Research indicates that the inclination to be physically active or sedentary has a biological foundation. Twin and family studies confi rm that physical activityrelated traits are characterized by familial aggregation and infl uenced by genetic factors. Results from animal model studies indicate that single genes can markedly infl uence physical activity-related behavior. The fi rst molecular genetic studies on physical activity traits in humans have been published during the last few years.

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... y support the notion that it is possible to detect relatively small, yet biologically important genetic effects impacting the tendency to be sedentary or habitual physical activity at the molecular level. We are beginning to appreciate that in utero environment and epigenetic events may play a role in postnatal physiology and behavior but their impact on physical inactivity or physical activity level remains to be determined.