Based on factorial method and accelerometry combined with heart rate, we found that PALs among Tsimane men and women are about 0.1–0.3 units higher than those of industrialized populations, but similar to those of other subsistence-oriented populations. A difference of only 0.1 PAL might seem small, but for a typical Tsimane man (62 kg) or woman (56 kg) amounts to an additional 144 and 120 calories expended per day in activity, respectively. Despite some seasonality in production tasks, physical activity remains high throughout the year. Tsimane men show higher PALs than women at all ages and in different geographic regions of their territory; multiple regression analysis, however, showed that women display greater activity than men in the wet season when rice is harvested, but in general, men's total activity is more seasonal than women's. Men are most active during the dry season months from May to August, especially in the forest region when hunting and logging activities are common. At the end of the dry season, male work effort also intensifies during field clearance of large trees with axes and underbush with machetes.
Factorial method and accelerometry-HR methods yielded similar estimates of PAL at the population level, whereas accelerometry alone gave significantly lower PAL estimates. Other comparisons of physical activity show a similar pattern where accelerometry alone underestimates free-living activity. For example, a recent study of activity among Shuar forager-horticulturalists of Ecuador using an Actical accelerometer reported low mean PALs of 1.54 for men and 1.42 for women 
. A PAL of 1.4 is close to the minimum measured for healthy humans in affluent societies confined to bed-rest or respiratory chambers, and so is likely an underestimate for healthy Tsimane 
. Our study and other recent studies 
therefore suggest that accelerometry-HR is a relatively cheap, easy, and field-friendly approach for more accurately measuring activity.
Despite greater physical activity than U.S. adults, evidence of extensive vigorous activity among Tsimane was scant, especially among women. Most physical activity instead ranges from lifestyle to moderate level, with relatively little time spent “sedentary”. Similar patterns were suggested by studies among Gambian farmers 
, Aymara agropastoralists 
, and subsistence societies more generally 
. Our results are consistent with the growing body of evidence that shows many benefits of exercise at relatively low to moderate intensity 
, and with assertions that among hunter-gatherers, the diversity of activities performed are of moderate and not vigorous intensity 
. These activity profiles nonetheless exceed the activity recommendations by the CDC, which advocate a mix of 150 minutes per week of moderate and vigorous aerobic activity combined with muscle-strengthening activity for at least two days per week, and those of the American Heart Association and the American College of Sports Medicine recommending only 30 minutes of moderate activity at least five days per week to “promote and maintain health”. Our results showing lower rates of sedentary behavior are also instructive, in light of growing evidence showing separable effects of time spent sedentary and average energy expenditure on weight gain, metabolism and cardiorespiratory fitness (e.g. active couch potato or weekend warrior syndrome) 
The effects of modernization on activity levels in a subsistence economy are modest. Activity among adults living near town is no different than those living in remote villages along the Maniqui River. Adults living in remote forest villages show the highest activity levels, although much of this difference is due to logging-related wage labor, which is restricted to men. Hunting among men is also more common in forest villages, whereas fishing (less physically intensive) is more common in riverine villages. Schooling was unrelated to activity patterns, while Spanish fluency was positively associated with greater activity. Spanish fluency is associated with greater wages among Tsimane, whereas schooling does not necessarily lead to fluency nor employment opportunities 
. Wage opportunities for Tsimane primarily include working as ranch hands, collecting and weaving palm thatch panels, cash cropping and working for logging companies. Each of these involves extensive activity; furthermore, transport to town is often done by bicycle, walking and poling dugout canoes. Cash cropping of rice and corn also involves the clearing and weeding of larger fields. Except for cash cropping and palm thatch manufacture, most wage labor opportunities are currently restricted to men.
Modernization may also not have a substantial net effect on Tsimane PALs closer to town because Tsimane in these villages still fish and farm, sometimes hunt, play soccer, visit other villages by foot or canoe. Cash cropping is also more common close to town, because of the relative ease by which Tsimane can transport their goods to market. Interaction with the market over the past half century since roads were built, however, has been mostly sporadic, rather than intense and sustained, and has not yet led to any secular changes in height 
. Electricity has existed since 2010, but only in a couple of villages, and so television is scarce even in the most acculturated Tsimane villages.
