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Genetic Inheritance Effects on Endurance and Muscle Strength An Update

Aldo M. Costa,1,2 Luiza Breitenfeld,3,4 António J. Silva,2,5 Ana Pereira,2,5 Mikel Izquierdo6

and Mário C. Marques1,2

1 Department of Sport Sciences at the University of Beira Interior, Covilhã, Portugal

2 Centre for Research in Sport, Health and Human Development, Vila Real, Portugal

3 Faculty of Health Sciences at the University of Beira Interior, Covilhã, Portugal

4 Research Centre for Health Sciences, Covilhã, Portugal

5 Department of Sport Science, Exercise and Health at the University of Trás-os-Montes and Alto Douro,

Vila Real, Portugal

6 Department of Health Sciences, Public University of Navarre, Navarre, Spain

Abstract Top-level sport seems to play a natural Darwinian stage. The most out- standing athletes appear to emerge as a result of exogenous influences of nature and/or coincidence, namely, the contingency of practicing certain sport for which their talents best fit. This coincidence arises because certain individuals possess anatomical, metabolic, functional and behavioural char- acteristics that are precisely those required to excel in a given sport. Apart from the effects of training, there is strong evidence of genetic influence upon athletic performance. This article reviews the current state of knowledge re- garding heritable genetic effects upon endurance and muscle strength, as re- ported by several twin and family studies. Due, probably, to the inaccuracy of the measurement procedures and sampling error, heritability estimates differ widely between studies. Even so, the genetic inheritence effects seem incon- trovertible in most physical traits: ~40–70% for peak oxygen uptake and car- diac mass and structure, and ~30–90% for anaerobic power and capacity, ranging according to the metabolic category. Studies in development by sev- eral researchers at this present time seem to guarantee that future reviews will include twins and family studies concerning genes associated with the adaptive processes against hormetic agents, such as exercise, heat and oxidative stress.

1. Introduction

The expression of a gene is a dynamic process modulated by neural and hormonal mechanisms activated by growth needs and environmental particularities, as well as other requirements of cellular regulation.[1] Thus, the study of pheno-

type variation and its response to different en- vironmental interactions allows us to assess the role of genes. The heredity factor, for example, in a specific physical domain can be estimated by analysing the degree of similarity between closely related individuals, compared with other geneti- cally distant subjects.

In the context of physical performance, the determinants of phenotypic variation are still not fully known (i.e. age, sex or conditions of training, along with others, have important roles). Addi- tionally, social learning and parental role model- ling are relevant factors in explaining familial similarity and familial aggregation in patterns of sedentary and physically active behaviour.[2-5]

Hence, the model proposed by Bouchard et al.[6]

is now widely accepted and shows physical activity and sports performance as an entire phenotype influenced by a complex, hypothetical multifactorial scenario. As such, human physical performance, like any other individual character- istic, is dependent on interactions between genes and environment.[7] However, various scientific studies suggest a significant effect of genetics on athletic performance, even when adjusted for the manifest effect of the environment.[8,9] Indeed, relevant traits, such as aerobic power, explosive strength or anaerobic capacity have been reported by several studies as parameters of high herit- ability.[10-12] For measures such as upper body strength, the variability within the population is higher than the improvements to be expected from an optimal training programme.[13,14]

Recently, Buxens et al.[15] found that 21.4% of genetic factors contribute to sports performance. In this context, knowing which genetic profile contributes most to athletic performance is now becoming the main focus. Due to the fact that the athletic status phenotype is a complex trait, it is likely the effect of single gene variant is small.[16]

Ruiz et al.,[17] for example, have identified a poly- genic profile that distinguishes elite power from endurance athletes and nonathletic population. However, the proportion of each gene variant in explaining the variance of a complex trait such as this is totally unknown. From our point of view, some challenging ethical dilemmas will likely arise very soon, such as genetic testing for athletic talent or even the modification of the human genome.

