Accelerometer-measured Sedentary Time Research Articles

Association Between Self-Reported and Objective Activity Levels by Demographic Factors: Ecological Momentary Assessment Study in Children.

BackgroundTo address the limitations of the retrospective self-reports of activity, such as its susceptibility to recall bias, researchers have shifted toward collecting real-time activity data on mobile devices via ecological momentary assessment (EMA). Although EMA is becoming increasingly common, it is not known how EMA self-reports of physical activity and sedentary behaviors relate to the objective measures of activity or whether there are factors that may influence the strength of association between these two measures. Understanding the relationship between EMA and accelerometry can optimize future instrument selection in studies assessing activity and health outcomes.ObjectiveThe aim of this study was to examine the associations between EMA-reported sports or exercise using the accelerometer-measured moderate-to-vigorous physical activity (MVPA) and EMA-reported TV, videos, or video games with the accelerometer-measured sedentary time (ST) in children during matched 2-h windows and test potential moderators.MethodsChildren (N=192; mean age 9.6 years; 94/192, 49.0% male; 104/192, 54.2% Hispanic; and 73/192, 38.0% overweight or obese) wore an accelerometer and completed up to 7 EMA prompts per day for 8 days during nonschool time, reporting on past 2-h sports or exercise and TV, videos, or video games. Multilevel models were used to assess the relationship between the accelerometer-measured ST and EMA-reported TV, videos, or video games. Given the zero-inflated distribution of MVPA, 2-part models were used assess the relationship between the accelerometer-measured MVPA and EMA-reported sports or exercise.ResultsEMA-reported TV, videos, or video games were associated with a greater accelerometer-measured ST (beta=7.3, 95% CI 5.5 to 9.0, P<.001). This relationship was stronger in boys (beta=9.9, 95% CI 7.2 to 12.6, P<.001) than that in girls (beta=4.9, 95% CI 2.6 to 7.2, P≤.001). EMA-reported sports or exercise was associated with a greater accelerometer-measured MVPA (zero portion P<.001; positive portion P<.001). This relationship was stronger on weekends, in older children, and in non-Hispanic children (zero portion all P values<.001; positive portion all P values<.001).ConclusionsEMA reports highly relate to accelerometer measures. However, the differences in the strength of association depending on various demographic characteristics suggest that future research should use both EMA and accelerometers to measure activity to collect complementary activity data.

Youth Participation in Competitive Sports Associates with Midlife Lean Body Mass and Physical Activity

Benefits of life-long physical activity (PA) are widely recognized. Fewer studies have investigated how being young competing athlete reflects to later life body composition and PA. PURPOSE: To investigate the impact of participation in competitive sports in youth on the body composition and subjectively and objectively measured PA at middle- age. METHODS: The study participants are 47-to-55-year old Finnish women (n=985) attending to the Estrogenic Regulation of Muscle Apoptosis study (ERMA). Their participation in competitive sports at the age of 13-to-16 was assessed with retrospective self-report. Midlife lean body mass (LBM), skeletal muscle mass (SMM) and fat mass (FM) were measured with bioimpedance (Inbody 720) after overnight fast (n=866). Midlife PA included 7-scaled self-estimate that was reclassified to form sedentary, light, moderate and vigorous PA categories (n=985), a detailed assessment of leisure-time PA volume, duration and intensity (a modified Kuopio Ischemic Heart Disease Risk Factor Study Questionnaire, n=982) enabling calculation of mean daily metabolic equivalent (MET-h/day) index as a proxy of the previous 12 months PA and objectively measured total PA over seven-days by hip-worn accelerometer (Actigraph GT3X+ or wGT3X+, n=734). Statistical differences were tested using chi-squared test, independent samples t-test or Mann-Whitney U test. RESULTS: Participants were assigned into youth athlete (n=136) or non-athlete (n=849) groups based on their participation in competitive sports at the age of 13-to-16. Youth athlete group had 1.5 kg higher LBM (p=0.002) and 1.0 kg higher SMM (p=0.001) at middle-age, but no statistically significant difference in FM. Youth athletes reported more vigorous PA (32.4% vs. 20.3%, p=0.007) and higher leisure-time MET index (4.3 vs. 3.4 h/day, p<0.001) than non-athletes. No difference was observed in accelerometer-measured sedentary time, light PA, moderate-to-vigorous PA or total PA (counts) between groups. CONCLUSION: Participation in sport competitions as a teenager has long lasting health benefits that associates with healthier body composition and higher leisure-time PA later in life. However, the discrepancy between self-reports and objective measures needs further studies to delineate the benefits of different components of PA.

