Figure 1: Participants in the Finnish twin studies originally described by Aaltonen et al. Genetic studies are one of the new areas of physical activity research. This is logical because individual's genetic characteristics seem to be a possible determinant of physical activity  and advances in genetic technologies permit identification of individual genes or gene systems associated with a trait such as physical activity. These studies have attempted to determine the genetic architecture of factors contributing to an individual's propensity to be physically active. This includes estimating the overall role of genetic factors in contrast to all nongenetic factors.
If genetic factors are shown to be relevant, work is done to identify the genes and the mode of action of the genes in physical activity. The overall contribution of genetic factors to variation in physical activity is often examined by conducting twin studies.
Twin study designs are popular in behavioral genetics, as they provide an opportunity to disentangle the effects of genes from those of the environment [12, 13]. In addition to genetics, motivation is a personal characteristic that also may be one of the key factors to help understand why some people spend their leisure time undertaking physical activity.
This may be the reason why motives have been widely studied.
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Although there are cross-sectional studies examining the associations between the genetic and environmental influences, motives, and leisure-time physical activity, longitudinal studies have been less frequently conducted. However, the advantages of longitudinal study designs are that causal associations can be better revealed and that the true effects of aging maybe demonstrated. To date, little is also known about whether the motives for physical activity change over the life course. Another poorly characterized area is the difference in motivational factors between active and inactive individuals.
The Finnish twin cohorts offered a great opportunity to utilize longitudinal study design and conduct comparison between physically active and inactive twins. The main aim. Furthermore, the motives for leisure-time physical activity among consistently physically active and inactive people from the Finnish twin studies are presented.
The present paper is based on the Ph.
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Physical activity has been defined to be body movements produced by the skeletal muscles, which cause a substantial increase in energy demands over resting energy expenditure . However, the term physical activity is often used interchangeably with the terms exercise or sports although that is not correct or recommended . The choice of term physical activity, exercise, or sports may impact the results of the genetic analyses and motivational studies.
In this review, we have therefore used the same terms used in the original papers. In quantitative genetic modeling, physical activity is assumed to be made up of genetic and environmental contributions.
Environmental influences can be divided into shared environmental influences, representing the effects of environmental factors shared, for example, by the cotwins in a pair. Specific environmental influences represent unique environmental influences and specific environmental influences result in differences between the cotwins of a pair . A number of twin studies using the quantitative genetic. However, the different studies have found very different patterns.
As this investigation indicates, it is clear that there is heterogeneity in the results of studies related to genetic influences on leisure-time physical activity. It can be assumed that a significant proportion of the heterogeneity may derive not only from changes in the genetic contribution to this trait in different aged individuals but also from culture-specific, sex-specific, and period-specific effects.
Physical activity assessment methods may also have an influence on the heterogeneity of results. Heritability is always assessed at a particular time and age, and above all, heritability is an estimate of the genetic influences to individual differences on a population level.
Longitudinal study designs are needed to reveal the age-specific genetic influences on leisure-time physical activity. However, only a few studies have investigated the genetic and environmental influences on longitudinal leisure-time physical activity before the Finnish twin studies were published [27, 28]. Simonen et al. A recent comparative study in twins aged 19 to 50 from seven countries that collaborated in the GenomEUtwin project was not a pure longitudinal study, but it revealed also age-related changes in heritability .
Earlier studies have also reported a shift between genetic and environmental influences in the time periods between childhood and adolescence and between adolescence and young adulthood, although at different times in different studies and in different directions. In Dutch boys, genetic influences on leisure-time exercise behavior were fluctuating from age of 7 years to age of 12 years, while in girls genetic influences were more stable . In this study, shared environmental influences mainly explained the largest part of the variance in leisure-time exercise behavior between childhood and early adolescence.
The decline in the heritability estimate was noted in longitudinal studies by both van der Aa et al. Genetic influences on leisure-time physical activity declined from early adolescence to late adolescence in both sexes in Dutch twins  and decline was also seen during a 4-year followup among young Swedish men in their twenties . In contrast to these studies, Stubbe et al. The participants of the Finnish twin studies examined for genetic. Both cohorts were identified from the Central Population Registry of Finland with the purpose of forming a national resource for genetic epidemiological studies .
