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International Journal of Obesity Supplements logoLink to International Journal of Obesity Supplements
. 2015 Dec 8;5(Suppl 2):S100–S106. doi: 10.1038/ijosup.2015.26

Active school transport and weekday physical activity in 9–11-year-old children from 12 countries

K D Denstel 1, S T Broyles 1, R Larouche 2, O L Sarmiento 3, T V Barreira 1,4, J-P Chaput 2, T S Church 1, M Fogelholm 5, G Hu 1, R Kuriyan 6, A Kurpad 6, E V Lambert 7, C Maher 8, J Maia 9, V Matsudo 10, T Olds 8, V Onywera 11, M Standage 12, M S Tremblay 2, C Tudor-Locke 1,13, P Zhao 14, P T Katzmarzyk 1,*; for the ISCOLE Research Group
PMCID: PMC4850627  PMID: 27152177

Abstract

OBJECTIVES:

Active school transport (AST) may increase the time that children spend in physical activity (PA). This study examined relationships between AST and weekday moderate-to-vigorous physical activity (MVPA), light physical activity (LPA), sedentary time (SED) and total activity during naturally organized time periods (daily, before school, during school and after school) in a sample of children from 12 countries.

METHODS:

The sample included 6224 children aged 9–11 years. PA and sedentary time were objectively measured using Actigraph accelerometers. AST was self-reported by participants. Multilevel generalized linear and logistic regression statistical models were used to determine associations between PA, SED and AST across and within study sites.

RESULTS:

After adjustment for age, highest parental educational attainment, BMI z-score and accelerometer wear time, children who engaged in AST accumulated significantly more weekday MVPA during all studied time periods and significantly less time in LPA before school compared with children who used motorized transport to school. AST was unrelated to time spent in sedentary behaviors. Across all study sites, AST was associated with 6.0 min (95% confidence interval (CI): 4.7–7.3; P<0.0001) more of weekday MVPA; however, there was some evidence that this differed across study sites (P for interaction=0.06). Significant positive associations were identified within 7 of 12 study sites, with differences ranging from 4.6 min (95% CI: 0.3–8.9; P=0.04, in Canada) to 10.2 min (95% CI: 5.9–14.4; P<0.0001, in Brazil) more of daily MVPA among children who engaged in AST compared with motorized transport.

CONCLUSIONS:

The present study demonstrated that AST was associated with children spending more time engaged in MVPA throughout the day and less time in LPA before school. AST represents a good behavioral target to increase levels of PA in children.

Introduction

Physical activity (PA) guidelines recommend that children engage in at least 60 min of moderate-to-vigorous PA (MVPA) each day to lower their risk of negative health consequences; yet globally, many children do not meet these guidelines.1, 2, 3 The journeys to and from school provide an opportunity to establish habitual PA patterns.4 Previous research suggests that children who engage in active school transport (AST), such as walking or biking, accumulate more PA and experience better cardiorespiratory, muscular and metabolic fitness and a lower likelihood of diabetes and obesity than those who travel using motorized transport alternatives including by car or bus.5, 6, 7, 8

Despite the documented benefits of AST, the prevalence has declined in many countries over recent decades. In the United States, the proportion of children who walked or bicycled to school declined from 48% to 13% between 1969 and 2009.9 Similar declines have been documented in the United Kingdom,10 Australia,11 Brazil,12 Canada,13 Kenya14 and Switzerland.15 The most common parent-reported barriers to AST include distance from home to school, traffic concerns and crime.16, 17

While a positive association between AST and MVPA among children has been shown,4, 13 the majority of evidence has been limited to developed countries. In addition, few studies have examined associations between AST and light-intensity PA (LPA) or sedentary time (SED). Further, little is known about the contribution of AST to PA (MVPA or other intensities) or SED during different naturally organized time periods throughout the segmented school day. It is important to more fully understand how AST contributes to PA of various intensities and SED time globally in order to plan and implement effective interventions, programs and policies in a variety of settings.

Given these research gaps, the aims of this study were to examine: (i) relationships between AST and objectively measured weekday MVPA, LPA, total activity and SED; (ii) how these relationships differ across different time periods of the school day; and (iii) associations between children's AST and accumulating an average of at least 60 min per weekday (Monday through Friday) of MVPA in a sample of 9–11-year-old children from 12 countries ranging in level of socioeconomic and human development.

