Physical fitness can be defined as a set of components that determine exercise ability and influence performance in sports. This study investigates the genetic and environmental influences on individual differences in explosive leg strength (vertical jump), handgrip strength, balance, and flexibility (sit-and-reach) in 227 healthy monozygotic and dizygotic twin pairs and 38 of their singleton siblings (mean age 17.2 ± 1.2). Heritability estimates were 49% (95% CI 35-60%) for vertical jump, 59% (95% CI 46-69%) for handgrip strength, 38% (95% CI 22-52%) for balance, and 77% (95% CI 69-83%) for flexibility. In addition, a meta-analysis was performed on all twin studies in children, adolescents and young adults reporting heritability estimates for these phenotypes. Fifteen studies, including results from our own study, were meta-analyzed by computing the weighted average heritability. This showed that genetic factors explained most of the variance in vertical jump (62%; 95% CI 47-77%, N = 874), handgrip strength (63%; 95% CI 47-73%, N = 4516) and flexibility (50%; 95% CI 38-61%, N = 1130) in children and young adults. For balance this was 35% (95% CI 19-51%, N = 978). Finally, multivariate modeling showed that the phenotypic correlations between the phenotypes in current study (0.07 < r < 0.27) were mostly driven by genetic factors. It is concluded that genetic factors contribute significantly to the variance in muscle strength, flexibility and balance; factors that may play a key role in the individual differences in adolescent exercise ability and sports performance.
Keywords: Exercise ability; Heritability; Meta-analysis; Physical fitness; Twin study.