The main objective of the present study was to determine whether a structured, laboratory-based exercise task would modify the energy expenditure (EE) and the pattern of spontaneous physical activity (PA) of obese boys on the day of an exercise laboratory visit and on the following day. Fourteen 10- to 15-y-old moderately obese (36.6+/-3.3% fat) boys volunteered. They each had three laboratory visits, I wk apart. In one visit, they performed a strenuous 50-min cycling task; in another, a 30-min medium-intensity cycling task; and in another (which served as placebo), they did not exercise. PA was monitored the day before (d 1), during (d 2), and after (d 3) each laboratory visit by use of a heart rate monitor and a 12-h recall interview. EE was calculated from minute-by-minute heart rate and each child's predetermined relationship between oxygen uptake and heart rate. EE and PA were analyzed from 1300 to 1900 h each day using 15-min intervals. EE tended to decrease (p < 0.087) in the afternoon of all d 2 compared with d 1, and it increased on d 3 after the medium-intensity exercise (p < 0.0005). EE during d 2 and 3 combined, compared with d 1, decreased after the high-intensity exercise (534.2 versus 564.3 kJ/h, p < 0.05). It increased after the medium-intensity exercise (561.8 versus 526.7 kJ/h, p = 0.052) and was not affected after the placebo visit (589.4 versus 574.3 kJ/h). Time spent outdoors was consistently reduced on the day of laboratory visit compared with the day before and after the visit, regardless of the contents of intervention. In conclusion, a single laboratory visit is followed by a reduction in EE and PA on the day of intervention. However, its effect on EE the following day may be dose dependent: medium-intensity exercise induces an increase in EE, but high-intensity exercise causes a decrease in EE. One implication is that intervention by physical training should employ medium-intensity exercise to enhance the EE of obese boys.

Although portable heart rate (HR) monitors are commonly used to assess energy expenditure, little is known about their suitability for preschoolers. To validate the Polar Vantage XL monitor (XL), the HRs of twenty-seven 3- to 5-year-old girls and boys were measured using the XL and ECG simultaneously. During rest, values for both methods were virtually identical (ECG = 97.3 ± 7.5, XL = 97.9 ±7.2, r = .99 for lying, and ECG = 111.1 ± 16.5, XL = 110.4± 16.3 for sitting). XL nonsignificantly underestimated HR during 1–2 min of non-steady-state cycling (ECG = 142.7 ± 11.0, XL = 140.2 ± 11.5, r = .93) and significantly overestimated it during recovery (ECG = 112.4 ± 12.8, XL = 118.0 ± 12.3, r = .92). To assess social acceptability of wearing the XL, twenty-three 3- to 5-year-old girls and boys were observed twice for 60–90 min. Approximately 90% of the time, their responses were rated as enthusiastic/positive or agreed. Only 3–5% of children refused to wear the XL. In conclusion, the XL is highly valid and socially acceptable, when used with 3- to 5-year-old children.