Physical fitness including muscle strength and endurance capacity varies substantially among individuals. Physical activity level and exercise behavior also vary among individuals. Many family studies and studies on twins have reported that genetic factors are responsible for heterogeneity in fitness-related traits. However, there is much dispersion in heritability, as previously reported. A systematic review was performed to determine the extent of heritability in physical fitness. A literature search was conducted through PubMed using the following keywords: (heredity or heritability) and (“muscle strength” or “muscle contraction” or endurance or athlete* or fitness or exercise) and (twin* or family). Finally, 43 articles that included 137 phenotypes met the criteria. Only one article focused on the heritability of athletic status. With respect to muscle strength, 23 articles and 54 phenotypes were collected, and showed a range of heritability of 0%-98% (mean: 55±22%). This heterogeneity was partly explained by the ages of subjects. With respect to endurance capacity, 13 articles and 28 phenotypes showed heritability of 0%-93% (mean: 54±25%). The considerable degree of variability in heritability of fitness-related traits may depend on age, sex, race, and environmental factors. Therefore it is necessary to investigate the interaction between genetic factor and factors other than genetic factor. On the other hand, with respect to exercise behavior or physical activity level which is important for improving the physical fitness or health status, the heritability of 0%-85% has also been reported. That is, the genetic factor plays a role in not only physiological phenotypes but also behavioral phenotypes. Understanding of these genetic factors and their mechanisms will lead to improvement in physical fitness or encouragement of physical activity/exercise behavior.

Background: Although previous reports have shown a lower proportion of the ACTN3 XX genotype (R577X nonsense polymorphism) in sprint/power athletes compared with controls, possibly attributed to the importance of skeletal muscle function associated with alpha-actinin-3 deficiency, the findings on association between endurance athlete status and R577X genotype are equivocal. Purpose: The present study was undertaken to examine association of ACTN3 R577X genotype with elite Japanese endurance athlete status. Subjects and Methods: Subjects comprised 79 elite Japanese endurance runners (E) who participated in competition at national level and 96 Japanese controls (C). We divided endurance runners into two groups, i.e., 42 national level runners (E-N) and 37 international level runners (E-I) who had represented Japan in international competition. R577X genotype (rs1815739) was analyzed by direct sequencing. Frequency differences of polymorphisms between athletes and controls were examined by Chi-square tests. Result: The R allele frequency tended to be higher in E group than in C group (P=0.066). When we divided E into two groups, the R allele frequency in E-I group was significantly higher than that in C group (P=0.046); whereas there were no significant differences between E-N and C groups (p=0.316). Then, the three genetic models were tested. In the additive genetic model (RR>RX>XX), there were significantly differences between E-I and C (P=0.038), but not the dominant (RR vs. RX+XX) and the recessive (RR+RX vs. XX) genetic models. Conclusion: R allele of the R577X genotype in the ACTN3 gene was associated with elite Japanese endurance athlete status.