Comparison of accelerometer measured levels of physical activity and sedentary time between obese and non-obese children and adolescents: a systematic review

BackgroundObesity has been hypothesized to be associated with reduced moderate-to-vigorous physical activity (MVPA) and increased sedentary time (ST). It is important to assess whether, and the extent to which, levels of MVPA and ST are suboptimal among children and adolescents with obesity. The primary objective of this study was to examine accelerometer-measured time spent in MVPA and ST of children and adolescents with obesity, compared with MVPA recommendations, and with non-obese peers.MethodsAn extensive search was carried out in Medline, Cochrane library, EMBASE, SPORTDiscus, and CINAHL, from 2000 to 2015. Study selection and appraisal: studies with accelerometer-measured MVPA and/or ST (at least 3 days and 6 h/day) in free-living obese children and adolescents (0 to 19 years) were included. Study quality was assessed formally. Meta-analyses were planned for all outcomes but were precluded due to the high levels of heterogeneity across studies. Therefore, narrative syntheses were employed for all the outcomes.ResultsOut of 1503 records, 26 studies were eligible (n = 14,739 participants; n = 3523 with obesity); 6/26 studies involved children aged 0 to 9 years and 18/26 involved adolescents aged 10.1 to19 years. In the participants with obesity, the time spent in MVPA was consistently below the recommended 60 min/day and ST was generally high regardless of the participant’s age and gender. Comparison with controls suggested that the time spent in MVPA was significantly lower in children and adolescents with obesity, though differences were relatively small. Levels of MVPA in the obese and non-obese were consistently below recommendations. There were no marked differences in ST between obese and non-obese peers.ConclusionsMVPA in children and adolescents with obesity tends to be well below international recommendations. Substantial effort is likely to be required to achieve the recommended levels of MVPA among obese individuals in obesity treatment interventions.This systematic review has been registered on PROSPERO (International Database of Prospective Register Systematic Reviews; registration number CRD42015026882).

Accelerometer-measured sedentary time and physical activity—A 15 year follow-up of mortality in a Swedish population-based cohort

ObjectivesTo investigate the associations of objectively assessed sedentary time, light intensity physical activity (PA), moderate to vigorous intensity PA (MVPA), and total PA with all-cause mortality and mortality from cardiovascular disease (CVD) or cancer in a Swedish population-based cohort with 15 years follow-up time. DesignLongitudinal prospective cohort study. MethodsData from 851 persons (56% women) ≥35 years at baseline were included. Primary exposure variables were time (min/day) spent sedentary, in light intensity PA and in MVPA, and total counts from an Actigraph 7164 accelerometer. Data on all-cause mortality and mortality from CVD or cancer were obtained from Swedish registers. Cox proportional hazards models estimated hazard ratios (HR) of mortality with 95% confidence intervals (CI). ResultsCompared with the least sedentary participants, those in the most sedentary tertile had an increased risk of all-cause mortality, HR: 2.7 (1.4, 5.3), CVD mortality, HR: 5.5 (1.4, 21.2) and cancer mortality, HR: 4.3 (1.2, 16.0). For all-cause mortality, those in the highest light intensity PA tertile had a HR 0.34 (0.17, 0.67) compared with the lowest tertile. A similar pattern was found for CVD and cancer mortality. More time spent in MVPA was associated with the largest risk reduction for CVD mortality, with an almost 90% lower risk in the tertile with the most time in MVPA. ConclusionsThis study confirms a strong inverse relationship between MVPA and mortality, and adds new insight for the understanding of the associations between sedentary time and light intensity PA and mortality.

Accelerometer measured levels of moderate-to-vigorous intensity physical activity and sedentary time in children and adolescents with chronic disease: A systematic review and meta-analysis.

