Eighty healthy youngsters, aged 10-18 years, will be equally randomized between the blinded and the non-blinded group. The blinded subjects will be informed that we were testing the reliability of a new device for body posture assessment and these youngsters will did not receive any information with regards to physical activity. Conversely, the non-blinded subjects were informed that the device was an accelerometer that assessed physical activity levels and patterns. Participants will be instructed to wear the accelerometer for 4 consecutive days and to keep a non-wear log diary over the 4 day-monitoring. The overall duration and the duration relative to the quality of the physical activity patterns (sedentary, light, moderate, vigorous and moderate to vigorous) were computed for each group during the 4-day monitoring, then compared between the two groups using the Student's t test.

Inclusion Criteria:

• 10-18 years old adolescent
• normal clinical examination,
• normal nutritional status at baseline defined by a weight / height ratio between - 2 and + 2 DS DS

Exclusion Criteria:

• abnormal ECG
• known chronic disease
• acute infection dating back at least a week.