The Paris Descartes University (Paris, France) approved the design, while the Secretaría de Educación de Bogotá, the application. None of the schools refused to participate and informed consent was requested from all students and their parents.

The proposed questionnaire
Different questionnaires have been developed for measuring PA (Aaron et al., 1993; 1995; Bouchard et al., 1983; Kriska, 1997). QAPACE (see Appendix), is self-administered and supervised and 18 questions were developed according to 13 categories (Table 1) of daily physical activities, covering all the possible school or vacation activities of the youth over the past year.

Construction of the questionnaire
Several questionnaires and methods were developed until the best ways of approach and introduction in a school were found. Volunteers from among the senior physicians, belonging to the permanent staff of the departments of either Community Medicine or Pediatrics or Sport Medicine from the El Bosque University were invited to participate in a course on the QAPACE study during a full week. During this course, each participant was requested to ask young relatives to test the questionnaire and to bring their comments back. Future supervisors were selected from this group. The latter were secondarily instructed, trained and motivated during a three-day workshop devoted to the management of the questions on the questionnaire by the pupils and to the means for minimizing biases in the questionnaire. The supervisors received specific training for assisting the children and giving them a proper understanding of the questions in the questionnaire for minimizing biases. They were particularly instructed on how to read out loud and explain the written foreword comment of the questionnaire about the specific meaning of an average day when answering the different items.

Three schools were randomly selected from among the 460 schools of Bogotá not participating in a planned future bigger survey, urban co-ed regular diurnal schools, with both elementary and secondary grades, and with a physical educator, one from each from high, middle and low socio-economic strata, a formal written acceptance by the parents, students and administrative staff of the school was requested. The acceptance criteria were: 1) 8-16 yr old; 2) born and living in Bogotá; 3) healthy, without disabilities; 4) non pregnant. Six students were randomly selected in each of the nine age groups (8-16 yr) for each gender. Hence, a total of 3 x 6 x 9 x 2 = 324 children were exposed to the initial version.

The students were invited in groups of 10-15 to receive instructions on how to identify only the activities they had participated in on at least ten occasions over the last year, and to complete the questionnaire. For each of those activities they had to specify the corresponding estimated average duration of each session in hours and minutes per day and the frequency of these sessions expressed in days per week. The questionnaires were collected and checked for inconsistencies. Less than 40 minutes were necessary to complete them.

The reproducibility
Three schools were selected from among the remaining 457 schools. Three students were randomly selected for each of the schools, nine grades and two genders, totalizing 162 students. Out of the randomly selected sample of 162 children, only 121 were included; 41 were not included due to non compliance of the criteria. All the 121 subjects filled the same questionnaire 90 days later (McDowell, 1987).

At the time of the surveys, the students were weighed in light clothing on a calibrated platform (Seca Digital Floor Scales - Model 7700). Height was measured to the nearest 0.1cm using a free standing Holtain Anthropometer. All data was stored in a Visual FoxPro 6.0 database.

Estimation of the energy expenditure using the questionnaire
The Daily Energy Expenditure (DEE), for school and vacation, along with the PA compendium (Ainsworth et al., 2000) were used to obtain the annual mean DEE.

The sum is extended over all possible activities i (i=1 to 13). For each activity i (i=1 to 13), f(i) corresponds to its daily frequency, d(i) to its mean duration and m(i) to its intensity according to the compendium. SP corresponds to School period and VP corresponds to Vacation period.

Validation methods
The direct and indirect peak VO₂ methods were used on 36 subjects from the reliability study who were randomly selected and whose results were plotted with the Bland-Altman method.

Indirect method: aerobic fitness
The peak VO₂ was assessed using the Lèger Test. In compliance with the protocol, the laps were counted. Stage and level were converted into peak VO₂ mL·kg^-1·min^-1, using the following formula (Leger and Lambert, 1982):

Peak VO₂ = 31.025+3.238*speed (km·h^-1)-3.248*age (y)+0.1536* speed (km·h^-1)* age (y)