ContextModerate-to-vigorous physical activity (MVPA) and sedentary time (ST) are important for child and adolescent health.ObjectiveTo examine habitual levels of accelerometer measured MVPA and ST in children and adolescents with chronic disease, and how these levels compare with healthy peers.MethodsData sources: An extensive search was carried out in Medline, Cochrane library, EMBASE, SPORTDiscus and CINAHL from 2000–2017.Study selection: Studies with accelerometer-measured MVPA and/or ST (at least 3 days and 6 hours/day to provide estimates of habitual levels) in children 0–19 years of age with chronic diseases but without co-morbidities that would present major impediments to physical activity. In all cases patients were studied while well and clinically stable.ResultsOut of 1592 records, 25 studies were eligible, in four chronic disease categories: cardiovascular disease (7 studies), respiratory disease (7 studies), diabetes (8 studies), and malignancy (3 studies). Patient MVPA was generally below the recommended 60 min/day and ST generally high regardless of the disease condition. Comparison with healthy controls suggested no marked differences in MVPA between controls and patients with cardiovascular disease (1 study, n = 42) and type 1 diabetes (5 studies, n = 400; SMD -0.70, 95% CI -1.89 to 0.48, p = 0.25). In patients with respiratory disease, MVPA was lower in patients than controls (4 studies, n = 470; SMD -0.39, 95% CI -0.80, 0.02, p = 0.06). Meta-analysis indicated significantly lower MVPA in patients with malignancies than in the controls (2 studies, n = 90; SMD -2.2, 95% CI -4.08 to -0.26, p = 0.03). Time spent sedentary was significantly higher in patients in 4/10 studies compared with healthy control groups, significantly lower in 1 study, while 5 studies showed no significant group difference.ConclusionsMVPA in children/adolescents with chronic disease appear to be well below guideline recommendations, although comparable with activity levels of their healthy peers except for children with malignancies. Tailored and disease appropriate intervention strategies may be needed to increase MVPA and reduce ST in children and adolescents with chronic disease.

Associations of Accelerometer-Measured and Self-Reported Sedentary Time With Leukocyte Telomere Length in Older Women.

Few studies have assessed the association of sedentary time with leukocyte telomere length (LTL). In a cross-sectional study conducted in 2012-2013, we examined associations of accelerometer-measured and self-reported sedentary time with LTL in a sample of 1,481 older white and African-American women from the Women's Health Initiative and determined whether associations varied by level of moderate- to vigorous-intensity physical activity (MVPA). The association between sedentary time and LTL was evaluated using multiple linear regression models. Women were aged 79.2 (standard deviation, 6.7) years, on average. Self-reported sedentary time was not associated with LTL. In a model adjusting for demographic characteristics, lifestyle behaviors, and health-related factors, among women at or below the median level of accelerometer-measured MVPA, those in the highest quartile of accelerometer-measured sedentary time had significantly shorter LTL than those in the lowest quartile, with an average difference of 170 base pairs (95% confidence interval: 4, 340). Accelerometer-measured sedentary time was not associated with LTL in women above the median level of MVPA. Findings suggest that, on the basis of accelerometer measurements, higher sedentary time may be associated with shorter LTL among less physically active women.

Association of Maximum Temperature With Sedentary Time in Older British Men.

BackgroundSedentary behavior is very common in older adults and a risk factor for mortality. Understanding determinants of sedentary behavior may help in defining strategies aimed to reduce the time spent sedentary. The degree of difference in sedentary time attributable to varying temperatures has not been yet estimated in older men.MethodsMen aged 71 to 91 years participating in an established UK population-based cohort study were invited to wear an Actigraph GT3X accelerometer for 1 week in 2010–12. Outcome was sedentary time (<1.5 Metabolic Equivalent of Task) in minutes per day. Associations between daily outdoor maximum temperature and accelerometer-measured sedentary time were estimated using multilevel models.Results43% (1361/3137) of invited men participated in the study and provided adequate data. Men spent on average 615 minutes in sedentary time per day (72% of the total accelerometer-wear time). After adjusting for covariates, men spent 26 minutes more per day (P < .001) in sedentary time when temperatures were in the lowest (–3.5; 9.2°C) versus highest quintile (19.1; 29.5°C).ConclusionsSedentary time in older adults is highest at lowest temperatures, typically recorded in winter. Findings are relevant for guidelines: interventions may consider targeting older men in winter providing recommendations for minimizing sedentariness on daily basis.

Accelerometer-measured physical activity among adolescents in a multicultural area characterized by low socioeconomic status.

