Relationship between TFAM Gene Polymorphisms and Endurance Capacity in Response to Training

The aim of this study was to explore whether polymorphisms in mitochondrial transcription factor A ( TFAM) gene are associated with endurance capacity in a pretraining state (baseline) and/or in response to a supervised 18-wk endurance training (changes) in 102 young Chinese males (nonathletes). Phenotypes measured were running economy (RE) and V(.)O (2max). Genomic DNA was extracted from white blood cells and the genotypes were analyzed by PCR-RFLP in single nucleotide polymorphisms (SNP) rs1937, rs2306604 and rs1049432. Genotype distributions were in Hardy-Weinberg equilibrium at three loci (p > 0.05). When the three polymorphisms were considered together, three haplotypes were estimated, i.e., G (rs1937)-A (rs2306604)-G (rs1049432) (49 %), G (rs1937)-G (rs2306604)-G (rs1049432) (33 %) and C (rs1937)-G (rs2306604)-T (rs1049432) (18 %). SNPrs1937 and rs1049432 achieved near complete linkage disequilibrium (LD) (D' = 1 and r (2) = 0.903). We found no significant differences in baseline levels of V(.)O (2max) and RE between TFAM genotypes or haplotypes. Similarly, we found no differences for the training-induced changes of both variables. It was concluded that the three polymorphisms in TFAM gene rs1937, rs2306604 and rs1049432 do not predict endurance capacity/trainability, at least in Chinese males.

PGC-1β is downregulated by training in human skeletal muscle: no effect of training twice every second day vs. once daily on expression of the PGC-1 family

We hypothesized that the peroxisome proliferator-activated receptor-γ coactivator-1 (PGC-1) family of transcriptional coactivators (PGC-1α, PGC-1β, and PRC) is differentially regulated by training once daily vs. training twice daily every second day and that this difference might be observed in the acute response to endurance exercise. Furthermore, we hypothesized that expression levels of the PGC-1 family differ with muscular fiber-type composition. Thus, before and after 10 wk of knee extensor endurance training, training one leg once daily and the other leg twice daily every second day, keeping the total amount of training for the legs equal, skeletal muscle mRNA expression levels of PGC-1α, PGC-1β, and PRC were determined in young healthy men ( n = 7) in response to 3 h of acute exercise. No significant difference was found between the two legs, suggesting that regulation of the PGC-1 family is independent of training protocol. Training decreased PGC-1β in both legs, whereas PGC-1α was increased, but not significantly, in the leg training once daily. PRC did not change with training. Both PGC-1α and PRC were increased by acute exercise both before and after endurance training, whereas PGC-1β did not change. The mRNA levels of the PGC-1 family were examined in different types of human skeletal muscle (triceps, soleus, and vastus lateralis; n = 7). Only the expression level of PGC-1β differed and correlated inversely with percentage of type I fibers. In conclusion, there was no difference between training protocols on the acute exercise and training response of the PGC-1 family. However, training caused a decrease in PGC-1β mRNA levels.

Similarity of polygenic profiles limits the potential for elite human physical performance

Human physical capability is influenced by many environmental and genetic factors, and it is generally accepted that physical capability phenotypes are highly polygenic. However, the ways in which relevant polymorphisms combine to influence the physical capability of individuals and populations are unknown. Initially, the literature was searched to identify associations between 23 genetic polymorphisms and human endurance phenotypes. Next, typical genotype frequencies of those polymorphisms in the general population were obtained from suitable literature. Using probability calculations, we found only a 0.0005% chance of a single individual in the world having the 'preferable' form of all 23 polymorphisms. As the number of DNA variants shown to be associated with human endurance phenotypes continues to increase, the probability of any single individual possessing the 'preferable' form of each polymorphism will become even lower. However, with population turnover, the chance of such genetically gifted individuals existing increases. To examine the polygenic endurance potential of a human population, a 'total genotype score' (for the 23 polymorphisms) was calculated for each individual within a hypothetical population of 1000 000. There was considerable homogeneity in terms of genetic predisposition to high endurance potential, with 99% of people differing by no more than seven genotypes from the typical profile. Consequently, with population turnover world and Olympic records should improve even without further enhancement of environmental factors, as more 'advantageous' polygenic profiles occasionally, though rarely, emerge. More broadly, human potential appears limited by the similarity of polygenic profiles at both the 'elite sport' and 'chronic disorder' ends of the performance continuum.

