Validation of Fitbit-Flex as a measure of free-living physical activity in a community-based phase III cardiac rehabilitation population

Background Accurate physical activity monitoring is important for cardiac patients. Novel activity monitoring devices may enable precise measurement of physical activity. This study aimed to validate Fitbit-Flex against Actigraph accelerometer for monitoring physical activity. Design A validation study with a comparative design. Methods Cardiac patients and family members participating in community-based exercise programs wore Fitbit-Flex and Actigraph simultaneously over four days to monitor daily step counts and minutes of moderate to vigorous physical activity (MVPA). Results Participants (N=48) comprised 52.1% males, with a mean age of 65.66.9 years and 58.9% had a cardiac diagnosis. Fitbit-Flex and Actigraph were significantly correlated in males, females, total participants and cardiac patients for step counts (r=.96; r=.95; r=.95; r=.95), though less so for MVPA (r=.81; r=.65, r=.74; r=.71). As step counts increased the differences between Fitbit-Flex and Actigraph also increased. Fitbit-Flex over-estimated step counts in females (556 steps/day), males (1462 steps/day) and total participants (1038 steps/day) as well as for minutes of MVPA in females (4min/day), males (15min/day) and total participants (10min/day). Fitbit-Flex had high sensitivity and specificity in classifying participants who achieved the recommended physical activity guidelines. Conclusion Fitbit-Flex is accurate in assessing attainment of physical activity guideline recommendations and is useful for monitoring physical activity in cardiac patients. The device does, however, slightly over-estimate step counts and MVPA.