The aim of this cross-sectional study was to describe and analyze accelerometer-measured sedentary time and physical activity (PA) among adolescents in a multicultural area characterized by low socioeconomic status (SES). Seventh-graders (n=114 (girls n=66), mean age: 12.8±0.5 y) were recruited from three schools in a multicultural area of the city of Gothenburg, Sweden. Sedentary time and PA were measured with ActiGraph™ accelerometers. Of total wear-time, 70 (±6)% was sedentary, with girls being more sedentary than boys. Girls had less light PA (LPA) and moderate-to-vigorous PA (MVPA) than boys. Similar patterns were shown during in-school and out-of-school hours. During wear-time, 53% had a mean of ≥60 min of MVPA per day, but only 6% of the girls and 24% of the boys were sufficiently physically active every day. Girls had more sedentary bouts of ≥10 min and fewer MVPA bouts of ≥5 min per day than boys. Those who participated in organized sports spent a mean of 15 more minutes of MVPA per day compared to those who did not. No association was observed between body mass index (BMI) and sedentary time and PA. Only a few adolescents from a Swedish multicultural area characterized by low SES met the PA recommendations every day, and girls were more sedentary and less physically active than boys. Adolescents involved in organized sports had more of MVPA per day than their non-involved peers. Sedentary time and PA were not related to BMI.

Validation of parent-reported physical activity and sedentary time by accelerometry in young children.

BackgroundIt is unknown if young children’s parent-reported physical activity and sedentary time are correlated with direct measures. The study objectives were to compare parent-reported physical and sedentary activity versus directly measured accelerometer data in early childhood.MethodsFrom 2013 to 2014, 117 healthy children less than 6 years of age were recruited to wear Actical accelerometers for 7 days. Accelerometer data and questionnaires were available on 87 children (74 %). Average daily physical activity was defined as the sum of activity ≥100 counts per minute, and sedentary time as the sum of activity <100 counts per minute during waking hours. Parents reported daily physical activity (unstructured free play in and out of school, and organized activities) and selected sedentary behaviors (screen time, stroller time, time in motor vehicle). Spearman correlation coefficients and Bland–Altman plots were used to assess the validity of parent-reported measures compared to accelerometer data.ResultsTotal physical activity was significantly greater when measured by accelerometer than parent-report; the median difference was 131 min/day (p < 0.001). Parent-reported child physical activity was weak to moderately correlated with directly measured total physical activity (r = 0.39, 95 % CI 0.19, 0.56). The correlations between types of physical activity (unstructured free play in and outside of school/daycare, and organized structured activity) and accelerometer were r = 0.30 (95 % CI 0.09, 0.49); r = 0.42 (95 % CI 0.23, 0.58); r = 0.26 (95 % CI 0.05, 0.46), respectively. There was no correlation between parent-reported and accelerometer-measured total sedentary time in children (r = 0.10, 95 % CI −0.12, 0.33). When the results were stratified by age group (<18, 18–47, and 48–70 months of age) no statistically significant correlations were observed and some inverse associations were observed.ConclusionsThe correlation between parent-report of young children’s physical activity and accelerometer-measured activity was weak to moderate depending on type of activity and age group. Parent-report of children’s sedentary time was not correlated with accelerometer-measured sedentary time. Additional validation studies are needed to determine if parent-reported measures of physical activity and sedentary time are valid among children less than 6 years of age and across these young age groups.Electronic supplementary materialThe online version of this article (doi:10.1186/s13104-015-1648-0) contains supplementary material, which is available to authorized users.

Self-reported sitting time in New York City adults, the Physical Activity and Transit Survey, 2010-2011.