Loss of ACTN3 gene function alters mouse muscle metabolism and shows evidence of positive selection in humans

More than a billion humans worldwide are predicted to be completely deficient in the fast skeletal muscle fiber protein alpha-actinin-3 owing to homozygosity for a premature stop codon polymorphism, R577X, in the ACTN3 gene. The R577X polymorphism is associated with elite athlete status and human muscle performance, suggesting that alpha-actinin-3 deficiency influences the function of fast muscle fibers. Here we show that loss of alpha-actinin-3 expression in a knockout mouse model results in a shift in muscle metabolism toward the more efficient aerobic pathway and an increase in intrinsic endurance performance. In addition, we demonstrate that the genomic region surrounding the 577X null allele shows low levels of genetic variation and recombination in individuals of European and East Asian descent, consistent with strong, recent positive selection. We propose that the 577X allele has been positively selected in some human populations owing to its effect on skeletal muscle metabolism.

NRF2 Genotype Improves Endurance Capacity in Response to Training

The aim of this work was to examine the association between the polymorphisms in nuclear respiratory factor (NRF2) gene and endurance capacity measured prior to and after an 18-wk endurance training program in young Chinese men. The phenotypes measured were running economy (RE) and VO(2max). The RE was determined by measuring submaximal VO(2) for 5 min at a constant running speed of 12 km x h (-1) and VO(2max) was measured during an incremental test to volitional exhaustion. Genomic DNA was extracted from white cells of peripheral blood and the genotypes were examined in SNPrs12594956, rs8031031 and rs7181866 by PCR-RFLP. Genotype distributions were in Hardy-Weinberg equilibrium at three loci, and linkage disequilibrium was observed (LD D' = 1 and r (2) = 0.903) between rs8031031 and rs7181866. The VO(2max) was associated with rs12594956 at baseline while the training response of VO(2) at RE, was associated with rs12594956, rs8031031 and rs7181866. When the three SNPs were considered together, those carrying the ATG haplotype had 57.5 % higher training response in VO(2) at RE (p = 0.006) than non-carriers. In conclusion, polymorphisms in NRF2 gene may explain some of the between-person variance in endurance capacity.

Mitochondrial haplogroup T is negatively associated with the status of elite endurance athlete

Mitochondrial function is absolutely necessary to supply the energy required for muscles, and germ line mutations in mitochondrial genes have been related with impaired cardiac function and exercise intolerance. In addition, alleles at several polymorphic sites in mtDNA define nine common haplogroups, and some of these haplogroups have been implicated in the risk of developing several diseases. In this study, we analysed the association between mtHaplogroups and the capacity to reach the status of elite endurance athlete. DNA was obtained from blood leukocytes of 95 Spanish elite endurance athletes and 250 healthy male population controls. We analysed eight mitochondrial polymorphisms and the frequencies were statistically compared between elite athletes and controls. Haplogroup T, specifically defined by 13368A, was significantly less frequent among elite endurance athletes (p =0.012, Fisher's exact test). Our study suggests that allele 13368A and mitochondrial haplogroup T might be a marker negatively associated with the status of elite endurance athlete. This mitochondrial variant could be related with a lower capacity to respond to endurance training, through unknown mechanisms involving a less efficient mitochondrial workload.

[Association of PPARD gene polymorphism with human physical performance]

Peroxisome proliferator-activated receptor delta (PPARdelta) regulates expression of genes involved in lipid and carbohydrate metabolism. To examine the association of a functional +294T/C polymorphism of PPARD gene with human physical performance, we have studied the distribution of PPARD alleles and genotypes in a cohort of athletes (n=1256; stratified by specialty and skill level) and controls (n=610). We found that the frequency of PPARD C allele (with higher transcriptional activity compared to T allele) in a group of endurance-oriented athletes (n=898) is significantly higher than in controls (18.3% vs. 12.1%; p < 0.0001). Moreover, in the group of endurance-oriented athletes with cyclic activity we revealed an increasing frequency of PPARD C allele with the rising of athletes' skill level. Thus, PPARD C allele is associated with predisposition to endurance performance.

Do skeletal muscle phenotypic characteristics of Xhosa and Caucasian endurance runners differ when matched for training and racing distances?