Materials and methods

Study design and participants

The International Study of Childhood Obesity, Lifestyle and the Environment (ISCOLE) is a cross-sectional, multinational study conducted in sites in 12 countries including Australia, Brazil, Canada, China, Colombia, Finland, India, Kenya, Portugal, South Africa, the United Kingdom and the United States. The ISCOLE protocol was approved by the ethics committee at the Pennington Biomedical Research Center and each participating institution. Written informed consent was obtained from parents or legal guardians and child assent, if required by the site's local ethics committee, was also obtained before children participated in the study. Data were collected from September 2011 through December 2013. Detailed information on the study design and methods can be found elsewhere.18

A total of 7372 children 9–11 years of age from 256 schools participated in ISCOLE. The present analytical sample includes 6224 children after excluding participants missing AST (n=62), valid accelerometry (n=798), parental education (n=283) and body mass index (BMI) z-score (n=5). Participants who were excluded in this study did not differ from those included with respect to the proportion in AST, but were more likely to be older, male, have a higher BMI and have parents who achieved less than a college education. The number of participants per site with complete data for each of the variables of interest for this study ranged from 400 in South Africa to 856 in Colombia. The primary sampling frame was schools which were stratified by an indicator of socioeconomic status. Classrooms within the recruited schools were the secondary sampling frame chosen to yield a sample of children with a mean age of 10 years.

Measurements

Accelerometry

The full accelerometer protocol has been previously reported.19 Briefly, time spent in MVPA, LPA, SED and total activity counts were obtained from 24-h accelerometry. Children were encouraged to wear an Actigraph GT3X+ accelerometer (ActiGraph LLC, Pensacola, FL, USA) on a belt around the waist at the right mid-axillary line 24 h per day for at least 7 consecutive days. A valid accelerometer record required ⩾4 days with ⩾10 h of waking wear time per day, including at least one weekend day. Sleep and non-wear time were identified using an automated algorithm prior to identification of other activities.20, 21 A waking non-wear period began with 20 consecutive minutes of 0 activity counts.20 Activity cut points were based on Evenson.22 Total activity was expressed as the number of activity counts per day. MVPA was defined as all activity greater than or equal to 574 activity counts per 15 s, LPA was defined as activity from 26 to 573 counts per 15 s and SED was defined as all activity ⩽25 counts per 15 s22 not including the sleep period and non-wear time. Minutes spent in MVPA, LPA, SED and total activity counts were assigned to the before school, during school and after school time periods using school day schedules provided by each participating school. The before school time period was considered wake time (established using a validated algorithm20) until school start time, during school was defined as the time between school start and end time and the after school period was considered school end time through the child's bed time (also determined by our validated algorithm20) determined objectively from accelerometry. Only weekday accelerometer data were used in these analyses to more accurately capture the effect of AST on PA.

In order to determine whether or not children met PA guidelines, the number of valid weekdays contained in the accelerometer file was determined and the mean number of MVPA minutes per valid weekday that children accumulated was calculated. Children were then classified as meeting PA guidelines (yes or no) if they obtained a mean of ⩾60 min of MVPA per weekday over the measurement period.

Active school transport

AST was self-reported by participants using a diet and lifestyle questionnaire18 with the question ‘In the last week you were in school, the main part of your journey to school was by'. Response options included ‘walking', ‘bicycle, roller-blade, skateboard or non-motorized scooter', ‘bus, train, tram, underground or boat', ‘car, motorcycle or moped' and ‘other'. A participant was considered to have engaged in AST if they reported walking, running, bicycle, roller-blades, skateboard or non-motorized scooter. Motorized transportation response options included bus, train, tram, underground, boat, car, motorcycle or moped. These options were collapsed into a binomial variable indicating AST or motorized transport to school.

Covariates

Standing height and body weight were measured using standard procedures across study sites.18 Detailed measurement procedures have been published elsewhere.18 BMI (weight (kg)/height (m)2) was derived from the average standing height and weight and BMI z-score was computed using age- and sex-specific reference values from the World Health Organization (WHO).23 A demographic questionnaire completed by parents captured the participant's date of birth, sex and parents' educational attainment. Age was computed from the date of birth and date anthropometric measurements were performed. Highest parental educational attainment was created based on the highest education level completed by either parent (did not complete high school, completed high school or some college, completed bachelor's degree or postgraduate degree).

