You know how you compare to others in your ability to run a mile, compete on the racquetball court or in triathlons, or perhaps to generate muscular strength. You have probably asked yourself why your capabilities differ from others. Of course, it is easy to explain your weaknesses as a reflection of a poorly dealt set of genetic cards. Conversely, when explaining your superiority, it is even more tempting to claim that you train harder and have a better mental attitude during competition. Obviously, some characteristics of the healthy human body have a clear genetic input, such as body height, physique, and the color of the skin, eyes and hair. For events where physique is important, such as long distance cycling, marathon running, power lifting, and body building, genetics is obviously important. However, how important is genetics to the development of fitness components ? Are differences in fitness components between individuals a reflection of training, genetic characteristics, or a combination of both ? Furthermore, if both training and genetics are important, which component is more influential for given components of fitness, and why ? The purpose of this article is to present a concise explanation of the genetic contributions to exercise performance.

Genetic Limitation: A Lame or Valid Argument ?
The contribution of genetics to athletic performance is difficult to apply to a broad population. There is no question that genetic differences separate elite athletes from those of us who no matter how much we train, are restricted to compete against our previous personal best time. However, this does not detract from the performance feats of elite athletes. Sedentary individuals differ in their fitness level because of physical inactivity and not genetic capacities. Training is required to exploit the genetic potential of any individual. Elite athletes, although genetically gifted, need to be respected for the training they have completed in order to achieve their genetic potential. The difficulty lies in quantifying how much of an influence genetics has on specific components of fitness, and how certain genetic traits enable a given individual to respond more to a given training stimulus than another.

The role of genetics in determining exercise performance is best viewed at different levels of influence (Figure 1). The very nature of genetics is based on the expression of genetic information (i.e. from genes) that directs the cellular development of an organism. In humans, we can exemplify cellular genetic regulation in the type and concentration of certain enzymes in skeletal muscle, which in turn influences the metabolic capacity of the muscle, which in turn influences adaptability to training, which in turn will determine the potential to excel during exercise. A similar approach can be taken for other tissues that are known to be important during exercise, such as the heart, lungs, blood, and nervous system. However, before discussing how genetics influences exercise performance, it is important to understand how research is conducted to study the genetic contribution, and the limitations associated with these approaches.

Research methods used to study genetic contributions to exercise performance
The most common models to study the role of genetics in the development of fitness are listed in Table 1. Of these models, the most informative approach is to compare the differences in fitness components between twins to differences between other individuals. Furthermore, a comparison between identical twins (from one egg, or monozygous) to non-identical twins (from different eggs, or dizygous) better separates inherited genetic traits from the influences of the environment (which includes training).

Table 1: Models used in research of genetic influences to exercise performance.

Model Example Question

Family members and relatives Do all family members have the same body composition ?

Family vs adopted members Do family members differ from adopted children ?

Monozgous vs dizygous twins Are monozygous twins more similar in certain capacities than dizygous twins ?
Is there a more similar training potential between
monozygous twins than dizygous twins ?

Molecular biology Do individuals with similar exercise performance have similar genetically determined cellular characteristics?
Can a given exercise capacity be traced to the presence of specific genes ?

However, even twin studies have their limitations due to small sample sizes (only a few twins can be studied) and the difficulties in generalizing the results from these subjects to the whole population. Consequently, even the world's leading authority on the study of the genetic influences on exercise performance, Claude Bouchard, has stated, "....the heritability of most performance is only low to moderate, with no evidence for a strong effect" (Bouchard, Dionne, Simoneau, & Boulay, 1992). It remains unclear whether our current knowledge of genetic influence to exercise is limited by inadequate research. It is apparent that research has not yet been able to identify the genetic influences on specific functional capacities, or the detection of genes that foster the development of given capacities of human function. Perhaps, from a moral and philosophical perspective, this is a good thing !!!!

Genetics of motor development, strength, balance
Motor development refers to the process of acquiring movement skills and patterns, and generally occurs during childhood. Many basic movement patterns are established during the first 7 years of a child's life. There may be considerable variation in the skill level of these patterns between children. However, studies have demonstrated that heredity only plays a moderate factor with motor characteristics (Malina & Bouchard, 1986) . It is interesting to note that brothers tend to resemble each other in motor tasks and in strength tests, more than sisters resemble each other in these characteristics. However, this gender difference has been explained more to social and familial pressures, and less to a genetic influence.

