<qa>  How many people do not engage in regular physical activity?As many as 75 of Americans do not engage in regular physical activity and 25 are completely inactive. The risk of cardiovascular disease is almost doubled among people who are physically inactive, comparable to the risk associated with increased systolic blood pressure, cigarette smoking, and elevated serum cholesterol. To make an impact on physical activity prevalence at the population level, research must focus at the interface between clinical efficacy trials and large-scale dissemination studies. Studies at this interface must compare proven interventions with new innovations that have the potential to reach large numbers of sedentary Americans. 