JMIR Publications

ABSTRACT

Background:

Wearable device–based monitoring of sedentary behavior is increasingly used in digital cardiovascular care; however, the validity of consumer-grade wrist-worn trackers in older cardiac patients remains unclear.

Objective:

This study evaluated the agreement between sedentary time measured by a consumer-grade wrist-worn activity tracker (Fitbit Inspire 3) and a research-grade waist-worn accelerometer (ActiveStyle Pro) in Japanese cardiac outpatients under free-living conditions.

Methods:

Fifty-one outpatients with heart failure or ischemic heart disease (mean age 78.4 years) simultaneously wore both devices for 7 consecutive days. Sedentary time was assessed within a standardized 12-hour daytime window (09:00–21:00). Agreement was examined using Bland–Altman analysis, Lin’s concordance correlation coefficient (CCC), and Cohen’s kappa for tertile classification.

Results:

The Fitbit systematically overestimated sedentary time by 62.1 minutes per day compared with the ActiveStyle Pro, with significant fixed bias but no proportional bias. Correlation between devices was strong (r = 0.905), while uncorrected concordance was moderate (CCC = 0.71). After applying a simple constant correction for the fixed bias, concordance improved substantially (CCC = 0.904). Tertile classification showed moderate agreement (κ = 0.588), with all misclassifications occurring between adjacent categories.

Conclusions:

In older Japanese cardiac outpatients, a consumer-grade wrist-worn tracker systematically overestimated sedentary time by approximately 62 minutes/day compared with a research-grade waist-worn accelerometer, with fixed bias but no proportional bias. A simple constant correction markedly improved concordance (CCC ≈0.90), supporting calibrated use of consumer wearables for scalable remote monitoring and behavioral interventions in ambulatory cardiac care while underscoring the need for population-specific validation.