Although East African black athletes dominate endurance running events, it is unknown whether black and white endurance runners with similar racing ability, matched for training, may differ in their skeletal muscle biochemical phenotype. Thirteen Xhosa (XR) and 13 Caucasian (CR) endurance runners were recruited and matched for 10-km performance, average preferred racing distance (PRDA), and training volume. Submaximal and maximal exercise tests were done, and vastus lateralis muscle biopsies were taken. XR were significantly lighter and shorter than CR athletes but had similar maximum oxygen consumption corrected for body weight and peak treadmill speed (PTS). XR had lower plasma lactate concentrations at 80% PTS ( P < 0.05) compared with CR. Also, XR had more type IIA (42.4 ± 9.2 vs. 31.3 ± 11.5%, P < 0.05) and less type I fibers (47.8 ± 10.9 vs. 63.1 ± 13.2%, P < 0.05), although oxidative enzyme activities did not differ. Furthermore, XR compared with CR had higher lactate dehydrogenase (LDH) activity in homogenate muscle samples (383 ± 99 vs. 229 ± 85 μmol·min−1·g dry weight−1, P < 0.05) and in both type IIa ( P < 0.05) and type I ( P = 0.05) single-fiber pools. A marked difference ( P < 0.05) in the composition of LDH isoform content was found between the two groups with XR having higher levels of LDH5-4 isoforms (skeletal muscle isozymes; LDH-M) than CR, which was not accounted for by fiber-type differences alone. These results confirm differences in muscle phenotype and physiological characteristics, particularly associated with high-intensity running.

TheACEdeletion allele is associated with Israeli elite endurance athletes

An Alu insertion (I)/deletion (D) polymorphism in the angiotensin I converting enzyme (ACE) gene has been associated with ACE activity. Opposing effects on elite athletic performance have been proposed for the I and D alleles; while the D allele favours improved endurance ability, the I allele promotes more power-orientated events. We tested this hypothesis by determining the frequency of ACE ID alleles amongst 121 Israeli top-level athletes classified by their sporting discipline (marathon runners or sprinters). Genotyping for ACE ID was performed using polymerase chain reaction on DNA from leucocytes. The ACE genotype and allele frequencies were compared with those of 247 healthy individuals. Allele and genotype frequencies differed significantly between the groups. The frequency of the D allele was 0.77 in the marathon runners, 0.66 in the control subjects (P = 0.01) and 0.57 in the sprinters (P = 0.002). The ACE DD genotype was more prevalent among the endurance athletes (0.62) than among the control subjects (0.43, P = 0.004) and the power athletes (0.34, P = 0.004). In the group of elite athletes, the odds ratio of ACE DD genotype being an endurance athlete was 3.26 (95% confidence interval 1.49-7.11), and of ACE II genotype was 0.41 (95% confidence interval 0.14-1.19). We conclude that in Israeli elite marathon runners the frequency of the ACE D allele and ACE DD genotype seems to be higher than in sprinters, suggesting a positive association between the D allele and the likelihood of being an elite endurance athlete in some ethnic groups.

[Association of regulatory genes polymorphisms with aerobic and anaerobic performance of athletes]

The aim of study was to investigate an allelic distribution of PPARA (G/C polymorphism), PPARG (Pro/Ala), PPARD (+294T/C) and PGCIA (Gly482Ser) genes in rowers (n=205) and controls (n=659), and to find correlation between genotypes and physiological parameters. Genotyping was performed by restriction fragment length polymorphism analysis. Physiological parameters were evaluated by PM 3 Rower Ergometer and MetaMax 3B Gas Analyzer. The frequencies ofPPARA G (90.1% vs. 83.6%) and PPARG Ala (23.1% vs. 16.2%) alleles in elite athletes, and of PPARD C (19.1% vs. 10.5%) and PGC1A Gly (75.4% vs. 66.5%) alleles in sub-elite athletes were significantly higher than in controls. Moreover, PPARA G (when oxygen pulse was measured) and PGC 1A Gly (when maximal aerobic power and anaerobic threshold (%) of VO2max were measured) alleles were associated with high values of aerobic performance. Thus, PPARA G, PPARG Ala, PPARD C and PGCIA Gly alleles can be considered as genetic markers associated with enhanced physical performance.

