When comparing the count output between models, a small but statistically significant bias of 2.7% (CI = 0.8, 4.7) occurred across all speeds (range 0.22 m·s−1 to 3.8 m·s−1; 0 to 14593 cnts.min−1). This bias is equivalent to the AM2 recording 58 cnts.min−1 higher than the average count value of 2152 cnts.min−1 recorded by AM1 (). Overall, the AM1 and AM2 count output was highly correlated (r = 0.99) () (p<0.05).
Mean and standard deviation (SD) of count output and step output for each monitor model at self-selected slow, medium and fast speeds.
Correlation between AM1 (ActiGraph model 7164) and AM2 (ActiGraph model GT1M) count output (cnts.min−1).
The average count output for each model was compared at self-paced slow, medium, and fast speeds (). Additional analysis was done by absolute speed tertile, and the relationship between AM2 and AM1 output did not change. Thus, all values reported are based on self-selected speeds. The coefficient of variation for each model at each speed was less than 3.2%. The count output for AM2 relative to AM1 at each speed is presented in . The count output from the models was not significantly different at self-paced slow speeds or fast speeds. However, a significant positive bias occurred at medium speeds, where AM2 count output was 5.3% higher than that of AM1 (CI = 3.4, 7.2). At all speeds the monitors were significantly correlated (r = 0.96, 0.95, and 0.97 at slow, medium, and fast speeds, respectively) (p<0.05). shows monitor output comparison at slow speeds (<1000 cnts.min−1, <0.52 m·s−1). At very slow speeds, AM1 recorded counts when the AM2 recorded zeros.
Bias for AM2 relative to AM1 expressed as percent difference and differences in count output
Correlation between AM1 (ActiGraph model 7164) and AM2 (ActiGraph model GT1M) count output (cnts.min−1). when counts are less than 1000 (approx. 0.52m·s−1)
Cross-classification of count output for the categorical ranges (e.g., <1000, 1000–1999 cnts.min−1
) agreed 85.5% of the time. For 11.2% of the minutes, the AM2 output was higher than that of AM1. AM1 recorded higher output than AM2 for 3.2% of the minutes (). As shown in , when the monitors were cross-classified based on the Freedson cut-points (7
) to estimate activity intensity (i.e., light, moderate, and vigorous), the models agreed 96.1% of the time. Estimated activity intensity from AM2 was higher than AM1 for 2.3% of the minutes while the AM1 estimated a higher intensity category for less than 1% of the minutes recorded.
Cross-comparison of count output for AM1 compared to AM2.
Cross-comparison of minutes in intensity categories from the two monitor models based on the Freedson cut-points
The step output was significantly higher for AM1 than AM2 across all speeds (bias = −19.8% CI: −23.2, −16.4), which was due to a large bias at slow speeds (bias = −59.5%, CI: −50, −72.2). There was no difference in steps per minute between models for medium or fast speeds, as shown in . shows that the differences in steps per minute were large for speeds less than 0.89 m·s−1, while at faster speeds there was no difference between models.
Bias for AM2 relative to AM1 expressed as percent difference and differences in step output
Comparison of step output (steps.min−1) between AM1 (ActiGraph model 7164) and AM2 (ActiGraph model GT1M) across all speeds.
For the normal-weight group, AM2 recorded significantly higher count output than AM1 for both medium (bias = 5.7%, CI = 3.4, 0.8) and fast speeds (3.1% CI = 1.3, 4.8). Within the overweight or obese group, the average count output for AM2 was higher at medium speeds (bias 4.8%, CI = 1.6, 8.1) but not at other speeds. However, since the confidence intervals overlapped at all speeds, there were no significant differences between BMI groups. Count output was significantly higher with the AM2 for females at medium (bias = 7.4%, CI = 4.3, 10.5) and fast speeds (bias = 3.8%, CI = 1.1, 6.5). For males, AM2 counts were significantly higher only at medium speeds (bias = 3.0%, CI = 1.0, 5.0). Similar to BMI groups, the differences in count output between sexes were not statistically significant at any speed. Step output was significantly lower for AM2 at slow speeds for all sex and BMI groups. There were no differences in step output for medium or fast speeds by sex or BMI. There were no statistically significant differences between males and females or between BMI categories for step output.