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Background: Activities generating relatively high ground-reaction forces produce greater gains in bone mass than low- to moderate-intensity forces, especially when imposed during early childhood. We investigated in a pediatric population the amount of time spent in Vigorous Physical Activity (VPA), that is currently used to estimate high-intensity forces beneficial to skeletal health. The objectives of this study were: 1) to quantify the proportion in a general pediatric population that do not accumulate at least 20 minutes of VPA daily, which is currently recommended for bone health; 2) and to identify any age and gender-based differences.
Methods: Physical activity wasrecorded during 10 days using an uniaxial accelerometer in 255 children and adolescents (117 females; 138 males). According to Ekelund et al.,VPA was defined as > 3000 counts.min-1. The time spent at VPA intensity was expressed as minutes per day, and the proportion of subjects who accumulated less than 20 minutes of VPA daily was assessed considering gender and age.
Results: Our cohort of healthy children and teenagers had a normal mean value of VPA. However, 31.8% of the participants (31.9% of males and 31.6% of females) did not accumulate at least 20 minutes of VPA daily. Vigorous PA was 31.7% lower in females compared to males.Finally, the time spent in VPA decreased significantly with age in the male group (t=-2.5, p=0.014), but not in the female group (t=-1.6, p=0.105).
Conclusions: Vigorous PA measured by accelerometer is a useful and valid tool to assess the amount of highintensity forces beneficial to skeletal health during growth. Around 30% of children and teenagers of both genders do not accumulate at least 20 minutes of VPA. We may hypothesise that the gender-related differences in BMD may be due partially to the difference in VPA observed in this study, females being less vigorously active. Further research is needed to establish the relationship between the VPA level and bone mineral accretion.