ABSTRACT
The evidence—at least in adults—is beyond compelling: engaging in regular physical activity engenders health and emotional well-being. Particularly striking is the diversity of the sources of this information, which includes evidence from human and animal activity levels over the life span, genetic studies of twins, biochemical effects, outcomes from surgical interventions in animals, and even intrauterine sex differences in physical activity. Virtually from the time that a toddler becomes fully ambulatory, the level of daily energy expenditure in physical activity, expressed in respect to body size, steadily declines throughout the human lifetime. Traditionally, it has been assumed that such differences in habitual physical activity by sex are explained by psycho-social factors, reflecting cultural stereotypes of masculinity and femininity. It should be recognized that evidence for genetic control of physical activity strongly supports, but does not necessarily imply, the existence of a central nervous system controller of physical activity in humans.