With muscular strength measurements, three kinds of strength are commonly denoted: 1) isometric strength, 2) concentric and eccentric contraction strength, and 3) explosive strength, which measures the muscles' ability to create maximal force in the shortest possible time. The limited data on muscular strength tends to show that there is a significant degree of similarity in strength among siblings. More extensive research has been completed with parent-offspring comparisons in strength. Grip strength, arm strength, and relative body strength appear to have strong resemblances between parents and their offspring (Malina & Bouchard, 1986) . Some studies indicate that female offspring tend to resemble their parents more than the male children in strength characteristics. Also, Malina cites some investigations that indicate that male offspring resemble their father's strength whereas females are more similar to their mother.

Information on parent-child similarities in running, jumping and throwing is limited. The difficulty with these investigations is that many of the dynamic strength tests used for the youth are not suitable for adults. However, evidence does seem to suggest some strong father-son similarities in sprinting, running and jumping.

Balance is a skill that requires a combination of fine and gross motor control in maintaining equilibrium. It is an essential component of performance in specialized skills, such as gymnastics and diving events. Available investigations seem to indicate a fairly strong parent to off-spring similarity in beam walking balance tests (Wolanski & Kasprzak, 1979) .

Genetics of body size, composition and muscle tissue
Each of body size, physique, body composition and biological maturation share similarities in genetic influences. Although human stature is mainly determined by genetics, it is also influenced by the environment (i.e., malnutrition). Segmental body lengths and bone-related mineral mass show a high degree of genetic control. Body weight, skinfolds and body circumferences show a lesser degree of genetic inheritance due primarily to changes occurring in the environment such as nutritional intake and variation in physical activity. However, fat patterning is apparently a highly heritable trait, which shows ethnic and racial variation (Mueller & Wohlleb, 1981) .

Medical geneticists have clearly shown that all muscular properties are subject to inherited influences. Muscle fiber numbers are presumably determined by the second trimester of fetal development (McArdle, Katch, & Katch, 1991) . The genetic contributions to muscle tissue fiber composition and size are significant. However, physical training may play a significant role in modifying fiber size and area, and the relative area composed of Type I (slow twitch, oxidative muscle) and Type II (fast twitch, glycolitic) fibers as well as their metabolic capacities. However, as will be discussed in the sections to follow, the proportion of slow and fast twitch muscle fiber types is genetically determined and can not be influenced by training (Costill, Fink, & Pollack, 1976; Gollnick, et al., 1972; Pette & Staron, 1990).
Genetics of blood, arterial blood pressure and heart structure
Investigations have examined the genetic effect of hemoglobin (the oxygen carrying component in the blood) and hematocrit (the relation of red blood cells to total plasma volume). There appears to be a significant inherited pattern of variation in hemoglobin concentration while the hematocrit shows a lower genetic effect (Bouchard & Malina, 1983) . Differences in hemoglobin concentration and total blood volume between the genders adds to the evidence of genetic control of these variables.

Studies of the genetic effect of arterial blood pressures and hypertension have been of great concern. Clearly, factors such as a person's body weight, age, gender, level of stress, salt intake, and socioeconomic condition are associated with systolic (the pumping cycle of the heart) blood pressure. These factors are proposed to only account for 30% of the variation in blood pressure. A summary of the trends in the literature suggest that 50% to 60% of the variation in resting systolic blood pressure, in normotensive individuals (people who have average blood pressure) is due to a genetic effect while 40% of the variation in diastolic blood pressure is due to genetics (Bouchard & Lortie, 1984) .

In regards to heart structure, investigations suggest a strong inheritance with vascular wall thickness of the coronary arteries and in the branching patterns of the coronary arteries. Non-invasive measurements of heart structures, size and functions suggest a significant familial resemblance (Bouchard & Malina, 1983) . Since the size of the heart is an important determinant of stroke volume, which is a limiting factor to aerobic performance, it appears that genetics may play a crucial role in determining one's aerobic capacity potential.


Genetics of endurance performance
The cardiorespiratory, muscular, orthopaedic, and body composition components of human structure and function that influence cardiorespiratory endurance and prolonged exercise performance are illustrated in Figure 2. These determinants can be grouped into three components: peripheral, central, and other. Peripheral components refer to those within skeletal muscle, whereas central components are those that concern the heart and cardiovascular system.

