The benefits of a physically active lifestyle are diverse and well known yet surveillance systems continue to track low levels of prevalence of this health-promoting behaviour . Of all types of physical activity (PA), walking is both most commonly encouraged  and most commonly reported [3, 4], especially in the form of walking for exercise. Walking for exercise has also been described in terms of "leisure-time" walking and is reasonably similarly distributed across social groups and by gender and age [3, 4]. Regardless of the applied label, this mode of walking characteristically represents that which is typically undertaken for its own sake (i.e., purposeful) and directly to improve some aspect of health. The Behavioral Risk Factor Surveillance System (a large, telephone-based survey) has been used to examine walking for exercise (their specific terminology is leisure-time walking) trends among American adults . Prevalence of (any) walking for exercise increased absolutely 3.8% (from 26.2% to 30.1%) in men and 6.6% (from 40.4% to 46.9%) in women between 1987 and 2000. The median frequency of walking for exercise remained stable at 2.9 times/week and 30 minutes/session; public health guidelines promote at least 5 days/week (of any PA of at least moderate intensity, including walking) at this duration.
Another mode of walking, walking for transportation, has more recently entered the public health arena as a plausibly important contributory source of PA that can be achieved easily within the Westernized and accepted culture of multi-tasking. Walking for transportation appears to meet the minimal requirements for health-enhancing PA: bouts are of sufficient duration  known to elicit cardirorespiratory benefits [7–9]; self-selected walking paces appear to be naturally of moderate intensity [10, 11], and walking for transportation appears to be more commonly (i.e., a greater proportion of the population on any given day and more days of the week) performed than walking for exercise . Further, empirical evidence of the health benefits associated explicitly with walking for transportation is accumulating [12–14]. In the USA, Healthy People 2010 Objective 22–14 calls for increasing the proportion of (short) trips made by walking . Specifically, the 2010 target for adults is 25% of all trips ≤ 1 mile and for children and adolescents it is 50% of trips to school ≤ 1 mile. A limited examination of walking for transportation trends (amalgamating 2001 National Household Travel Survey and the 1995 Nationwide Personal Transportation Survey) suggests that American adults walked more short trips in 2001 (21.2%) compared to 1995 (16.7%); a similar result was found for youth (35.9% in 2001 vs. 31.3% in 1995) . Differences in methods between the two survey administrations likely temper interpretation of time trends.
Few rigorous data exist to directly compare trends in walking for exercise and transportation to general sports/exercise participation or other potential sources of health-promoting walking such as walking the dog . Recently, however, a harmonised historical data file of time use by adults in the USA., covering surveys collected in 1965–66, 1975–76, 1985, 1992–94, and 2003, has been made freely available to the research community at www.timeuse.org/AHTUS. This data set, known as the American Heritage Time Use Study (AHTUS), was developed for the Yale University Program on Non-Market Accounts with funding from the Glaser Progress Foundation. Time-diary methods have an extensive global history in the social sciences  and are distinctive because they collect a complete log of the respondents' (or diarists', applying time use terminology conventions) activities over a 24-hour period. Time-use data typically capture starting and ending times, diarists' primary activity, 'what else' they were doing at the same time (i.e., secondary activity, although this is not uniformly captured in all surveys), the location of the activity (e.g., outside), and the presence of others during the activity.
Collecting the time allocated to walking (regardless of purpose) by using time-diaries has some clear advantages over conventional activity surveys relying on respondents' self-report of targeted behaviours. Firstly, the time-use surveys require diarists to report all their activities in their own words for a full 24-hour period. This yields, among other things, a comprehensive picture of walking as a deliberate form of exercise, as a mode of transport, or even as a form of pet care within the context of daily life and competing activities (including other sports/exercises). Secondly, precisely because diarists are asked for a comprehensive account of activities, their responses are far less affected by well-known social desirability biases more commonly associated with self-reported 'typical' behaviours . Further, faking a time-diary entry requires careful consideration of the plausibility of what was recorded before and after the fallacious activity, and deliberately falsifying activity accounts takes more effort than recording legitimate accounts (except in the rare case of very low episode diaries), giving respondents an additional internal incentive to recall their actual activities . Although Levin et al.  have shown that intra-individual variability is sufficient to warrant multiple repetitions of 48-hour physical activity records to achieve reliable estimates of habitual physical activity, time use diaries differ in that: 1) they do not focus specifically on physical activity but may be mined for physical activity in the context of all activities undertaken; and 2) they are not intended to be used to interpret individual behaviour but are rather to be used to identify population (or subgroup) time use patterns "on any given day." Time use researchers recognize that optimal diary duration (or length of coverage) varies across different types of activity, as a function of the "cycle" time between repetitions of the target event. For example, estimations of meal times (or repetitive habitual excursions of walking for transportation), require only a short diary duration. In contrast, occasional performance of recreational hikes or walking around the zoo, for example, logically require longer durations. Regardless, a succession of validation studies has confirmed that time allocation data collected by time-diaries are more accurate than the estimates derived from more general queries of time spent in a single or a group of listed activities [19, 20, 22].
As social surveys, the primary purposes of time-use studies has ranged from evaluating household and other unpaid production of goods and services, to monitoring of media use, to comparing lifestyles of more and less privileged social groups, or to tracking broad shifts in social behaviour . These data also represent a rich and largely untapped resource for exploring health-related behaviours, including participation in sports/exercise and different sources of walking. Previously, we took advantage of the 1997 Australian Bureau of Statistics (ABS) Time Use Survey to describe nationally representative patterns of walking for transport and for exercise in Australian adults . The harmonized American data set further provides an opportunity to look into historic trends in these behaviours among Americans, to the extent to which component surveys comparatively captured and coded these specific variables. Therefore, the purpose of this paper is to describe the process and utility of using the AHTUS to describe any walking, but specifically walking for exercise and transportation as well as dog walking and sports/exercise participation (i.e., this latter to contextualize the contribution of walking to PA), and all PA collected using time-use survey methods over almost 40 years in the USA. We discuss the extent to which these variables can be confidently compared across surveys and the limitations confronted.