Statistical analysis

Means and standard deviations were computed for variables by study site and sex for participants with complete data for accelerometry, AST and covariates. AST was the independent variable in all analyses. Separate multilevel generalized linear mixed models (SAS version 9.4 (SAS Institute, Cary, NC, USA), PROC MIXED) were used to examine the associations between mean daily, before school, during school and after school MVPA, LPA, SED, total activity counts and AST (0=no, 1=yes). All linear models were stratified by sex and adjusted for mean waking accelerometer wear time, highest parental educational attainment, average BMI z-score and a site by AST interaction. Study site and school nested within study site were treated as fixed effects to account for the clustering of the data (weekday MVPA intra-class correlation; ICC=0.14 at site; ICC=0.26 at school nested within site). Results are presented as adjusted means using the LSMEANS option. The PDIFF option was used to obtain the difference in the number of weekday MVPA minutes obtained by children who used AST versus motorized transport alternatives within each site. Multilevel logistic mixed models (PROC GLIMMIX) with the SLICEDIFFS and ODDSRATIO options specified were used to obtain the within-site odds of children meeting PA guidelines (0=no, 1=yes) for those who engaged in AST compared with those who used motorized transport. Logistic models were adjusted for mean waking accelerometer wear time, highest parental educational attainment, average BMI z-score and a site by AST interaction. All data management and statistical analyses were conducted using SAS version 9.4. The significance level was set at P<0.05.

Results

The descriptive characteristics of the study sample, stratified by study site and sex, are provided in Table 1. The mean±s.d. age of the full sample was 10.4±0.6 years and 54.4% of the participants were girls. The prevalence of AST was 43.1% among boys and 41.8% among girls and this ranged across sites from 7.2% to 76.2% among boys and 3.7% to 81.0% among girls. Approximately 37% of children traveled to school by walking, 5% by bicycle, roller-blade, skateboard, or non-motorized scooter, 22% by bus, train, tram, underground, or boat, 35% by car, motorcycle, or moped and 1% traveled by some other mode. More detail regarding mode of transportation prevalences by site can be found elsewhere.24 Across sites, the mean time spent in daily MVPA ranged from 50.9±17.6 min per day to 85.8±29.2 min per day among boys and 38.9±14.8 min per day to 66.3±21.6 min per day among girls.

Table 1. Descriptive characteristics of ISCOLE participants stratified by sex and study site (n=6224).