Direct method: ergo-spirometry
The Hebestreit protocol was used (Heberstreit et al., 2000) on a calibrated cycle ergometer (ER900 Jaeger, Würzburg, Germany). The work rate was initially set up to 0 W·kg^-1 body mass for 2 minutes, and then increased every 2 min by 1 W·kg^-1 body mass. After a total of three 2 min-stages, including the initial stage, work rate was increased every min by 0.5 W·kg^-1 until exhaustion. The peak oxygen uptake, expressed in mL·kg^-1·min^-1, was determined from expired air sampled at 30 seconds intervals using an Oxycon Delta Jaeger. The measured outcomes were the averaged cardiac frequency (CF) over the duration of the experiment (beats.min^-1), the respiratoryquotient (RQ), the Minute Respiratory Volume (MRV) both at the maximal load, and the peak VO₂. The values of the peak VO₂ were secondarily converted into kJ.kg^-1.h^-1 using 1 Met = 4.184 kJ.kg^-1.h^-1 =1 kcal= 0.207 L of O₂ [Ainsworth et al., 2000,].

Statistical analysis
Means and standard deviations were used for both the test- retest reproducibility between the two periods, and also the Validity study, using Pearson's ICC with its 95% confidence interval (SPSS 11.0 statistical Software, Chi, Ill) and the Bland & Altman plot (Bland et al., 1986). For validation, two methods were used for, 1) the concordance between direct and indirect peak VO₂, and 2) the concordance between direct or indirect peak VO₂ and the DEE over a one year period.

Study 1: Questionnaire comprehension
Between August 2001 and October 2001 the comprehension study was undertaken with a sample of the 324 remaining students, i.e. 162 boys and 162 girls, aged 8-16 years (Table 2).

It was necessary to modify Distance and Mode of Transportation and Sports, and, "Child care" was added to home activities, because among girls from low income families, this activity is common (Table 1).

Study 2: Test re-test study
The study was carried out from February to March 2002. The time needed for each session was shortened to 30 minutes average for all age groups. A general description of the study sample of the 121 is given in Table 2. The ICC of the group sorted by age, grade, gender, etc. is in Table 3. The corresponding Bland & Altman DEE plots are shown in Figures 1, 2 and 3. The first one corresponds to the total population. The lower and upper limits of two standard deviations below and above the mean difference (8.5 kJ) were -515.5 and 532.5 kJ·24^-1 hours. The mean absolute EE of the two questionnaires was 7566 kJ·d^-1 (SD 3521). In boys (Figure 2), the mean difference was 10.8 with lower and upper limits of -546.2 and 567.8 kJ·24^-1 hours. Whereas in girls (Figure 3), the mean difference was 4.5 with lower and upper limits of -487.5 and 496.5.

In order to estimate the variability capacity of the DEE according to the daily activities as recorded by the questionnaire, the mean durations for each of the thirteen categories of activities are shown (Table 4).

In Table 4, the categories 1- 3, 9, 11-13 correspond to rather stable activities all year around, whereas the categories 4-7 and 8 correspond to stable activities during the school period. In the 7 x 24 hours = 168 hours of a school or vacation week, a mean of 157 (93.5 %) and 145.2 (86.4 %) hours/week, respectively, correspond to routinely fixed activities, leaving only 6.4% and 22.8 %, for variability in physical activities during the periods.

It can be noted that the highest correlation correspond to the rather fixed activities (Table 4: activities 1-3 and 5-12), whereas the lowest ones correspond to more variable activities (Distance and mode of Transportation and Home Activities).

Study 3: Validation study
The mean values of the observed CF, RQ and MRV suggest the rather good quality of the load reached by the students during the peak VO₂ Test.

The overall mean DEE was 125.7 (SD 24.9) kJ·kg^-1·min^-1, whereas it was 135.5 (SD 28.8) (boys) and 115.9 (SD 15.6) (girls). The mean values of DEE were higher in boys than in girls, whether they were considered over one year or separately over the school or vacation periods (Table 5).

The relationship between the direct and indirect measurements of peak VO₂ is shown on Figure 4. The general ICC is 0.91 (p < 0.01), whereas it was 0.96 (boys) and 0.67 (girls).

The ICC between the indirect measurement of VO₂ uptake and the overall mean DEE was 0.56 (p < 0.0001). The corresponding ICC between direct measurement of peak VO₂ and the respective mean DEE was 0.69 (p < 0.00001). The ICC remained high for both school and vacation. They were lower in the female group as compared with the male group (Table 6), and increased with age category. The Bland-Altman plot between the DEE and the direct and indirect peak VO₂ are shown in Figures 5 and 6.