Despite all this, the phenotypic expression of a hereditary factor precedes the study of the polymorphic variation. Although an individual’s potential for excelling in endurance or power sports can be partly predicted based on specific

and/or complex gene-gene interactions, environ- mental factors and epigenetic mechanisms are also important contributors of being an athletic champion.[15,18,19]

In brief, the purpose of this article is to provide a balanced review of the literature concerning the relative contributions of genetic and environ- mental factors to sports performance traits such as aerobic endurance, muscular strength and power. Using keywords, a comprehensive search was con- ducted on MEDLINE and SPORTDiscus� data- bases. Only twins and family data-based studies in peer-reviewed journals were included.

2. Genetic Inheritance Effects on Aerobic Endurance

The hereditable factor in aerobic endurance has commonly been ascertained by a classical twin model study on a single period of assessment. How- ever, a wide diversity in sample size and different phenotype measurement procedures may be no- ticed, which often leads to conflicting results.

As is common knowledge, the maximal oxy- gen uptake (

. VO2max) parameter is the result of the

joint efficiency of the cardiovascular, respiratory and musculoskeletal systems; together they de- termine a particular ability to capture, transport and use oxygen. Early studies on the role of he- redity in the variability of the

. VO2max indicate the

almost unconditional influence of genetics in this parameter.[20,21] Other twin studies confirmed a strong genetic influence, although results showed a considerably lower contribution (table I).[13,22,23,28-30]

The first genetic studies of . VO2max based on

family data,[31,32] also suggested a significant in- heritance component (about 40%). More recent family studies[24,26] suggest that genetic factors could account for about 50% of the variance in this parameter (table I). According to Bouchard et al.,[24] the maternal influences reach about 30%, in part potentially associated with mitochondrial inheritance. The trainability of

. VO2max is also

highly familiar and includes a significant genetic component.[25] The study of genetic inheritance effects on submaximal aerobic fitness is also note- worthy; although, as yet, the scientific evidence is scarce. The study of Gaskill et al.,[27] using

450 Costa et al.

familial data from the HERITAGE Family Study, is one of the few recognized contributions to this area. The authors used the oxygen uptake (

. VO2)

[mL�min-1] at the ventilatory threshold (adjusted for body weight) as a valid parameter to estimate submaximal sustainable aerobic work capacity and aerobic fitness. Results show maximal heri- tability effects of 58% and 54% in samples for White and Black subjects, respectively. However, the response of this submaximal parameter to training appears characterized by apparently di- vergent familial components (22% and 51%) in White and Black subjects, respectively. Accord- ing to the authors, the familial effect for ventila- tory threshold/

. VO2max in Black subjects appeared

to be accounted for by fat and fat-free mass. This study underlines an important and controversial issue in subject recruitment for human genetics studies – race and ethnicity, as a classifying vari- able. In one perspective, race by definition is not an inherently biological trait (but rather a social construct), and also an imperfect means of de- termining geographic ancestry.[33] On the flip side of the controversy, scientists argue that genetic differences do exist among the various popula-

tions. Geographic ancestry could influence some genetic and physiological traits and this could indicate that the distribution exhibits clear di- versity between different ethnic groups. In con- sequence, geographic ancestry variation could explain some of the non-consensual results that are present in the literature.

Because there is no consensus regarding this issue, future studies should at least consider the collection of information about ethnicity when recruiting subjects.

With regard to the cardiovascular system, trained athletes generally have higher rates of ventricular mass, stroke volume, cardiac output, bradycardia and lower resting heart rate under conditions of submaximal exercise.[6] Regarding this, genetic heritage is clearly evident in the struc- ture, mass and function of heart muscle (table II), resulting in a hereditary influence of 29–83%[34-40] depending on the ethnicity of subjects. Moreover, this large range of heritability values is also de- pendent on the measurement accuracy of pheno- types; cardiac magnetic resonance[40] is superior to echocardiography for quantitatively imaging the heart.