Country (site)     Mean (s.d.)
  n Active transport Yes (%) Age (years) BMI (kg m−2) BMI z-score MVPAa (min per day) LPAa (min per day) SEDa (min per day) Total activitya (counts per day)
Boys
 Australia (Adelaide) 220 33.0 10.8 (0.4) 18.6 (2.9) 0.6 (1.1) 79.9 (24.3) 317.6 (48.6) 465.8 (63.9) 564 028.9 (132 446.3)
 Brazil (São Paulo) 217 39.2 10.5 (0.5) 19.9 (4.8) 1.0 (1.5) 74.1 (28.0) 347.4 (53.9) 495.4 (78.5) 536 820.8 (147 588.2)
 Canada (Ottawa) 213 44.1 10.5 (0.4) 18.5 (3.4) 0.6 (1.2) 72.2 (20.8) 318.7 (43.6) 500.8 (66.7) 513 246.8 (105 426.6)
 China (Tianjin) 258 31.4 9.9 (0.5) 19.8 (4.5) 1.1 (1.6) 50.9 (17.6) 307.5 (56.9) 569.1 (72.1) 401 917.5 (98 427.1)
 Colombia (Bogotá) 422 69.4 10.5 (0.6) 17.8 (2.6) 0.3 (1.1) 79.3 (27.4) 344.4 (51.1) 502.5 (71.1) 549 247.5 (138 015.7)
 Finland (Helsinki, Espoo and Vantaa) 218 76.2 10.5 (0.4) 17.6 (2.5) 0.3 (1.1) 85.8 (29.2) 303.0 (44.1) 528.1 (73.0) 578 639.7 (156 260.4)
 India (Bangalore) 249 7.2 10.4 (0.5) 17.6 (3.3) 0.1 (1.5) 62.0 (20.0) 356.1 (51.5) 497.0 (64.1) 489 561.4 (104 436.9)
 Kenya (Nairobi) 230 48.3 10.2 (0.7) 17.1 (2.8) 0.02 (1.2) 78.9 (31.1) 328.6 (56.6) 507.0 (78.6) 546 932.8 (147 917.5)
 Portugal (Porto) 269 29.7 10.4 (0.2) 19.3 (3.3) 1.0 (1.2) 73.6 (24.6) 310.6 (50.8) 539.2 (69.3) 507 698.7 (122 343.3)
 South Africa (Cape Town) 159 52.2 10.3 (0.7) 17.8 (3.3) 0.3 (1.3) 73.6 (25.9) 320.9 (54.0) 495.6 (72.4) 524 922.9 (131 996.5)
 UK (Bath and NE Somerset) 185 66.0 10.9 (0.4) 18.1 (2.6) 0.4 (1.0) 76.7 (24.8) 289.7 (46.5) 496.1 (63.0) 526 362.0 (131 745.9)
 USA (Baton Rouge) 195 8.7 10.0 (0.6) 18.5 (3.3) 0.7 (1.3) 54.1 (18.0) 318.0 (54.6) 533.9 (65.4) 439 918.9 (107 853.4)
 All sites 2835 43.1 10.4 (0.6) 18.4 (3.4) 0.5 (1.3) 72.0 (26.8) 324.0 (54.5) 511.8 (74.7) 515 841.0 (137 393.5)
Girls
 Australia (Adelaide) 258 27.9 10.7 (0.4) 19.0 (3.5) 0.6 (1.1) 59.9 (19.8) 308.4 (49.1) 489.5 (59.2) 473 728.8 (119 392.7)
 Brazil (São Paulo) 228 41.2 10.4 (0.5) 19.7 (4.3) 0.8 (1.3) 49.6 (19.4) 336.3 (55.8) 519.2 (67.5) 418 194.0 (108 757.1)
 Canada (Ottawa) 304 29.9 10.5 (0.4) 18.2 (3.3) 0.3 (1.2) 55.1 (18.3) 308.1 (47.2) 516.3 (63.5) 439 736.4 (100 951.3)
 China (Tianjin) 238 34.0 9.8 (0.4) 18.0 (3.6) 0.3 (1.3) 41.6 (15.1) 281.9 (57.1) 598.8 (72.2) 345 893.1 (89 348.4)
 Colombia (Bogotá) 434 75.6 10.4 (0.6) 17.4 (2.4) 0.1 (1.0) 62.3 (21.5) 333.7 (53.0) 526.4 (70.0) 470 448.4 (111 298.7)
 Finland (Helsinki, Espoo and Vantaa) 252 81.0 10.5 (0.4) 17.9 (2.7) 0.2 (1.0) 66.3 (21.6) 295.9 (46.7) 541.4 (65.3) 486 068.3 (131 095.2)
 India (Bangalore) 297 3.7 10.4 (0.5) 18.2 (3.3) 0.3 (1.3) 38.9 (14.8) 336.1 (53.5) 556.3 (67.0) 370 217.0 (89 399.1)
 Kenya (Nairobi) 266 41.7 10.2 (0.7) 17.3 (3.3) -0.02 (1.2) 63.0 (30.8) 329.2 (61.5) 521.1 (72.5) 481 052.9 (151 122.6)
 Portugal (Porto) 349 27.5 10.4 (0.3) 19.4 (3.4) 0.8 (1.1) 50.5 (16.6) 301.3 (52.8) 571.3 (60.1) 399 559.8 (89 138.0)
 South Africa (Cape Town) 241 62.7 10.2 (0.7) 18.3 (3.8) 0.4 (1.3) 57.1 (22.6) 319.2 (56.9) 515.5 (64.5) 457 427.3 (123 594.2)
 UK (Bath and NE Somerset) 243 63.8 10.9 (0.5) 18.7 (3.2) 0.4 (1.1) 59.2 (18.7) 288.3 (47.6) 506.9 (57.3) 444 690.1 (100 906.2)
 USA (Baton Rouge) 279 7.9 9.8 (0.5) 19.0 (4.0) 0.7 (1.3) 42.8 (15.4) 306.9 (53.8) 550.5 (64.4) 385 268.2 (100 745.5)
 All sites 3389 41.8 10.4 (0.6) 18.4 (3.5) 0.4 (1.2) 54.1 (21.7) 313.2 (55.8) 535.0 (71.3) 431 804.9 (118 704.3)
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Abbreviations: BMI, body mass index; MVPA, moderate-to-vigorous physical activity; LPA, light physical activity; SED, sedentary time.

a

Weekday (Monday through Friday) average. Results presented as mean (s.d.) unless otherwise specified.

