studies. The most promising genetic markers for endurance athlete status include AMPD1 rs17602729 C, CDKN1A rs236448 A, HFE rs1799945 G, MYBPC3 rs1052373 G, NFIA-AS2 rs1572312 C, PPARA rs4253778 G, and PPARGC1A rs8192678 G alleles. Genetic markers associated with power include ACTN3 rs1815739 C, AMPD1 rs17602729 C, CDKN1A rs236448 C, CPNE5 rs3213537 G, GALNTL6 rs558129 T, IGF2 rs680 G, IGSF3 rs699785 A, NOS3 rs2070744 T, and TRHR rs7832552 T alleles. Finally, genetic markers associated with strength include ACTN3 rs1815739 C, AR ≥21 CAG repeats, LRPPRC rs10186876 A, MMS22L rs9320823 T, PHACTR1 rs6905419 C, and PPARG rs1801282 G alleles.
It is important to note that elite performance cannot be accurately predicted using only genetic testing. While genetic factors undoubtedly contribute to athletic performance, few genes are consistently associated with elite athletic performance.
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Environmental factors influencing muscle composition
While muscle composition is influenced by genetics, environmental factors also play a significant role in muscle development and performance. These factors interact with genetics to determine an individual's athletic ability and muscle composition. Here are some key environmental factors that influence muscle composition:
Training and Exercise
The type and intensity of physical activity an individual engages in can impact their muscle composition. Training and exercise stimulate muscle growth, adaptation, and repair, leading to changes in muscle fibre composition and overall muscle health. Different types of exercises target different muscle fibre types. For example, endurance activities like long-distance running favour slow-twitch muscle fibres, while activities requiring power or strength, such as sprinting, rely on fast-twitch muscle fibres.
Nutrition
Nutritional habits play a crucial role in muscle health and composition. Proper nutrition provides the body with the necessary building blocks for muscle growth and repair. For instance, consuming adequate protein is essential for muscle development and maintenance. Additionally, specific nutritional interventions can optimise muscular health and performance, depending on an individual's genetics and exercise routine.
Lifestyle Factors
Lifestyle choices, such as smoking, can negatively impact muscle composition and performance. Smoking has been associated with a reduced muscle protein fractional synthetic rate, leading to decreased lean muscle mass. On the other hand, certain lifestyle factors like oral contraceptive use or a history of pregnancy may positively influence skeletal muscle phenotypes in women, potentially increasing calf muscle cross-sectional area.
Support and Circumstances
The amount of support an individual receives, such as encouragement from family and coaches, can influence their athletic performance and, consequently, their muscle composition. Additionally, economic circumstances and other factors that enable an individual to pursue specific activities or access certain resources can also play a role in muscle development and performance.
Age and Sex
Age and sex-specific genetic factors can influence muscle composition. For example, differences in bone mineral density between men and women have been linked to specific genetic loci. Additionally, the loss of muscle mass with ageing is a well-known phenomenon that can be influenced by environmental factors interacting with genetics.
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Muscle composition and chronic diseases
Muscle composition is influenced by both genetic and environmental factors. Skeletal muscles are made up of two types of muscle fibres: slow-twitch fibres and fast-twitch fibres. Slow-twitch muscle fibres contract slowly but can work for a long time without tiring, enabling endurance activities like long-distance running. Fast-twitch muscle fibres contract quickly but tire rapidly and are good for sprinting and other activities requiring power or strength.
Genetic markers have been identified that are associated with athletic performance, including ACTN3 and ACE, which influence the fibre type that makes up muscles and have been linked to strength and endurance. The ACE gene, for example, helps control blood pressure and may influence skeletal muscle function. The ACTN3 gene provides instructions for making a protein called alpha-actinin-3, predominantly found in fast-twitch muscle fibres.