No association of the ACTN3 gene R577X polymorphism with endurance performance in Ironman Triathlons

Alpha-actinins are major structural components of the Z-discs in skeletal muscle. Alpha-actinin 3 is encoded by the ACTN3 gene and is expressed only in type II muscle fibres. Homozygosity for the nonsense mutation, 577X, within ACTN3 results in deficiency of alpha-actinin-3 but does not result in an abnormal muscular phenotype. Previous research has found an association of the 577R allele with sprinting and/or power performance. It has also been suggested that the 577X allele may confer an advantage during endurance events. Four hundred and fifty seven Caucasian male triathletes who completed either the 2000 and/or 2001 226 km South African Ironman Triathlons, and 143 Caucasian controls, were genotyped for the R577X mutation within the ACTN3 gene. There were no significant differences in either the genotype (P = 0.486) or allele (P = 0.375) frequencies within the fastest, middle of the field or slowest Caucasian male finishers and the control population. In conclusion, the R577X polymorphism within the ACTN3 gene was not associated with ultra-endurance performance in the 2000 and 2001 South African Ironman Triathlons.

Artificial selection for high-capacity endurance running is protective against high-fat diet-induced insulin resistance

Elevated oxidative capacity, such as occurs via endurance exercise training, is believed to protect against the development of obesity and diabetes. Rats bred both for low (LCR)- and high (HCR)-capacity endurance running provide a genetic model with inherent differences in aerobic capacity that allows for the testing of this supposition without the confounding effects of a training stimulus. The purpose of this investigation was to determine the effects of a high-fat diet (HFD) on weight gain patterns, insulin sensitivity, and fatty acid oxidative capacity in LCR and HCR male rats in the untrained state. Results indicate chow-fed LCR rats were heavier, hypertriglyceridemic, less insulin sensitive, and had lower skeletal muscle oxidative capacity compared with HCR rats. Upon exposure to an HFD, LCR rats gained more weight and fat mass, and their insulin resistant condition was exacerbated, despite consuming similar amounts of metabolizable energy as chow-fed controls. These metabolic variables remained unaltered in HCR rats. The HFD increased skeletal muscle oxidative capacity similarly in both strains, whereas hepatic oxidative capacity was diminished only in LCR rats. These results suggest that LCR rats are predisposed to obesity and that expansion of skeletal muscle oxidative capacity does not prevent excess weight gain or the exacerbation of insulin resistance on an HFD. Elevated basal skeletal muscle oxidative capacity and the ability to preserve liver oxidative capacity may protect HCR rats from HFD-induced obesity and insulin resistance.

Muscle Na+-K+-ATPase activity and isoform adaptations to intense interval exercise and training in well-trained athletes

The Na+-K+-ATPase enzyme is vital in skeletal muscle function. We investigated the effects of acute high-intensity interval exercise, before and following high-intensity training (HIT), on muscle Na+-K+-ATPase maximal activity, content, and isoform mRNA expression and protein abundance. Twelve endurance-trained athletes were tested at baseline, pretrain, and after 3 wk of HIT (posttrain), which comprised seven sessions of 8 × 5-min interval cycling at 80% peak power output. Vastus lateralis muscle was biopsied at rest (baseline) and both at rest and immediately postexercise during the first (pretrain) and seventh (posttrain) training sessions. Muscle was analyzed for Na+-K+-ATPase maximal activity (3- O-MFPase), content ([3H]ouabain binding), isoform mRNA expression (RT-PCR), and protein abundance (Western blotting). All baseline-to-pretrain measures were stable. Pretrain, acute exercise decreased 3- O-MFPase activity [12.7% (SD 5.1), P < 0.05], increased α1, α2, and α3 mRNA expression (1.4-, 2.8-, and 3.4-fold, respectively, P < 0.05) with unchanged β-isoform mRNA or protein abundance of any isoform. In resting muscle, HIT increased ( P < 0.05) 3- O-MFPase activity by 5.5% (SD 2.9), and α3 and β3 mRNA expression by 3.0- and 0.5-fold, respectively, with unchanged Na+-K+-ATPase content or isoform protein abundance. Posttrain, the acute exercise induced decline in 3- O-MFPase activity and increase in α1 and α3 mRNA each persisted ( P < 0.05); the postexercise 3- O-MFPase activity was also higher after HIT ( P < 0.05). Thus HIT augmented Na+-K+-ATPase maximal activity despite unchanged total content and isoform protein abundance. Elevated Na+-K+-ATPase activity postexercise may contribute to reduced fatigue after training. The Na+-K+-ATPase mRNA response to interval exercise of increased α- but not β-mRNA was largely preserved posttrain, suggesting a functional role of α mRNA upregulation.