The adjusted mean minutes of weekday MVPA, LPA, SED and total activity counts by mode of transport to school are presented in Table 2. After adjustment for age, highest parental educational attainment, BMI z-score, accelerometer wear time and an AST by site interaction, AST was associated with obtaining more minutes of MVPA and a higher number of total activity counts for boys at all time periods (daily, before, during and after school) compared with motorized transport alternatives. Results were similar among girls, but the associations apparent during the school time period were not significant. Girls who engaged in AST accumulated less LPA time daily and also before, during and after school compared with those who did not. Among boys, daily and before school LPA was significantly lower among those who reported AST compared with those who did not. There were no significant differences in the amounts of SED time.

Table 2. Adjusted meana minutes of weekday MVPA, LPA, SED and total activity counts by active transportation status and gender among 6224 9–11-year-old children.

  Boys Girls
Active transportation Yes No Yes No
MVPA, min per day 77.1 (75.1–79.1) 70.4 (68.8–72.0)*** 58.1 (56.7–59.6) 53.2 (52.0–54.3)***
 Before school 8.8 (8.3–9.2) 6.1 (5.7–6.4)*** 7.1 (6.8–7.4) 5.0 (4.7–5.2)***
 During school 32.3 (31.4–33.2) 30.6 (29.9–31.4)* 22.0 (21.3–22.6) 21.3 (20.8–21.8)
 After school 36.2 (34.9–37.6) 33.8 (32.7–34.9)* 29.0 (28.0–30.0) 26.8 (26.1–27.6)**
         
LPA, min per day 320.7 (316.6–324.9) 326.3 (323.0–329.7)* 310.7 (306.6–314.8) 319.0 (316.0–322.0)**
 Before school 35.9 (35.1–36.7) 38.5 (37.9–39.2)*** 36.1 (35.3–36.8) 38.8 (38.2–39.4)***
 During school 138.6 (136.3–140.9) 140.2 (138.4–142.1) 125.4 (123.1–127.6) 128.5 (126.8–130.1)*
 After school 147.9 (146.1–149.7) 147.5 (145.6–149.3) 148.9 (146.7–151.2) 151.7 (150.1–153.4)*
         
SED, min per day 509.8 (504.7–515.0) 510.9 (506.8–515.1) 533.5 (528.7–538.3) 530.2 (526.7–533.7)
 Before school 54.7 (53.7–55.7) 54.8 (54.0–55.6) 56.6 (55.7–57.5) 56.0 (55.3–56.6)
 During school 223.7 (221.0–226.5) 223.8 (221.6–226.0) 242.1 (239.5–244.6) 239.6 (237.8–241.5)
 After school 230.7 (227.7–233.7) 231.9 (229.5–234.3) 235.3 (232.5–238.1) 234.7 (232.6–236.8)
         
Total activity, counts per day 536 816 (526 375–547 258) 513 420 (504 991–521 849)** 449 700 (441 270–458 131) 433 500 (427 275–439 725)*
 Before school 58 551 (56 519–60 583) 49 463 (47 815–51 110)*** 51 308 (49 748–52 868) 44 619 (43 465–45 773)***
 During school 226 377 (221 540–231 214) 220 347 (216 433–224 261)* 173 113 (169 291–176 934) 171 169 (168 340–173 997)
 After school 253 150 (246 040–260 260) 244 356 (238 618–250 094)* 224 823 (218 999–230 647) 217 264 (212 953–221 576)*
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Abbreviations: BMI, body mass index; MVPA, moderate-to-vigorous physical activity; LPA, light physical activity; SED, sedentary time data shown as adjusted mean (95% confidence interval).

***P<0.0001, **P<0.001 *P<0.05.

a

Multilevel models with site and school nested within site treated as fixed effects; adjusted for age, BMI z-score, highest parental education level, mean accelerometer wear time and site by active transportation interaction.

