Interactive effects of habitual physical activity and calcium intake on bone density in boys and girls.

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.

Intermonitor variability of the RT3 accelerometer during typical physical activities.

PURPOSE: To evaluate the reliability and variability of eight RT3 accelerometers. METHODS: The RT3 were subjected to two repeated trials of six activities: rest, walking (4 and 6 km.h-1), running (8 and 10 km.h-1), and sit-stand position (20 min). One person performed all trials (female: age 24 yr, height 158.0 cm, mass 48.2 kg). Each activity lasted 12 min. The middle 10 min were taken from each 12-min trial and used as the output measure (cts.min-1). Data were analyzed for activity ( 6), monitor ( 8), and trial ( 2) effects using four three-way ANOVA: vector magnitude, X (vertical), Y (anterioposterior), and Z (mediolateral) axes. RESULTS: Intermonitor coefficient of variation was <6% during locomotive activities, however, increased to 8-25% during sit-stand. A three-way interaction was found for vector magnitude (F35,315=88945.7, P < 0.001) and Y (F35,315=978435.6, P < 0.001) and Z axes (F35,315=103802.8, P < 0.001). A two-way activity x monitor interaction was found for the X axis (F35,315=1037787.0, P < 0.001). Follow-up tests revealed no differences between trials 1 and 2 for vector magnitude, X and Z axes. One monitor recorded significantly lower activity counts in trial 1 compared with trial 2 along the Y axis. Intermonitor differences were evident at 4, 6, 8, and 10 km.h-1 for the Y and Z axes, and at 6, 8, and 10 km.h-1 for the vector magnitude and X axis. Variability between monitors at each activity increased as intensity increased. CONCLUSION: 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.

Technical variability of the RT3 accelerometer.

PURPOSE: To evaluate the technical performance of the RT3 triaxial accelerometer. METHODS: Twenty-three RT3 accelerometers were subjected to a specific vibration along each sensitive axis in isolation, using a motorized vibration table that produced frequencies of 2.1, 5.1, and 10.2 Hz, respectively. Data were analyzed for frequency and axis effects and inter- and intra-instrument variability. RESULTS: ANOVA showed a frequency by axis interaction (F2.1,36.8 = 19.9, P < 0.001). Post hoc tests revealed the Y axis count to be significantly higher than the X and Z axes counts at 5.1 and 10.2 Hz. There was no difference in counts between axes at 2.1 Hz. Interinstrument coefficients of variation (CV) decreased as frequency increased (21.9 to 26.7% at 2.1 Hz, 6.3 to 9.0% at 5.1 Hz, and 4.2 to 7.2% at 10.2 Hz). The intraclass correlation (ICC) between RT3s was 0.99, regardless of the axis. Intra-instrument CV also decreased as frequency increased (2.1 to 56.2%, 0.3 to 2.5%, and 0.2 to 2.9% at 2.1, 5.1, and 10.2 Hz, respectively. CONCLUSION: 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.