The effects of physical activity (PA) on bone mass gained during growth in Japanese are not well understood. The purpose of this study was to examine whether changes in PA affected bone mass gained by Japanese schoolchildren, as measured by calcaneus quantitative ultrasound (QUS). Three hundred and seven children aged 9–13 years participated in the study and were followed for a 2-year period. The stiffness index (Stiffness) of the calcaneus was measured by QUS, and PA evaluated by a questionnaire. Participants were divided into two groups according to PA: high (≥ 7 hours/week, H) or low (< 7 hours/week, L). Participants were also divided into four groups according to their 2-year change in PA: consistently high (HH), consistently low (LL), changed from high to low (HL), and changed from low to high (LH). Analysis of covariance was used to compare adjusted Stiffness across all four groups. The adjusted 2-year changes in Stiffness ranked in decreasing order among girls: HH (20.8 %), HL (17.6 %), LH (14.3 %), and LL (12.2 %), respectively (trend test, P = 0.027). This trend was not observed among boys. These results suggest that changes in PA significantly affected bone mass gain among peripubertal girls, and that a continuing PA of more than 7 hours a week (approximately ≥ 60 min/day) from a young age is effective in increasing peak bone mass. However, given the limitations of this study, further robust studies which recruit representative samples and consistently employ validated measurement instruments are needed.