The differences in mean minutes of daily MVPA between children who engaged in AST compared with motorized transport to school adjusted for age, highest parental educational attainment, BMI z-score, accelerometer wear time and an AST by site interaction (P for interaction=0.06) are presented in Figure 1. Across all study sites, AST was associated with 6.0 min (95% confidence interval (CI): 4.7–7.3; P<0.0001) more of daily MVPA. The association was significant within 7 of 12 ISCOLE study sites, including Brazil, Canada, Finland, Kenya, Portugal, South Africa and the United Kingdom. Among those with a significant association, the effect size of AST on daily MVPA ranged from 4.6 min (95% CI: 0.3–8.9; P=0.04) in Canada to 10.2 min (95% CI: 5.9–14.4; P<0.0001) in Brazil.

Figure 1.

Figure 1

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The difference in mean school day MVPA among children who engage in AST compared with motorized transport to school in n=6224 9–11-year-old children. Means are adjusted for sex, BMI z-score, age, highest parental educational attainment, accelerometer waking wear time, site by AST interaction and school nested within site was treated as a fixed effect in the multilevel analysis. Error bars represent 95% confidence intervals.

The site-specific odds ratios for the association of AST and obtaining a weekday average of 60 or more minutes of MVPA adjusted for age, highest parental educational attainment, BMI z-score, accelerometer wear time and an AST by site interaction are shown in Figure 2. AST was associated with significantly higher odds of obtaining the recommended amount of MVPA across all sites (odds ratio (OR): 1.8; 95% CI: 1.5–2.1) and within 7 of 12 study sites, including Brazil (OR: 2.3; 95% CI: 1.4–3.8), Canada (OR:1.7; 95% CI: 1.0–2.8), Finland (OR: 2.1; 95% CI: 1.2–3.8), Kenya (OR: 1.9; 95% CI: 1.1–3.4), Portugal (OR: 2.5; 95% CI: 1.6–3.8), South Africa (OR: 2.8; 95% CI: 1.4–5.3) and the United Kingdom (OR: 2.4; 95% CI: 1.4–3.9).

Figure 2.

Figure 2

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Associations between AST and obtaining at least 60 min of MVPA per school day in n=6224 9–11-year-old children from 12 countries. Odds ratios are adjusted for sex, BMI z-score, age, highest parental educational attainment, accelerometer waking wear time, site by AST interaction and school nested within site was treated as a fixed effect in the multilevel analysis. Error bars represent 95% confidence intervals.

Discussion

The present study showed that AST was associated with children spending more time engaged in MVPA and less time in LPA throughout the weekday. Overall, children who undertook AST were 1.8 times more likely to meet daily PA guidelines, though this varied across different study sites.

Children who practiced AST recorded almost 10% more daily time in MVPA compared with those who did not. A multicenter study of adolescents from 10 European cities reported similar findings in that time spent walking and biking for travel was positively associated with objectively measured MVPA compared with those who reported less time walking and biking for travel.25 However, that study did not differentiate active travel to/from school from trips to other locations. Further, a recent review of 49 studies found that 82% of included studies showed a positive association between AST and daily PA levels (for example, MVPA, energy expenditure expressed as kilocalorie per day, steps, total PA and so on) as determined by accelerometry, pedometry or self-report.8 In addition, Lee and Li26 showed that children who practiced AST accumulated 6.9 min of MVPA during the trip contributing ~11% to daily time in MVPA. This agrees with our findings that children who engaged in AST obtained 6 additional minutes per day in MVPA. One study we identified examined AST and associations with MVPA at various time periods throughout the day among 10-year-old children in England and found that objectively measured MVPA was significantly higher before and after school among boys and before school among girls who participated in AST.27 These results were similar to the present study; however, our findings also suggest significant positive associations between AST and weekday MVPA during school among boys and after school among girls. Further, intervention studies provide causal evidence that introducing AST into children's daily routine can significantly increase MVPA. For example, in a randomized controlled trial among 149 4th graders, Mendoza et al.28 implemented a 5-week walking school bus intervention in which study staff walked the children to and from school. Children in the intervention group significantly increased their daily MVPA levels from 46.6 min per day to 48.8 min per day whereas control group children significantly decreased their daily MVPA levels from 46.1 min per day to 41.3 min per day.