The ability to perform aerobic or anaerobic exercise varies widely among individuals, and this is partially dependent on their muscle-fibre composition. Variability in the proportion of skeletal-muscle fibre types may also explain marked differences in certain chronic disease states, including obesity, insulin resistance, and hypertension.
Grip strength has attracted interest as a marker of muscle strength and is positively associated with cardio-metabolic function in children. In adults, it is negatively related to morbidity and mortality. It may provide a valuable proxy for several traits, each associated with chronic disease risk. Grip strength can simultaneously index both early-life development of metabolic capacity and current physical fitness, which is also important for health.
Multiple chronic conditions and low skeletal muscle mass are common features of ageing that are detrimental to physical performance. Studies have shown that older people with diabetes have less muscle strength and a slower gait than those without. Those with arthritis have less muscle strength than the general population of the same age. The co-occurrence of low muscle mass and chronic disease is common as people age, and both factors are associated with poor physical function.
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Methods for predicting muscle fibre composition
Muscle fibre composition is an important factor in determining athletic performance and physical strength. The human skeletal muscle is composed of two types of muscle fibres: slow-twitch (Type I) and fast-twitch (Type II) muscle fibres. Slow-twitch fibres contract slowly but can work for a long time without tiring, making them ideal for endurance activities like long-distance running. On the other hand, fast-twitch fibres contract quickly but fatigue rapidly, making them suitable for activities requiring power or strength, such as sprinting.
Genetics play a significant role in muscle fibre composition, with studies suggesting that genetic factors account for 30 to 80% of the differences in athletic performance between individuals. Genes such as ACTN3 and ACE are known to influence the type of muscle fibres an individual possesses. The ACTN3 gene, for example, is linked to the production of fast-twitch muscle fibres, while variations in the ACE gene are associated with higher levels of angiotensin-converting enzyme, which may influence skeletal muscle function and is linked to a higher proportion of fast-twitch fibres.
While genomic research can provide valuable insights, it requires specialist knowledge and resources. Therefore, alternative methods for predicting muscle fibre composition are often sought, especially those that are simple, cost-effective, and non-invasive. One such method is resistance exercises at 80% 1RM, which has been suggested as a way to indirectly estimate muscle fibre composition. However, this hypothesis has not been validated against direct methods like muscle biopsy, which is still considered the gold standard despite its invasive nature and high sampling variance.
Another non-invasive approach that has shown promise is proton magnetic resonance spectroscopy (1H-MRS). This method measures muscle carnosine content, which has been found to positively correlate with the percentage of Type II (fast-twitch) fibres. This technique has been applied to various groups, including control subjects, athletes, and ex-athletes, and may have applications in talent identification and sports discipline.
Additionally, multiple repetition testing has been explored as a practical approach to estimating muscle fibre composition. Studies have investigated the relationship between the number of repetitions at specific percentages of 1RM (such as 70%, 80%, or 85%) and muscle fibre composition. However, these studies have had limited sample sizes, and no significant correlations have been established between multiple repetition testing and direct estimates of muscle fibre composition. Nonetheless, they offer a potential alternative for individuals without access to the facilities and expertise required for more invasive procedures.
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Frequently asked questions
Muscle composition is influenced by genetics, with one study of twins finding that about 45% of the differences in muscle fibre composition are due to genetic factors. However, muscle fibres can shift from one type to another in response to changes in exercise patterns.
Muscle fibres are classified into two main types: slow-twitch (Type I) and fast-twitch (Type II). Slow-twitch muscle fibres contract slowly but can work for a long time without tiring, making them ideal for endurance activities like long-distance running, cycling, and swimming. Fast-twitch muscle fibres contract quickly but tire rapidly, making them better for building strength and muscle size, as well as activities that require power or strength, such as sprinting.
Athletic performance is influenced by both genetic and environmental factors. Variations in muscle fibre composition can impact an individual's ability to perform aerobic or anaerobic exercises. Additionally, certain genetic markers have been linked to endurance, power, and strength in athletes.