Table I. Heritability estimates of aerobic performance (family and twin studies)

Study (year) Subjects Parameter Heritability estimates

Bouchard et al.[22]

(1986)

172 young adult twins . VO2peak 40% (p > 0.05)

Fagard et al.[23]

(1991)

48 young adult twins . VO2peak 68% (p < 0.001)

40% (p = 0.05) after weight adjustment 26% (p > 0.05) after life-style factors adjustment

Maes et al.[13] (1996) 105 young child twins . VO2peak 65% (p < 0.05)

Bouchard et al.[24]

(1998)

429 sedentary adult individuals from

86 families

. VO2peak 50% (p < 0.05)

Bouchard et al.[25]

(1999)

481 adults from 98 two-generation

families Response of

. VO2peak to

training

47% (p < 0.05) adjusted for age and sex

Perusse et al.[26]

(2001)

483 individuals from two-generation

families (184 parents and 299 biological

offspring)

. VO2 at 50 W, 60% and 80%. VO2max

48–74% (p < 0.05) for baseline phenotypes

23–57% (p < 0.05) for the training response phenotypes

Gaskill et al.[27]

(2011)

199 nuclear families (100 White and

99 Black)

. VO2 at the ventilatory threshold 58% (p < 0.05) in White and 54%

(p < 0.05) in Black families (adjusted for weight, age, fat mass and

fat-free mass)

Mustelin et al.[28]

(2011)

304 young adult twins . VO2peak 71% (p < 0.05), after weight adjustment

. VO2 = oxygen uptake;

. VO2max = maximal

. VO2;

. VO2peak = peak

. VO2.

Endurance and Muscle Strength Genetic Inheritance Effects 451

With regard to cardiac dimension differences between athletes and non-athletes, the few studies published[41,42] suggest that no significant genetic component seems to exist (at least in resting state). This could mean that cardiac factors are not sig- nificantly involved in the inheritance of aerobic power but are instead due to inheritance of other cardiac features, which are only expressed during exercise.[41,43] Further studies are needed to better understand the inheritance of the qualities char- acteristic of an athlete’s heart during exertion.

Regarding pulmonary function, while the con- textual determinants associated with lung function have been extensively studied, genetic influence has only recently been given more attention.[44,45]

Chen[44] compiled some of the most relevant twin and family studies about pulmonary function. As these authors affirm, espirometric measures of lung function are heritable traits that reflect res- piratory health and predict morbidity and mor- tality. One of the measures is the forced expiratory volume during one second (FEV1.0) – on this parameter, the genetic inheritance effects range between 28%[46] and 47%.[47] Additionally, stud- ies in monozygotic twins have consolidated the evidence of genetic contribution to lung function variability, although with broad-spectrum esti- mates. Hubert et al.,[48] who studied 127 mono- zygotic twins and 141 dizygotic twins, estimated a hereditary influence of 77% for FEV1.0. In a study of 256 monozygotic twins and 158 dizygot- ic twins, Redline et al.[49] suggested that 40–75% of the lung function variability is due to genetic inheritance. Curiously, Ghio et al.[50] found no statistical significance of genetic contribution within this parameter; however, it must be em- phasized that the small sample studied may ex- plain part of the contradictory results achieved (74 twins). The strong genetic influence in pul- monary function phenotypes has recently been confirmed by genetic polymorphism studies,[51]

genome-wide association studies[52,53] and even meta-analyses approaches.[54]

Although lung function does not appear to bear upon sports performance, there are some relevant studies on the ventilatory response in case of hypoxia,[55,56] suggesting a hereditary in- fluence on physiological processes of adaptation

to high-altitude environments.[57,58] Moreover, the large intervariability of blood markers in elite athletes in response to acute hypoxic exposure, corroborating previous observations made in other populations, would seem to suggest a significant genetic inheritance.[55,59] Nevertheless, repeated short episodes of reduced oxygenation alone or in combination with intense endurance work is now understood to sustain exercise performance when atmospheric oxygen levels are low. Genome- mediated muscle plasticity is then controlled by feedback through constraints of the oxygen pathway.[56]