In contrast to our main findings, no association between AST and MVPA was identified for 5 ISCOLE study sites including Australia, China, Colombia, India and the United States. This may have been due to the low AST prevalence in India (5.3%) and the United States (8.2%) and lack of power to detect a difference. In addition, the average weekday MVPA time in our sample was high for Australian children irrespective of school transportation mode suggesting that behaviors other than AST may influence their activity levels. Alternatively, both active and motorized travelers in China spent less time than recommended in weekday MVPA (45.9 min per day versus 43.4 min per day) indicating that behavioral interventions in addition to those promoting AST may be needed to increase MVPA. In Colombia, car ownership is low and the majority of Colombian children in our sample either practices AST (72.6%) or uses a motorized form of public transportation (20.0%). In the present analysis, public transportation (‘bus, train, tram, underground or boat') was categorized as motorized, but this method likely includes some active travel, such as walking to bus stops, which may have resulted in an underestimation of weekday MVPA among Colombian children who use motorized transport to school.

Across all 12 study sites, AST was associated with 80% higher odds of children obtaining the recommended 60 min of MVPA on weekdays compared with motorized transport to school. Further, AST was associated with significantly higher odds of children obtaining a weekday average of at least 60 min per day of MVPA in 7 of 12 study sites. This result supports previous studies that have quantified the association between AST and PA guidelines. Although limited to Russian children, Tudor-Locke et al.29 found that omitting activity obtained through AST decreased the prevalence of meeting PA guidelines by 17–19% for boys and girls. Although in the same direction, our finding was greater in magnitude which may be owing to the use of different PA guidelines; we used an average of 60 min per day during the school week as our proxy for meeting PA guidelines while the aforementioned study utilized a different guideline of ‘at least 150 min per week or 30 min per school day'.30 In addition, Tudor-Locke et al.29 measured children's PA by parent-proxy report rather than objectively as was done in the present study.

In the present study, boys and girls who engaged in AST recorded ~7% less time in LPA before school compared with those who used motorized transport to school. No previous studies were found that examined the relationship between LPA time and AST, but our results suggest that children who participate in AST substitute minutes of LPA for MVPA. LPA confers some health benefits to children, especially those overweight or just beginning a PA routine; however, studies suggest greater health benefits are obtained from more intense PA (that is, MVPA). Carson, et al.31 found that each additional hour adolescents spent in objectively measured higher intensity LPA (800 counts per min: <4 METs) was associated with significantly lower diastolic blood pressure and higher high-density lipoprotein cholesterol while additional time spent in MVPA was associated with greater cardiometabolic benefits, including lower systolic blood pressure and insulin, smaller waist circumference and higher insulin sensitivity (HOMA-S%). In addition, Ekelund et al.32 examined associations between various intensities of objectively measured PA among children and adolescents from the European Youth Heart Study and found that MVPA was associated with better cardiometabolic risk profiles compared with LPA. Similar results were found in a sample of Spanish adolescents. Objectively measured MVPA was associated with better body composition measures whereas LPA was not.33 This suggests that children who engage in AST accumulate additional health benefits by replacing LPA with MVPA.

Insight into the relationship between AST and health in the present sample of children was beyond the scope of this study, but has been reported elsewhere.34 Briefly, Sarmiento et al. reported that ISCOLE children who engaged in AST had lower odds of obesity (OR=0.72; 95% CI: 0.60–0.87) and lower body fat percentage (β=−0.56 (0.22); P=0.002) compared with those that did not. These findings are in line with previous studies that demonstrated an inverse association between AST and objectively measured overweight and obesity among children.35, 36, 37 In contrast, a recent review8 concluded that the relationship between AST and body composition is inconsistent. Compared with children who engaged in motorized transport to school, 56% of included studies showed no difference in body composition among active travelers, 36% of studies found more favorable body composition among active travelers and 8% of studies observed a less favorable body composition among active travelers.8 In addition, a systematic review by Lubans et al.38 suggested a positive association between bicycling to school and better cardiorespiratory fitness. Finally, it is well accepted that higher levels of MVPA are associated with decreased risk of obesity and better cardiovascular and metabolic risk profiles.39, 40, 41, 42 Therefore, MVPA may be the mechanism by which AST leads to better health and lower odds of obesity, but intervention studies are needed to elucidate causality in these relationships.