Peripheral determinants
During the 1970's and 1980's, there were many scientists who strongly believed that the capacity of skeletal muscle to use oxygen was what limited VO2max and endurance performance. For example, blood leaving contracting skeletal muscle was known to still have considerable oxygen content, indicating that there may be a limitation in oxygen uptake from blood. In addition, endurance training was shown to cause large increases in muscle mitochondria (organelles that consume oxygen) and their enzymes, and the density of capillaries in skeletal muscle. These changes occurred in concert with increases in VO2max (Costill, Fink, & Pollack, 1976; Gollnick et al., 1972). Collectively, these facts indicated that increasing the ability of skeletal muscle to use oxygen increased VO2max.

The capacity of skeletal muscle to utilize oxygen has strong genetic and training components. The maximal ability of skeletal muscle to utilize oxygen will depend on the proportion of slow twitch muscle fibers in the working muscle, as well as the endurance training nature of the muscle. Slow twitch muscle has a higher capacity to consume oxygen than fast twitch muscle, and the proportions of these fibers are developed during fetal life and consolidated during infancy. Strength or endurance training can not change these proportions. However, strength and endurance training can alter the capacities of these fibers. For example, endurance training will increase the ability of certain fast twitch muscle fibers to use oxygen, thereby increasing the endurance potential of the muscle (Costill, Fink, & Pollack, 1976; Gollnick et al., 1972). Conversely, strength or sprint training will detrain the oxygen uptake capacity of slow twitch muscle, and train fast twitch muscle to better utilize muscle glycogen and the ability to continue generating ATP at high rates with minimal oxygen consumption.

Central determinants
During the last decade, numerous research studies have clearly revealed the importance of the cardiovascular system in determining a person's abilities to consume oxygen during exercise (Coyle, Hemmert, & Coggan, 1986; Cox, Bennett, & Dudley, 1986). For example, individuals with a large heart and large volume of each ventricle can pump more blood to contracting muscles per minute (cardiac output) during intense exercise, which allows them to have a higher maximal oxygen consumption (Cox, Bennett, & Dudley, 1986; Pellicia et al., 1991). In addition, when the blood's capacity to transport oxygen is increased without training (breathing pure oxygen, increasing hematocrit and hemoglobin, increasing plasma volume), the maximal rate of oxygen consumption (VO2max) is also increased (Coyle, Hemmert, & Coggan, 1986; Spriet, Gledhill, Froese & Wilkes, 1986). These research findings indicate that a person's maximal ability to transport oxygen is crucial to the body's ability to consume oxygen. Based on Figure 2, the genetic control of these capacities are strong, as each of heart size and dimensions, blood hemoglobin, and blood volume have a strong genetic influence. Nevertheless, endurance training can increase blood volume by increasing the plasma component of blood (Coyle, Hemmert, & Coggan, 1986), and heart function can improve with training causing increases in the maximal volume of blood pumped each beat (stroke volume), which in turn increases maximal cardiac output.

It is incorrect to assume that physiological capacities alone dictate how well a person will perform during endurance events. Differences in body physique, body composition, the distribution of fat, and gender also contribute to explain differences in VO2max and endurance performance. Added to these components are the neural influences to motor performance and sports skill, as well as the psychological issues pertaining to motivation, desire, concentration, competitiveness, and for some people and events, pain tolerance ! Except for body physique, where the genetic influence is obvious, the remainder of these variables have complicated interactions between genetics and the environment.

Summary of Genetics of Endurance Performance
The purpose of this section was not to discuss the controversy of central and peripheral limitations to VO2max and endurance exercise performance, but to use this organization to reveal how genetics and training combine to determine a person's suitability for endurance exercise. Clearly, the genetic control over central cardiovascular and peripheral muscle oxygen uptake capacities indicates that there is a strong genetic determinant over how much a person can train to increase VO2max and improve endurance exercise performance. Figure 3 reveals how each of the previous components relates to one another in determining the body's maximal ability to consume oxygen. Despite the pivotal position of many capacities that have a strong genetic component, it is also clear that any individual can train their body to cause dramatic improvements in their capacity to consume oxygen by a host of alternative mechanisms. Thus our body is highly adaptive to endurance training, and it is the extent of adaptation that appears to be determined by genetics.

Conclusions
The impact of genetics in exercise appears to have multiple influences. Its positive effect on exercise performance must be combined with effective training programs and favorable lifestyle habits for optimal success. Although this review shows the many interactions genetics does play on exercise, it also highlights how training and lifestyle can significantly affect exercise training and performance. As health and fitness practitioners, it is good to appreciate the interrelation that genetics plays in our profession. However, the best message we can share with our clients and students is that regardless of hereditary, regular participation in aerobics and resistance training will lead to remarkable improvements and enhancement of quality of life.

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