3. Heredity in Anaerobic Power and Capacity

The differences in the ability to perform anaer- obic motor tasks are due to several factors, in- cluding age, sex, metabolic capacity, muscle mass, higher muscle fibre recruitment and greater den- sity of fast fibres.[60] A classic example of this variation occurs among athletes of different sports’ specialties (e.g. sprinters vs distance runners), as result of the specific effect of training.[61] For ex- ample, differences in properties of isolated slow- twitch (ST) and fast-twitch (FT) muscle fibres and motor units have been well documented. The endurance capacity has been related to ST-fibre predominance (50%) whereas FT fibres are re- lated to power and speed capacity. Consistent with this evidence, power athletes and sprinters tend to have higher proportions of FT muscle fi- bres with low-oxidative capacity when compared with endurance athletes, plus higher percentages of ST muscle fibres.[62-64] Nevertheless, it has been widely suggested that muscle function is also markedly influenced by genetic factors.[65,66]

As such, current research has not yet clarified whether this variability is largely environmental or genetic in origin or even the result of gene-training interaction.

In substance, the functioning of human skele- tal muscle under anaerobic conditions requires analysis in at least three distinct metabolic cate- gories: maximal anaerobic work and/or explosive tasks; maximal tasks of short duration; and long- term maximal anaerobic tasks.[60]

452 Costa et al.

3.1 Anaerobic and Explosive Tasks

The literature on this subject reports a con- siderable influence of heredity in muscle function during anaerobic or explosive tasks, although there is a disparity in obtained results, which suggests that these traits should be further investigated.

With regard to eccentric force of elbow flexors, Thomis et al.[67] reported a genetic explanation of 65% to 77%. For maximum strength in leg ex- tension, the genetic contribution was estimated at 46% by Arden and Spector[68] and, later, at 42% by Zhai et al.[69] For maximum strength in arm flexion, the genetic role was estimated at 77% by Thomis et al.[67] and subsequently from 30% to 80% by Thomis and colleagues,[70] depending on the angle, type and contraction speed performed. According to Thomis and colleagues,[70] the im- portance of genetic factors in eccentric arm flexor strength (62–82%) would seem larger than for concentric flexion (29–65%), and therefore these may suggest that heritability estimation followed the torque-velocity curve contributing differently to eccentric and concentric torques. Further- more, the genetic contribution for static strength seems quite high: 69%[67] to 72%.[13] For isometric handgrip strength, the inheritance element is lower but remains a significant component, with a contribution of 30%.[68] With regard to max- imal isometric quadriceps strength, the study of Jones and Klissouras[71] reports a substantial hereditary coefficient of 0.83. However, it is in- teresting that Komi and Karlsson[72] have re- pudiated the existence of any significant influence of genetics in this phenotype.

In the elderly, the decline in muscle strength and power suggests that there may be changes in the relative contributions of genetic and environ- mental effects. New specific environmental effects could be due to the onset of new disease processes or changes in lifestyle.[73] Nevertheless, the con- tribution of genetic effects to muscle strength seems to remain significant for measures such as isometric muscle strength[12,73,74] or leg extensor power.[73] Unfortunately, very few longitudinal studies of elderly people have been conducted and further evidence must be accumulated.Ta

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Endurance and Muscle Strength Genetic Inheritance Effects 453

3.2 Short-Duration Anaerobic Tasks

The literature suggests that the genetic con- tribution in short-duration motor tasks is quite significant. The pioneering study by Komi and colleagues[75] reports an intraclass correlation coefficient (ICC) of 0.58 and 0.80 for mono- zygotic and dizygotic twins, respectively. Ad- ditionally, the dizygotic twins showed a higher variance (5-fold higher) between twin pairs. For instance, Simoneau et al.[76] involved a large sample of subjects (a total of 328 individuals) who were intensively studied for anaerobic capacity on a cycle ergometer. Surprisingly, the results were convergent with previous studies, suggesting that the fact of sharing ‘half of the genome’ and ‘living together’ translates into greater intraclass similarity than that seen in individuals who ‘live together’ but do not have genes in common. This is a variation on the typical twin study, which provides important insight into the role of genetic versus environmental factors. Studies with com- binations of family members can even extend beyond the use of twin pairs.[77] This approach should be considered for future replication studies.