ACTN3 genotype in professional soccer players

The authors studied the frequency distribution of alpha-actinin-3 (ACTN3) R577X genotypes in 60 top-level professional soccer players. The results were compared with those of 52 elite endurance athletes and 123 sedentary controls. The per cent distribution of RR and RX genotypes in soccer players (48.3% and 36.7%) was significantly higher and lower, respectively, than controls (28.5% and 53.7%) and endurance athletes (26.5% and 52%) (p = 0.041). Although there are notable exceptions, elite soccer players tend to have the sprint/power ACTN3 genotype.

Dysregulation of muscle lipid metabolism in rats selectively bred for low aerobic running capacity

As substrate for evaluation of metabolic diseases, we developed novel rat models that contrast for endurance exercise capacity. Through two-way artificial selection, we created rodent phenotypes of intrinsically low-capacity runners (LCR) and high-capacity runners (HCR) that also differed markedly for cardiovascular and metabolic disease risk factors. Here, we determined skeletal muscle proteins with putative roles in lipid and carbohydrate metabolism to better understand the mechanisms underlying differences in whole body substrate handling between phenotypes. Animals (generation 16) differed for endurance running capacity by 295%. LCR animals had higher resting plasma glucose (6.58 +/- 0.45 vs. 6.09 +/- 0.45 mmol/l), insulin (0.48 +/- 0.03 vs. 0.32 +/- 0.02 ng/ml), nonesterified fatty acid (0.57 +/- 0.14 v 0.35 +/- 0.05 mM), and triglyceride (TG; 0.47 +/- 0.11 vs. 0.25 +/- 0.08 mmol/l) concentrations (all P < 0.05). Muscle TG (72.3 +/- 14.7 vs. 38.9 +/- 6.2 mmol/kg dry muscle wt; P < 0.05) and diacylglycerol (96 +/- 28 vs. 42 +/- 8 pmol/mg dry muscle wt; P < 0.05) contents were elevated in LCR vs. HCR rats. Accompanying the greater lipid accretion in LCR was increased fatty acid translocase/CD36 content (1,014 +/- 80 vs. 781 +/- 70 arbitrary units; P < 0.05) and reduced TG lipase activity (0.158 +/- 0.0125 vs. 0.274 +/- 0.018 mmol.min(-1).kg dry muscle wt(-1); P < 0.05). Muscle glycogen, GLUT4 protein, and basal phosphorylation states of AMP-activated protein kinase-alpha1, AMP-activated protein kinase-alpha2, and acetyl-CoA carboxylase were similar in LCR and HCR. In conclusion, rats with low intrinsic aerobic capacity demonstrate abnormalities in lipid-handling capacity. These disruptions may, in part, be responsible for the increased risk of metabolic disorders observed in this phenotype.

ACE I/D gene polymorphism and aerobic endurance development in response to training in a non-elite female cohort

AIM

The aim of this study was to investigate the association between ACE gene polymorphism and short- and medium-duration aerobic endurance performance improvements in response to the same training regimen in a non-elite female cohort.

METHODS

Fifty-five female non-elite Caucasian Turkish athletes trained to enhance running speeds corresponding to 70% and 90% of heart rate reserve (V-HRR70 and V-HRR90 respectively) 30 min running speed performance (V-30min) 3 times per week, for 6 weeks. ACE gene polymorphisms studied by PCR analysis.

RESULTS

The distribution of genotypes in the whole cohort was 21.8%, 41.8%, 36.4% for II (n=12), ID (n=23) and DD (n=20), respectively. Subjects with ACE II genotype had significantly higher improvements in V-30min and V-HRR70 than the ACE DD group (P<0.05). However, in HRR90 ACE DD genotype had a better performance enhancement in running speed than others (P<0.05). Endurance improvements in the V-HRR70 and in the V-30min showed a linear trend as II>ID>DD (P<0.05 and P<0.01, respectively) while a linear trend as DD>ID>II (P<0.01) observed in V-HRR90.

CONCLUSION

ACE II genotype may related with better improvements in medium duration aerobic endurance performance whilst ACE DD genotype seems to be more advantageous in performance enhancement in shorter duration and higher intensity endurance activities.

Is there an association between PPARGC1A genotypes and endurance capacity in Chinese men?