The longitudinal quantitative genetic analyses of these cohorts published by Aaltonen et al. In the studies by Aaltonen et al. In addition, these results of the Finnish twin studies confirmed the existence of age-specific changes in the genetic and environmental influences on leisure-time physical activity.
The results revealed a change in the pattern of genetic and environmental influences in the progress of leisure-time physical activity: first, from adolescence to adulthood  and, second, from the age of thirty to the mid-thirties . The summary of the final models for leisure-time physical activity has been presented in Figure 2. This decrease in genetic influences is parallel to the indications that leisure-time physical activity level declines with age [8, ].
Shared environmental influences, in turn, also showed relative stability during adolescence, but in contrast to genetic influences they increased markedly in young adulthood, especially in women. Additive genetic, shared environmental, and specific environmental correlations between the baseline results in adolescence and follow-up results in young adulthood are shown in Figure 2. In this study, the additive genetic correlation for leisure-time physical activity was greater for men, 0.
The longitudinal phenotypic correlation in men was 0.
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Based on these longitudinal quantitative studies among Finnish twins, both shared and specific environmental influences affected leisure-time physical activity up to adulthood, but only specific environmental influences were further present in adulthood in the thirties and mid-thirties. In contrast to the consistent expression of an important group of genes observed in adulthood, new additive genetic,.
Figure 2: The summary of the final genetic models for leisure-time physical activity between both ages of It is important to note that the cohorts used in the models between ages of Genetic and environmental influences are shown as percentages; upper value is for men and lower value is for women. Additive genetic, shared environmental, and specific environmental correlations between the baseline and follow-up results are shown as curved arrows.
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The more detailed summaries for models are presented in the publications of Aaltonen et al. In addition to genetics, motivation is a personal characteristic that also may be one of the key factors for understanding why some people are physically active in their leisure time. Many studies have been published on what motivates individuals to undertake physical activity. Several of these studies have reported that, regardless of age, gender, or level of physical activity, health is an important factor motivating participation in leisure-time physical activity among adults . For instance, among the citizens of the European Union member states, almost half of those aged over 15 years reported good health as the most important reason for participation in physical activity .
Despite the general importance of health as a factor motivating leisure-time physical activity, it seems to be a factor which varies by region . In addition to health benefits, appearance , fitness , enjoyment , and body image  are features which are highly linked to physical activity among young adults.
However, it is important to remember that motives may change during the stages of adoption of some form of. Differences may also exist according to exercise type [54, 55], gender, and age [46, 53, 56, 57]. So far, only some of the published studies have examined differences in motivational factors between physically active and inactive people, but none of these studies has been longitudinal. Studies have been based on the hypothesis that the level of leisure-time physical activity is explained by differences in motivational factors. One study did indicate that physical activity was mostly associated with environmental factors, whereas inactivity was linked with sociodemo-graphic factors .
Overall, when physically active people were compared to physically inactive people, health, fitness, and enjoyment were identified as the major motivational factors for leisure-time physical activity in the active people [46, 48, 59].
Social reasons were highlighted by physically active and inactive people in the recent study of Costello et al. In this study, physically inactive people wanted leisure-time physical activity to be purposeful and fun, while the active participants enjoyed exercise regardless of its purpose. The randomized controlled study of Silva et al. The role of family and genetic factors in motivation for physical activity is poorly studied; further, links between physical activity, genetic influences, and motivational factors remain unraveled.
A recent animal study suggested that voluntary running motivation may be inherent . In a study by Huppertz et al. In bivariate modeling, all the genetic and all but two unique environmental correlations between attitude components and exercise behavior suggested a causal relationship between exercise attitude and leisure-time exercise behavior. The authors concluded that both exercise attitudes and exercise behavior are heritable and are partly correlated through pleiotropic genetic effects. It thus seems plausible that family and genetic factors influence motives for physical activity.