Despite minimal effects of modernization on activity, we found significant effects of Spanish fluency (but not education) on adult BMI, particularly among women, even though overall adult obesity prevalence was low (<3%). Spanish speakers are more likely to earn wages, and therefore may have a more energy-dense diet 
; in our sample, fluent Spanish speakers were more likely to eat market-purchased foods than non-Spanish speakers (5.4% vs.3.3%, p<0.01, t
2192.2, t-test). These items include sugar, cooking oil, bread, beef jerky (charqui
) and pasta. A separate Tsimane study similarly found that a human capital index was associated with greater adult BMI and body fat percentage, but that the magnitudes of the relationships were small 
. These patterns in the Bolivian Amazon contrasts with those observed elsewhere, where in the span of only a few decades, intense market integration has led to changes in body size, activity, and risk factors for chronic disease 
Men's PAL is greater than women's PAL in 11/15 of the subsistence societies listed in , including the Tsimane. Among Tsimane, the sex difference in male and female physical activity levels narrow considerably by age 70. Activity reaches a peak by mid-teens for females and by late twenties in males. Even though older men participate in a wide range of subsistence tasks (), their overall physical activity level declines by about 10–20% from the peak (; ); older women's physical activity level remains constant throughout adulthood. Among those living near town, there was no evidence of age-related declines in daily activity among those age 60+ (Table S1
). One of the few other studies to examine changes in activity profiles over the lifespan in the Netherlands observed age-declines only after age 52 
. The rate of decline in PALs among Dutch adults was 0.060 per decade for men and 0.023 per decade for women (adapted from from 32). The equivalent estimate for Tsimane (0.074 per decade for men, 0.030 per decade for women) reveals a similar rate of decline. This suggests that while Tsimane may remain reasonably active for much of their adult lives, they decrease the intensity of work effort with age. Activity at late ages likely declines as a result of diminishing strength, endurance, and functional and health status with age. Such a pattern suggests that late age activity is not by itself responsible for the minimal obesity, cardiovascular disease and diabetes documented among Tsimane.
BMI and other measures of body size were unrelated to physical activity in this study. This result is relevant in light of the current controversy over the relationship between activity and obesity. One prominent explanation for the obesity “epidemic” in the U.S., and increasingly throughout the world, is a reduction in physical activity throughout adulthood. The prevalence of obesity in the U.S. increased from 5% to 22% over the period of 1980 to 2005 
. Since 1960, the mean daily energy expenditure due to work-related activity is estimated to have dropped by over 100 calories per day for both men and women, due to the changing nature of occupations, and work-related transportation 
. The increase in adult weight over the same time period matches this decrease in energy expenditure, suggesting that changes in activity patterns may help explain the obesity epidemic, at least in the U.S. However, leisure-related activity may have increased over the same time period, thereby compensating for the decline in work-related activity. Indeed, there is no evidence for a decline in PAL over the same 25-yr period where obesity prevalence in the U.S. quadrupled 
Other evidence suggests that overconsumption of food may be more to blame for the upsurge in obesity than diminished energy expenditure. First, several studies have shown that obese individuals expend a similar amount of energy as thinner individuals and that activity levels do not consistently predict weight gain 
; though possessing lower PALs, movement costs among obese individuals are higher due to greater BMR, and so their active energy expenditure can resemble that of more active, but leaner individuals. Cross-cultural data on TDEE and PAL (used in ) also shows no statistically significant difference by low or high development (based on Human Development Index) 
. Second, Hadza hunter-gatherers of Tanzania show similar total daily energy expenditure as Westerners despite displaying higher PALs, and their body fat percentage was unrelated to PAL or energetic expenditure 
. Body fat percentage has been shown to be unrelated to PAL and energy expenditure in Western populations as well 
. Third, total daily energy expenditure of many human populations, including “sedentary” Westerners and people in developing countries, falls along the same regression line determined by body mass and ambient temperature, as 90 species of wild terrestrial mammals who are not obese 
. Such results support the claim above that TDEE among Westerners may not be unique nor is it too low. Lastly, evidence tracks a temporal relationship between changes in food supply at the population level (as proxy for food intake after adjusting for food wastage) and weight gain patterns over the past several decades 
. Together these arguments lend support to the notion that food intake may be a stronger candidate than diminished activity for the recent upsurge in obesity. Our results do not support claims that obesity is associated with reduced activity. Active Tsimane with BMI>25 may instead be “fit and fat” 
The main limitation of this study is its reliance on time allocation data and accelerometry with heart rate monitor to measure AEE and PAL, instead of the more precise and accurate doubly labeled water (DLW) method. However, DLW is expensive, and measures expenditure over a limited time period of up to two weeks. Despite not using DLW, the combined accelerometry-HR method provides the best alternative approach to measuring activity 
. The factorial method is an old approach to estimating activity and expenditure. Our factorial method presented here, however, does not suffer from standard shortcomings; it was not based on interview or recall, but instead used a large number of scan observations among 900+ individuals, which permitted year-long sampling. The main drawback of our factorial method is that it does not permit continuous observation of individuals throughout the day, and it relies on published tables to assign PARs to activities.
An additional limitation is that modernization among the Tsimane is still relatively modest and has not resulted in great changes in lifestyle or subsistence patterns as the majority of Tsimane still maintain fields for horticulture, engage in hunting and fishing activities, and wage labor opportunities are largely restricted to physical work such as logging or ranching. Thus, the Tsimane may not represent an ideal test of the hypothesis that increasing modernization leads to reduced physical activity, higher obesity, and the associated health risks.