Finally, we found no evidence of a relationship between AST and time spent in SED behaviors during any period of the day. A study of 9- and 15-year-old children in Europe similarly failed to identify a correlation between walking or biking to school and percent of time spent sedentary.43 The null findings between AST and SED in the present study support previous findings that correlates of children's MVPA appear distinct from those of SED.44 This is in line with the growing evidence base that SED and MVPA are independent behaviors.45, 46, 47 In addition, since children spend such a large amount of time in SED behaviors, it is unlikely that AST alone would be sufficient to significantly reduce SED time. However, Hinckson et al.48 found that among children who engage in AST, those living in a 1–2 km radius from their school accumulated less SED time than those living closer or further from school. This suggests that future studies should examine the relationship between AST and SED across various home-to-school distances.

Our study has several strengths including the large sample size consisting of children from all major world regions and various levels of development. In addition, the rigorous standardization of the protocol, measurements and quality control procedures facilitate comparisons across study sites and increase the integrity of our data. Further, ISCOLE was the first study to employ and analyze a 24-hour waist worn accelerometer protocol to objectively measure various intensities of activity, which led to higher wear time and compliance compared with other studies.49 In addition, time-stamped accelerometer data matched to local daily school start and end times enabled the present study to examine the associations between AST and activity levels during natural time periods before school, during school and after school time periods when such behaviors may especially contribute to MVPA, the first to our knowledge to do so in an international sample of children.

Our study also has several limitations. First, the cross-sectional study design impedes causal inference. Next, the large number of participants missing valid accelerometer data may have biased our results. However, the observed differences in PA between children who engage in AST and motorized transport alternatives are consistent with previous systematic reviews,8, 13 so it is unlikely that our results would have been drastically different if all participants had provided valid accelerometry data. Previous studies suggest MVPA from wheeled modes of AST, such as biking, may be underestimated by hip-worn accelerometers. A sensitivity analysis comparing MVPA obtained from wheeled modes (bicycle, roller-blade, skateboard or non-motorized scooter) to all other modes of AST and motorized transport revealed that the number of minutes of weekday MVPA obtained from wheeled modes of AST was significantly lower than that from other modes of AST and did not differ from motorized transport modes (data not shown). This suggests that we may be underestimating the association between weekday MVPA and AST in countries with a higher prevalence of wheeled AST, such as in Finland where nearly 25% of children in our sample bicycled to school. In addition, several sites had little variability in the prevalence of AST which may have limited our ability to detect significant within-country differences. Further, ISCOLE assessed method of transportation to school and children who participate in AST both to and from school may accumulate more MVPA than those only using AST to school.8 Future work should collect transport mode to and from school to avoid this potential source of bias. In addition, the literature suggests the distance between a child's home and school may affect both their PA levels and participation in AST.9, 50, 51, 52 This distance was not included in the present analyses as a potential moderator as it was not collected in ISCOLE. The between-country variation in the relationship between AST and weekday MVPA may be partially explained if the average home-to-school distance varied across ISCOLE sites. Finally, AST was captured by children's self-report which may be unreliable.53, 54 Nevertheless, a recent systematic review indicated that child-reported school travel mode showed substantial test-retest reliability and substantial convergent validity with parent reports.55

This study was the first to show associations between AST and weekday time in MVPA, LPA, SED and total activity counts in an international sample of 9–11-year-old children. It provided evidence that AST is associated with higher levels of objectively measured MVPA throughout the weekday. Future research should seek to elucidate factors that explain variability in MVPA and AST across various countries. In addition, there is a need for additional prospective and experimental studies to examine whether or not switching from motorized transport to AST increases MVPA as the cross-sectional design is the key limitation of our study. Finally, barriers to AST need to be further explored in order to develop effective interventions aimed at increasing AST prevalence among children.

Acknowledgments

We thank the ISCOLE External Advisory Board and the ISCOLE participants and their families who made this study possible. A membership list of the ISCOLE Research Group and External Advisory Board is included in Katzmarzyk et al. (this issue). ISCOLE was funded by The Coca-Cola Company. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Footnotes

MF has received a research grant from Fazer Finland and has received an honorarium for speaking for Merck. AK has been a member of the Advisory Boards of Dupont and McCain Foods. RK has received a research grant from Abbott Nutrition Research and Development. VM is a member of the Scientific Advisory Board of Actigraph and has received an honorarium for speaking for the Coca-Cola Company. TO has received an honorarium for speaking for the Coca-Cola Company. JZ has received a grant from The British Academy/Leverhulme Trust. The remaining authors declare no conflict of interest.

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