Our purpose was to determine the possible association between genotypes of three polymorphisms (Gly482Ser, Thr394Thr and A2962G) of the peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PPARGC1A) gene, on one hand, and both the pre- (baseline) and post-training levels of maximal (i.e., maximal oxygen uptake [VO2max]) and submaximal human endurance capacity (i.e., running economy [RE]). We studied 102 young males (physically active, non-athletes; age: 19+/-1 yrs) from Northern China (of Han origin) who underwent a 18-week endurance training (running) program and were tested on a treadmill (for VO2max and RE determination) before and after training. None of the VO2max and RE related traits were associated with the Gly482Ser and Thr394Thr polymorphisms at baseline or after training. The A2962G polymorphism was however associated with VO2max at baseline, as carriers of the G allele (AG+GG genotypes; n=49) had higher levels of VO2max than the AA group (n=53) (58.2+/-4.3 vs 56.3+/-3.9 mL/kg/min; P=0.017). Our results do not support previous data on Caucasians showing an association between the Gly482Ser variant and VO2max but suggest the potential role of another polymorphism (A2962G) to explain individual VO2max differences in Chinese men.

Quantitative trait loci associated with maximal exercise endurance in mice

The role of genetics in the determination of maximal exercise endurance is unclear. Six- to nine-week-old F2 mice (n = 99; 60 female, 39 male), derived from an intercross of two inbred strains that had previously been phenotyped as having high maximal exercise endurance (Balb/cJ) and low maximal exercise endurance (DBA/2J), were treadmill tested to estimate exercise endurance. Selective genotyping of the F2 cohort (n = 12 high exercise endurance; n = 12 low exercise endurance) identified a significant quantitative trait locus (QTL) on chromosome X (53.7 cM, DXMit121) in the entire cohort and a suggestive QTL on chromosome 8 (36.1 cM, D8Mit359) in the female mice. Fine mapping with the entire F2 cohort and additional informative markers confirmed and narrowed the QTLs. The chromosome 8 QTL (EE8(F)) is homologous with two suggestive human QTLs and one significant rat QTL previously linked with exercise endurance. No effect of sex (P = 0.33) or body weight (P = 0.79) on exercise endurance was found in the F2 cohort. These data indicate that genetic factors in distinct chromosomal regions may affect maximal exercise endurance in the inbred mouse. Whereas multiple genes are located in the identified QTL that could functionally affect exercise endurance, this study serves as a foundation for further investigations delineating the identity of genetic factors influencing maximum exercise endurance.

Citius and longius (faster and longer) with no alpha-actinin-3 in skeletal muscles?

The muscle protein alpha-actinin-3 (ACTN3) is normally thought to be expressed in type II muscle fibres and to be necessary for high-power, high-velocity muscle contractions, such as those typically seen in speed/power athletes. The authors report the case of a Spanish elite long jumper (two times Olympian, personal best of 8.26 m) whose genotype for the ACTN3 gene is 577XX (ACTN3 deficient). These data suggest that there might be notable exceptions to the concept that ACTN3 is the "gene for speed".

The human gene map for performance and health-related fitness phenotypes: the 2005 update

The current review presents the 2005 update of the human gene map for physical performance and health-related fitness phenotypes. It is based on peer-reviewed papers published by the end of 2005. The genes and markers with evidence of association or linkage with a performance or fitness phenotype in sedentary or active people, in adaptation to acute exercise, or for training-induced changes are positioned on the genetic map of all autosomes and the X chromosome. Negative studies are reviewed, but a gene or locus must be supported by at least one positive study before being inserted on the map. By the end of 2000, in the early version of the gene map, 29 loci were depicted. In contrast, the 2005 human gene map for physical performance and health-related phenotypes includes 165 autosomal gene entries and QTL, plus five others on the X chromosome. Moreover, there are 17 mitochondrial genes in which sequence variants have been shown to influence relevant fitness and performance phenotypes. Thus, the map is growing in complexity. Unfortunately, progress is slow in the field of genetics of fitness and performance, primarily because the number of laboratories and scientists focused on the role of genes and sequence variations in exercise-related traits continues to be quite limited.

ACTN3

A common variant of the ACTN3 gene, R577X, results in complete deficiency of the alpha-actinin-3 protein in the fast skeletal muscle fibers of more than a billion humans worldwide. We review the evidence that this genetic variant is strongly associated with elite athlete status and with normal variation in human muscle strength and sprinting speed.