IntroductionRecent studies have demonstrated the negative health consequences associated with extended sitting time, including metabolic disturbances and decreased life expectancy. The objectives of this study were to characterize sitting time in an urban adult population and assess the validity of a 2-question method of self-reported sitting time.MethodsThe New York City Health Department conducted the 2010–2011 Physical Activity and Transit Survey (N = 3,597); a subset of participants wore accelerometers for 1 week (n = 667). Self-reported sitting time was assessed from 2 questions on time spent sitting (daytime and evening hours). Sedentary time was defined as accelerometer minutes with less than 100 counts on valid days. Descriptive statistics were used to estimate the prevalence of sitting time by demographic characteristics. Validity of sitting time with accelerometer-measured sedentary time was assessed using Spearman’s correlation and Bland-Altman techniques. All data were weighted to be representative of the New York City adult population based on the 2006–2008 American Community Survey.ResultsMean daily self-reported sitting time was 423 minutes; mean accelerometer-measured sedentary time was 490 minutes per day (r = 0.32, P < .001). The mean difference was 49 minutes per day (limits of agreement: −441 to 343). Sitting time was higher in respondents at lower poverty and higher education levels and lower in Hispanics and people who were foreign-born.ConclusionParticipants of higher socioeconomic status, who are not typically the focus of health disparities–related research, had the highest sitting times; Hispanics had the lowest levels. Sitting time may be accurately assessed by self-report with the 2-question method for population surveillance but may be limited in accurately characterizing individual-level behavior.

Accelerometer-measured sedentary time among Hispanic adults: Results from the Hispanic Community Health Study/Study of Latinos (HCHS/SOL).

Excessive sedentary behavior is associated with negative health outcomes independent of physical activity. Objective estimates of time spent in sedentary behaviors are lacking among adults from diverse Hispanic/Latino backgrounds. The objective of this study was to describe accelerometer-assessed sedentary time in a large, representative sample of Hispanic/Latino adults living in the United States, and compare sedentary estimates by Hispanic/Latino background, sociodemographic characteristics and weight categories. This study utilized baseline data from the Hispanic Community Health Study/Study of Latinos (HCHS/SOL) that included adults aged 18–74 years from four metropolitan areas (N = 16,415). Measured with the Actical accelerometer over 6 days, 76.9% (n = 12,631) of participants had > 10 h/day and > 3 days of data. Participants spent 11.9 h/day (SD 3.0), or 74% of their monitored time in sedentary behaviors. Adjusting for differences in wear time, adults of Mexican background were the least (11.6 h/day), whereas adults of Dominican background were the most (12.3 h/day), sedentary. Women were more sedentary than men, and older adults were more sedentary than younger adults. Household income was positively associated, whereas employment was negatively associated, with sedentary time. There were no differences in sedentary time by weight categories, marital status, or proxies of acculturation. To reduce sedentariness among these populations, future research should examine how the accumulation of various sedentary behaviors differs by background and region, and which sedentary behaviors are amenable to intervention.

Accelerometer-measured sedentary time among Hispanic adults: Results from the Hispanic Community Health Study/Study of Latinos (HCHS/SOL)

Accelerometer-measured sedentary time among Hispanic adults: Results from the Hispanic Community Health Study/Study of Latinos (HCHS/SOL)

Patterns of Weekday and Weekend Sedentary Behavior Among Older Adults.

The purpose of this study was to compare estimates of sedentary time on weekdays vs. weekend days in older adults and determine if these patterns vary by measurement method. Older adults (N = 230, M = 83.5, SD = 6.5 years) living in retirement communities completed a questionnaire about sedentary behavior and wore an ActiGraph accelerometer for seven days. Participants engaged in 9.4 (SD = 1.5) hr per day of accelerometer-measured sedentary time, but self-reported engaging in 11.4 (SD = 4.9) hr per day. Men and older participants had more accelerometer-measured sedentary time than their counterparts. The difference between accelerometer-measured weekday and weekend sedentary time was nonsignificant. However, participants self-reported 1.1 hr per day more sedentary time on weekdays compared with weekend days. Findings suggest self-reported but not accelerometer-measured sedentary time should be investigated separately for weekdays and weekend days, and that self-reports may overestimate sedentary time in older adults.

Patterns of Sedentary Behavior and Weight Status Among Children

To make robust conclusions regarding the association between accelerometer-measured sedentary time and overweight and obesity among children, several gaps in the literature must be addressed. The purpose of this study was to examine associations between sedentary time, weekday sedentary time, weekend sedentary time, sedentary bouts, sedentary breaks, and BMI z-score among children and by low (bottom 50%) and high (top 50%) moderate- to vigorous-intensity physical activity (MVPA) participation. Results are based on 787 children aged 11 years living in Toronto, Canada. Children's physical activity and sedentary time were objectively assessed using ActiGraph accelerometers in 2010/11. Height and weight were measured and BMI z-scores were calculated based on the World Health Organization growth standards. When participants were stratified into low and high MVPA groups, sedentary bouts of 5-9 (β = 0.22 [95% CI: 0.01, 0.43]) and 10-19 (0.30 [-0.05, 0.55]) minutes for total days were associated with BMI z-score in the low MVPA group only. Similar trends were observed with the weekday but not the weekend variables. Therefore, in addition to increasing MVPA, reducing time spent in 5- to 19-min sedentary bouts may have important implications for weight status particularly for children with lower MVPA participation during the week.

