Prevention and modification of population obesity requires an understanding of the nature of the activities that comprise the current patterns of energy expenditure. These data present a graphic portrait of a society in which sedentary and low-intensity activities predominate. They also make clear that leisure time physical activity contributes a very small proportion (5%) of the total energy expenditure in the United States. The data also suggest strategies for preventing population weight gain that acknowledge and make use of the patterns described here.
It is clear that Americans need to be more physically active, and to increase the ratio of energy expended to energy consumed in order to control the increase in the prevalence of obesity. How this is to be achieved is less obvious. With such a small proportion of time and energy expended in leisure time physical activity, it is clear that this is of very low priority for most Americans. Perhaps this is in part simply a reflection of time constraints of modern life. Americans work more total annual hours than persons in most other developed countries – for example, in 2001, 1821 hours compared with 1467 hours in Germany . The problem of long working hours is often compounded by long commutes. In just the decade between 1990 and 2000, the proportion of workers who commuted for 30 or more minutes a day increased from 19.6% to 33.7% [12, 13]. Dual-earner families have also increased, and both husband and wife held jobs outside the home in two-thirds of married couples in the year 2000 . Further, a substantial number of others are single-parent households where the responsibilities of earning a living, raising children and maintaining a household fall on a single person. Given these time constraints, perhaps ways other than leisure-time physical activity should be found to encourage energy expenditure. These tables make clear that it is ordinary, day-to-day mundane activities that contribute the great majority of our current energy expenditure. It may be that we could achieve a greater effect by modest increases in the duration or intensity of those activities than we could by large increases in the very small percent of time spent on leisure time activities.
As with dietary interventions, it may be more successful to increase frequency or intensity of activities already commonly performed, rather than attempting to introduce new behaviors into established lifestyles. For example, at work people could be encouraged to "walk up two or down three flights". Commuters could be encouraged to park a slight distance away from their destination. This is consistent with numerous recent recommendations for an "active living" approach [1, 15]. The NHAPS data suggest that this approach could have a substantial population impact, since these activities comprise such a large proportion of total energy expenditure. Such an approach could have several benefits, as light-to-moderate activity has been found to be most effective in motivating sedentary individuals , to benefit sedentary and obese individuals , and frequent moderate activity was found to be associated with better control of body fat .
The other observation suggested by these data is that the only "available" time in which leisure time physical activity might be performed would come from the category "Watching TV/movie, at home or theater", which accounted for two hours and 50 minutes of the population's average day. Promotion of leisure time physical activity that was explicitly linked to a corresponding restriction in TV viewing might be more effective than unspecific promotion of increased activity.
The 24-hour recall approach reveals some important aspects of physical activity by gender and ethnic group. Women have often been reported to be less active than men, in research using self-report instruments . This finding could be influenced by the types of activities assessed . In the NHAPS 24-hour recall approach, all types of activities could be reported. The results indicate that women have only slightly lower activity levels than men in terms of kcal/kg body weight (medians 36.8, 36.5), although men must expend more total kilocalories due to their heavier body weights (Table 5). Women had substantially less leisure-time physical activity, but total energy expenditure was approximately compensated by a higher proportion of household activities. These results emphasize the importance of household activities, which provide approximately one-third of the daytime energy expenditure for women.
While all ethnic groups appear to have inadequate activity levels, African Americans in particular are consistently found to have the lowest percent of total energy expended in leisure time activities, and the highest proportion of the population reporting no leisure time activity. In addition, African Americans had lower total expenditure in kcal/kg than either Hispanics or non-Hispanic whites (Table 5). Minority groups have consistently been found to have relatively lower physical activity levels than other groups . However, these surveys have not always taken into account energy expenditure from occupational and household related activities. The data presented here take into account energy expenditure from all sources, and indicate that non-leisure activities are also lower among African Americans.
Younger age groups expended more kcal/kg body weight and a larger percentage of their energy in leisure time physical activities compared to older age groups. These results are similar to the trend described in the Surgeon General's Report in 1996, in which the prevalence of physical inactivity was noted to be higher in older groups . Interestingly, people aged 65–74 actually performed a higher percentage of leisure time physical activities and household related activities than did people aged 55–64 (Table 6). This may be due to the greater available time for these activities after retirement, and highlights the problem of time constraints in the promotion of physical activity.
Regions along the western and eastern coast had larger percentages of energy expenditure spent on leisure time physical activities than other groups. Nevertheless, even in the relatively more leisure-time-active regions 82.5% of persons reported no leisure-time activities.
The strength of this study lies in the large sample, the representativeness, and the 24-hour recall approach. The reasonable similarity of the energy expenditure estimates to the estimated energy requirements using the IOM formulae  suggest that activities shown here represent a reasonably complete capture of activities in this population, and thus may provide a valid picture of U.S. activity patterns. The physical activity methodology used here has been used in physical activity research  and in the social sciences and environmental exposure literature for at least 40 years, with several very large studies including a multinational study of 25,000 persons in 12 countries . In addition, the method as applied to assessment of dietary intake has been the subject of extensive study, by the U.S. Department of Agriculture and other researchers, including in minority populations . It is judged to be the most accurate method for the assessment of the intake of groups and the least subject to overreporting bias, and is the method used by national surveys such as the NHANES.
Response rate and missing data represent potential limitations of the study. The response rate of 63% is lower than desirable, although not too dissimilar to the response rate for the dietary data in NHANES III of 73%. As noted by Klepeis et al. , "When the number of interviews successfully completed (9,386) is divided by the number of interviews attempted [completed interviews (9,386) plus refusals (2,944) = 12,330], the resulting cooperation rate is over 76%. This cooperation rate is relatively high for a survey that did not utilize financial or other incentives to increase participation." No information is available on nonrespondents, but the likely bias is that persons who did participate were more interested in health, which implies that the data presented here are, if anything, optimistic regarding the level of physical activity of the U.S. population. Missing data are unlikely to have a notable impact on these results, since only 1% of all respondents had any time periods that were missing a description of the activity or location.
These results represent a snapshot in time. Subjects were surveyed throughout 1992–1994. As new technologies are developed and as demographics change, new activities are created and certain activities may increase or decrease in importance. For example, video and computer game use was not as common during the time period of the survey as it is today. In view of that, these results may underestimate how sedentary our society is now. Conversely, the importance of physical activity has been increasingly emphasized in the time since publication of the Surgeon General's report in 1996 , so it is conceivable that some Americans have become more active. That is not supported, however, by data from the Behavioral Risk Factor Surveillance System (BRFSS), which found that trends in physical activity remained stable over the period 1990–1998 .
The usefulness of the activity lists presented here will of course depend on the purposes to which they may be put. The primary use may be in identifying strategies for preventing population weight gain that acknowledge and make use of the population-based activity patterns described here. The activity lists presented here reveal common behaviors that could become the focus of intervention efforts to increase their frequency or intensity. In addition, these lists may be useful in the development of assessment instruments for total energy expenditure. The great majority of these activities (unfortunately) do not contribute to cardiovascular fitness. Researchers studying cardiovascular fitness will continue to use instruments developed for that purpose . However, like the dietary approach on which this study is based [23, 24], this method has been used by one of the authors (GB) to develop a Total Energy Expenditure Questionnaire.