Meanwhile, the classical twin study by Wolanski et al.[77] is an important reference, since it reports high heritability estimates for several phenotypes, such as handgrip strength and running speed (20 m, 30 m and 60 m). It is interesting to note that the inheritance effect seems to decrease as the duration of the event increases. The study of Calvo et al.,[11] with 32 Caucasian male twins subjected to various conventional anaerobic as- sessments, shows a significant hereditability in- dex of 0.74 for maximum strength at 5 seconds in the Wingate test. The study of Missitzi et al.[78]

should be also be noted because it suggests a strong genetic effect for neuromuscular coordina- tion in fast movements, reporting an ICC of 0.85 and 0.73 for monozygotic and dizygotic twins, respectively.

3.3 Long-Term Anaerobic Tasks

Concerning the heritability of lactic acid con- centrations after exercise, the literature remains sparse and quite divergent. Komi and Karlson[72]

and Prud’homme et al.[29] disclaim any influence of genetics in this metabolic parameter. Actually, one of the few conclusive studies on this matter was published by Calvo et al.,[11] who reported significant hereditability indexes in maximal lac- tate concentration (0.82) and lactate concentra- tion (0.84), as well as in the second (0.93) and third minute (0.92) of recovery after the deficit test. More recently, Maridaki[79] adds that the matura- tion of anaerobic metabolism and neuromuscular activation seem to induce some variation to the heritability of lactic acid concentrations (here- ditability index of 0.98 for pre-adolescents and of 0.73 for adolescents, p < 0.05). Regarding this, the information concerning genetic inheritance ef- fects on long-duration anaerobic tasks (up to 90 seconds) seems both conflicting and scarce. Re- sults here indicated a marked genetic influence in the 1000 m run performance, as the ICCs were 0.98 and 0.69 for monozygotic and dizygotic twins, respectively. In a previous study, the same author[20] had already studied the hereditary in- fluence on maximum lactate accumulation, re- porting an ICC of 0.93 for monozygotic twins and of 0.76 for dizygotic twins. These claims were later supported by Calvo et al.,[11] who showed a significant heritability index (0.84; p < 0.05) for total power in a 30 seconds Wingate test. Never- theless, the literature seems to suggest a significant genetic influence, although there are conflicting results that should clarified by further studies.

3.4 Other Determinants of Anaerobic Performance

It should be mentioned that skeletal muscle is the largest component of adipose tissue-free body mass in humans. As such, we may assume that the high heritability of lean body mass[11] will be ex- tended to skeletal muscle mass. The few studies on this issue have shown that skeletal muscle mass appears, in fact, a heritable phenotype. Loos et al.[80] reported that the circumference variance of the upper arm and calf (87–95%) is due to heredity. A study by Sanchez-Andres and Mesa[81]

reports, for various anthropometric and body com- position parameters, a significant hereditability index: 0.68 for the perimeter of the forearm, 0.58

454 Costa et al.

for the circumference of the thigh and 0.67 for the perimeter of the calf. Thomis et al.[70] also observed a very high genetic influence for anthropometric and arm cross-sectional area measurements (>85%), adding that common environmental factors are only significant for anthropometrically estimated mid-arm muscle tissue (48%).

Some studies also suggest the significant role of genetics in the distribution of muscle fibre types. One of the first studies was published by Komi et al.[82] and was based on a small sample of monozygotic (15 pairs) and dizygotic (16 pairs) twins. The results showed a surprising heredit- ability index (p < 0.05) for the proportion of type I fibres: 0.99 in men and 0.93 in women. These data suggest that the ratio of muscle fibre type is almost exclusively dependent on genes. However, these data should be treated with caution due to the very small sample (31 pairs). Indeed, declar- ing a difference or a significant effect that does not exist in reality is one of the consequences of an inadequate level of statistical power. In addi- tion, it should be noted that about 15% of the total variance in the proportion of type I muscle fibres could be explained by the error associated with muscle sampling and technical variance.[83]

