There were no differences in counts recorded on the X, Y, and Z axes at 2.1 Hz; however, the counts recorded along the Y axis were significantly higher than the counts at the X and Z axes at 5.1 and 10.2 Hz. Due to large coefficients of variation for both inter- and intra-instrument variability at 2.1 Hz, testing the inter- and intra-instrument variability of the accelerometers before use is recommended.
Reliability of the RT3 is good; however, intermonitor variability exists. The vertical axis of the RT3 accelerometer showed the least variability and was the most reliable. It is recommended that intermonitor variability and reliability of RT3 on each axis be assessed before use.
The purpose of this study was to assess the interactive effects of habitual physical activity (total and vigorous intensity) and calcium intake on bone mineral content (BMC) in prepubertal boys and girls. Seventy-six children, aged 8-11 yr, wore accelerometers for up to 7 days to assess activity. Calcium intake was estimated by a 4-day weighted food diary. BMC and areal density (bone mineral density) were measured at the total body, proximal femur, and femoral neck by using dual-energy X-ray absorptiometry. Moderated regression analyses were used to assess the contributions of physical activity (total and vigorous) and calcium intake to BMC, residualized for bone area and body mass. Interactive effects of vigorous activity (> or =6 metabolic equivalents) and calcium intake were found at the total body in boys (b = 2.90 x 10(-3)) and in girls (b = 6.58 x 10(-3)) and at the proximal femur (b = 9.87 x 10(-5)) and femoral neck (b = 2.29 x 10(-5); where b is the regression coefficient from final equation) in boys only; residualized BMC was high only if both vigorous activity and calcium intake were high. There were no interactive effects of total activity and calcium intake. This study provides evidence for synergistic action of habitual vigorous activity and calcium intake on bone mass in children. Recommendations for optimizing bone mass should reflect this synergism.
This study aimed to determine the relationship between bone mineral content, habitual physical activity, and calcium intake in children. Fifty-seven children, aged 8–11 years, wore pedometers for seven days to assess activity. Calcium intake was estimated by a 4-day food diary. Bone mineral content (BMC) and areal density (BMD) were measured at the total proximal femur and femoral neck using dual energy X-ray absorptiometry. Regression analysis was used to assess contributions of physical activity and calcium intake to BMC, residualized for bone area and body mass. Physical activity explained 11.6% of the variance in residualized BMC at the proximal femur and 14.3% at the femoral neck (p < 0.05). Calcium intake added to the variance explained at the proximal femur only (9.8%, p < 0.05). This study provides evidence for an association between BMC and habitual physical activity.
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