D Results

Participants were primarily white (74%), female (66%), and well-educated. The average age of participants was 31.8 (9.16) years, with an average reported BMI of 24.5 (5.43) kg/m2 (see Supplemental Table 4.1 in Appendix, which outlines participant demographics). Activity output was compared between the Fitbit and the ActiGraph (Table 1). The Fitbit recorded an average of 9286 (4134) daily steps and 915.2 minutes of total activity (adjusted for Sedentary time). In comparison, the ActiGraph recorded an average of 8591 (3883) steps and 913.3 minutes of total activity during the 24-hour study period.

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A total of 2099 15-minute blocks were included in the final analysis. The Fitbit recorded a mean of 152 (313) steps per 15-minute block, whereas the ActiGraph recorded 142(280) steps per 15-minute block. Six outlying observations were identified in the 15-minute block data. While the outlier values were high, they were not impossible and did not appear to result from monitor error. Analyses with and without these observations were nearly identical, therefore the observations were retained.

Comparing Total Steps and 15- minute Block Steps

On average, the Fitbit Zip recorded approximately seven percent (~676 steps) more steps than the ActiGraph. This difference varied slightly for days with less than 7000 Fitbit steps (n=12; 4.6% difference) compared to days with more than 10,000 Fitbit steps (n = 13; 8.6% difference). While the differences between these step levels were statistically significant (p = 0.044), both estimates and the difference are still considered small. Furthermore, there was a very strong correlation between the Fitbit and ActiGraph accelerometers (r=0.98), suggesting that the difference in step estimation is consistent. Additionally, Bland-Altman plots affirmed low levels of discrepancy with a 95% Limit of Agreement (LOA) of +/- 1625steps per day, and 75% LOA at +/- 953 steps per day (Fig. 4.1).

Results also showed that the Fitbit and ActiGraph 15-minute block step estimates were very strongly correlated (r= 0.985). Regression analyses and Bland-Altman plots again indicated strong agreement for the 15-minute step blocks, with an average deviation of ~10 steps per 15 minute block (Fig. 4.2).

To see if the difference between steps estimated by the Fitbit and ActiGraph varied by the number of steps taken during a 15-minute block, we examined deviations between the two step

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estimates in 10 steps per block categories. Observations were categorized based on the Fitbit step estimate for a given block. Grouping categories and deviations for this comparison can be found in Figure 3. While the coefficient of variation remained fairly consistent across categories, it is important to note that the average step counts for the ActiGraph were higher than the Fitbit below 100 steps per 15-minute block. However, this trend is reversed for steps per block above 100, such that the Fitbit recorded more steps than the ActiGraph.

Comparing Classification of Activity Intensity

Table 4.1 also compares Sedentary, Lightly Active, Fairly Active, and Very Active Fitbit minutes to minutes of activity recorded by the ActiGraph. On average, the Fitbit Zip reported approximately 540 more minutes of Sedentary time compared to the ActiGraph. However, after the Fitbit Zip Sedentary minutes were adjusted (Fitbit Zip Sedentary minutes minus ActiGraph non-wear minutes), sedentary minutes of activity became strongly correlated between the two monitors (r=0.96). Further confirmation for use of the Adjusted Sedentary minutes was observed with Bland-Altman plots which also demonstrated strong levels of agreement between the accelerometers (figure not shown).

In Table 4.1, it was also observed that Lightly Active minutes was strongly correlated with NHANES cut points for light activity (r=0.71), but actual differences were 67.6 minutes (deviation % about mean=98.1%). Fairly Active minutes, as recorded by the Fitbit Zip, was poorly correlated with the NHANES moderate cut point (r=0.26). However, a stronger correlation was observed between Fairly Active minutes and Matthews’ lifestyle cut point of 760cpm (r=0.50, deviation= -33.6mins, deviation % of mean =46.9%). Very Active minutes was moderately correlated with the NHANES vigorous cut point (r=0.54), but had a strong

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correlation with the NHANES MVPA cut point (r=.89, deviation=-17.7mins, deviation % about mean= 41.1%). Finally, the strongest correlation was seen between Fairly Active+ Very Active

minutes and Matthews’ Lifestyle MVPA cut point (r=.91, deviation=9.5mins, deviation % about mean=8.3%).

Estimation of ActiGraph Cut Points which most Closely Replicate Fitbit Estimated Physical Activity

Table 4.2 summarizes the results for the estimation of the ActiGraph cut points which most closely replicate the minutes in various levels of physical activity from the Fitbit Zip monitor. Cut points were evaluated at both the group and individual level, but ultimately the findings were very closely matched. We found that the best matched ActiGraph cut points for

Lightly Active, Fairly Active, and Very Active activity were 150cpm, 600cpm, and 2975cpm. The

Sedentary/Lightly Active cut point was very similar to the current 100cpm used in many studies, while the Lightly Active/Fairly Active cut point was very close to the lifestyle MVPA cut point proposed by Matthew (81).

Meeting National Recommendation for Steps

The agreement for classification of steps meeting or not meeting the national

recommendation of 10,000 daily steps was also examined in this study. It was found that the level of agreement was also high (88.6%) for the Fitbit Zip and ActiGraph. In total, both

monitors classified 22 participants reaching 10,000 steps, and nine participants reaching less than 10,000 steps. Thus, there were four disagreements on 10,000 step attainment, resulting in a kappa = 0.74. For each disagreement, fewer steps were recorded by the ActiGraph and were

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recorded as follows for ActiGraph vs. Fitbit Zip: 9589 vs. 10,094 steps, 8073 vs. 10,234 steps, 7340 vs. 10,010 steps, and 9096 vs. 11,218 steps.

Self-reported Activity Outcomes

All participants completed both the pre- and post- questionnaires. In response to the baseline question regarding activity in past week, 94% of participants reported engaging in moderate activity and 71% of participants reported engaging in vigorous activity. In the follow up survey, 80% of participants reported engaging in moderate activity the day of accelerometer wear, while 43% reported engaging in vigorous activity.

As additional validity evidence, we examined the correlations between various self-report physical activity outcomes and activity outcomes from the activity monitors (see Table 4.3). Correlations with step outcomes were nearly identical. Other relationships varied slightly between monitors when matched by intensity (Fairly+Very Active with MVPA, Very Active

with VPA, and Lightly Active+ Fairly Active+ Very Active with LMVPA). Ultimately, ActiGraph outcomes had slightly stronger relationships with self-report compared to those from the Fitbit.