The association between accelerometer-measured patterns of sedentary time and health risk in children and youth: results from the Canadian Health Measures Survey.

BackgroundSelf-reported screen time is associated with elevated health risk in children and youth; however, research examining the relationship between accelerometer-measured sedentary time and health risk has reported mixed findings. The purpose of this study was to examine the association between accelerometer-measured patterns of sedentary time and health risk in children and youth.MethodsThe results are based on 1,608 children and youth aged 6 to 19 years from the Canadian Health Measures Survey (2007–2009). Sedentary time was measured using the Actical accelerometer. Breaks in sedentary time and prolonged bouts of sedentary time lasting 20 to 120 minutes were derived for all days, weekend days and during the after-school period (i.e., after 3 pm on weekdays). Regression analyses were used to examine the association between patterns of sedentary time and body mass index (BMI), waist circumference, blood pressure and non-HDL cholesterol.ResultsBoys accumulated more sedentary time on weekdays after 3 pm and had a higher number of breaks in sedentary time compared to girls. Overweight/obese boys (aged 6–19 years) accumulated more sedentary time after 3 pm on weekdays (282 vs. 259 min, p < .05) and as prolonged bouts lasting at least 80 minutes (171 vs. 133 min, p < .05) compared to boys who were neither overweight nor obese. Prolonged bouts of sedentary time lasting at least 80 minutes accumulated after 3 pm on weekdays were positively associated with BMI and waist circumference in boys aged 11–14 years (p < .006). Each additional 60 min of sedentary time after 3 pm on weekdays was associated with a 1.4 kg·m-2 higher BMI and a 3.4 cm higher waist circumference in 11–14 year old boys. No sedentary pattern variables differed between girls who were not overweight or obese and those who were overweight/obese and none of the sedentary pattern variables were associated with any health markers in girls.ConclusionsThe findings confirm results of other studies that reported accelerometer-measured sedentary time was not associated with health risk in children and youth. Even when the pattern and timing of sedentary time was examined relative to health markers, few associations emerged and were limited to boys aged 11–14 years.

Television Viewing Time As A Marker Of Overall Sedentary Behavior: National Health And Nutrition Examination Survey

Television (TV) viewing time has previously been shown to be a potential marker of a broader pattern of leisure-time sedentary behavior in women, but not in men. However, the evidence on this relationship has been based on self-report measures of time spent in other sedentary behaviors, and requires confirmation using objective data. PURPOSE: To examine whether TV viewing time is a marker for accelerometer-measured total sedentary time for women and for men. Method: Data from the NHANES 2003/2004 survey (age range = 18-85 year; mean = 48 years) were used; the sample was restricted to those who wore the accelerometer for five or more days with 10 hours or more of wearing time each day. Self-reported TV time was compared with accelerometer-measured sedentary time (<100 counts per minute; expressed as a percentage of monitored time), separately for women (n=1,472) and for men (n=1,454). Association between the two variables was tested using regression models and agreement was assessed using weighted kappa. RESULTS: Some 29% of participants reported watching 2hrs TV/day; mean accelerometer-measured sedentary time was 8.1hrs/day out of an average 14.3hrs/day of monitored time. Regression models showed that reported TV time was positively associated with accelerometer-measured total sedentary time for both women (p<0.001) and men (p=0.002). Participants in the highest, compared with lowest, category of TV time (≥4hrs/day vs <1 hr/day) spent a greater percentage of total accelerometer-derived hours in sedentary time (61.6% versus 53.7% for women; 59.2% versus 54.4% for men). There was only fair agreement for women (k=0.21) and slight agreement for men (k=0.15) between participants' relative rankings in the distributions of self-report TV time and objective sedentary time (quintiles). CONCLUSIONS: Self-reported TV time is associated with objectively measured total sedentary time for women, and to a slightly lesser extent for men. However, the low agreement between the two measures suggests caution in using self-reported TV time as a proxy for total sedentary time.