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Muscular Strength and Power
Genetic contributions to muscular strength and muscle power have been recently summarized
[19]. Estimated h2 in five family studies varied between 0.27 and 0.58 for static
strength of different muscle groups and with adjustment for covariates such as sex, age
and sometimes body mass. Estimates tended to be higher in twin studies, although the
range of h2 in 15 studies, 0.14–0.83, was greater than in family studies (table 1). Apart
from limitations related to sample sizes and methodology used to obtain heritabilities,
it should be noted that these estimates are based on samples that span childhood
through adolescence into old age. While adjusting for age and sex is essential in family
studies, many twin studies encompass broad age ranges without correction for age and
do not always test for potential sex differences. Age-corrected heritabilities overlook
the fact that genetic and environmental influences can change with age per se and
also with biological maturation as genes can be switched on or off during childhood,
adolescence and adulthood. Moreover, individuals differ in the timing and tempo of
maturation and of aging. They also are exposed to different environmental influences
during the life cycle. The use of same-aged twins eliminates the need for age correction,
although it may be necessary to consider individual differences in the timing and
tempo of biological maturation rather than chronological age [4].
More recent twin studies based on structural equation modeling are consistent in
showing no evidence of a shared (familial) environment effect for static strength in
adolescents of both sexes [4, 20], young adult men [21, 22], and elderly women [23].
This observation is credible since multivariate analyses increase the power to detect
shared environmental influences. The heritabilities in Belgian adolescents ranged
from 0.52 to 0.82 for boys (from 1 year before PHV until 3 years after PHV) and
were somewhat lower in girls, 0.22–0.75 with one outlier of 0.07 [4]. The h2 at 3 years
after PHV, which approximates young adulthood, of 0.52 in boys and 0.48 in girls
were consistent with those for young adult Swedish, 0.50 and 0.60 [22], and Belgian,
0.70 [21] men, and for elderly women, 0.49 [23]. These estimates based on similar
analytical techniques suggest that the heritability for static strength may be somewhat
higher during adolescence than in young adulthood and old age. Systematic analyses
of sex differences in static strength in adulthood are limited with one study observing
no sex differences in the relative contribution of genes and environment in a large
sample of Danish twins aged 45–96 years, h2 = 0.52 [24].