This means that the hereditability index of fibre- type ratio reported by the literature must be viewed with discretion. In fact, Bouchard et al.[22]

suggests no significant genetic effect for muscle fibre types I, IIa and IIb distribution and fibre areas. Instead, according to the authors, genetic factors appear to be involved in the variation of regulatory enzymes of the glycolytic and citric acid cycle pathways, and in the variation of the oxidative to glycolytic activity ratio. Furthermore, there is also evidence of heredity in adaptation to training with regard to enzymatic activity asso- ciated with muscle contraction.[84,85]

The literature is limited regarding the quanti- fication of genetic and environmental factors in muscular strength adaptations. According to Thomis et al.,[67] there would seem to be a high pleiotropic gene action (20–77%) and a minor activation of training specific genes during strength training (20%). However, the genetic regulation during adolescence appears to be a factor im- portant enough to be a main cause of the ob-

served phenotypic stability in vertical jump per- formance.[86] The heritability and environmental contributions to skeletal muscle phenotypes may differ as a function of sex and age, at least in multigenerational families of African heritage.[87]

Rationally, we can expect that the younger the subjects studied, the higher will be the hereditary influence and, by consequence, the lower the en- vironment effect. Furthermore, it seems that the differential acute response among muscle fibre types may also play a critical role in the sub- sequent adaptations and, for that reason, in her- itability studies.[1] According to these authors’ results, basal gene expression has been shown to be variable among fibre types, which may influ- ence the response and eventual adaptations to an exercise stimulus. Therefore, it seems logical to expect that the hereditary influence will vary, depending also on the subject’s physical activity level. This is another important issue that war- rants consideration in future studies.

4. Conclusions

The sources of phenotype variability arise from three major factors: experimental errors, environ- ment stimuli and genetic factors. The environment stimuli interact with common genetic variants to determine individual characteristics including physical performance. Thus, the study of genetic inheritance effects allows for estimation of the genetic contribution to total variance. However, the literature estimation of this genetic contribu- tion is, to date, very uneven among studies, which precludes the drawing of any exact conclusions. These differences are a consequence of different phenotype evaluation procedures beyond the ex- perimental error associated with each measure- ment. Furthermore, the age, sex and ethnicity of studied subjects are other important factors that appear to contribute to the divergent results. The effect of different environmental circumstances on the relative contribution of genes is equally important. Indeed, gene expression can vary in response to changing environments, as well as in number and the epistasis processes involved. Since most studies did not report any effect size, future research should therefore focus on achieving

Endurance and Muscle Strength Genetic Inheritance Effects 455

adequate statistical power. There is also a need for replication studies using improved measure- ment precision of phenotypes and a more homo- geneous social environment shared across the study population (e.g. physical fitness levels).

In summary, there is an obvious multifactorial causation for the inexact nature of measuring inheritance in complex traits. Here, family studies provide more information that is precise for al- leles that are shared identically by descent. More complex analysis of families is possible (trans- mission disequilibrium test and the sibship dis- equilibrium test), but the use of actual DNA sequence variation data is required. Future models that take into account the association between athletic status and complex gene-gene and gene- environment interactions will have to be studied on the Hormetic Theory base. This approach rep- resents the biphasic dose-response patterns to toxic challenges[1] by exposure to hormetic agents at threshold levels (e.g. exercise, heat and oxida- tive stress).

Acknowledgements

No sources of funding were used to assist in the prepara- tion of this review. The authors have no conflicts of interest that are directly relevant to the content of this review.

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Correspondence: Dr Mikel Izquierdo, Department of Health Sciences, Public University of Navarre, Spain. Campus of Tudela, Av. de Tarazona s/n, 31500 Tudela, Navarre, Spain. E-mail: [email protected]

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

c.0007256-201242060-00001_6723.pdf

Genetic Inheritance Effects on Endurance and Muscle Strength

Abstract
1. Introduction
2. Genetic Inheritance Effects on Aerobic™Endurance
3. Heredity in Anaerobic Power and Capacity

3.1 Anaerobic and Explosive Tasks
3.2 Short-Duration Anaerobic Tasks
3.3 Long-Term Anaerobic Tasks
3.4 Other Determinants of Anaerobic Performance

4. Conclusions
Acknowledgements
References