Validity of Smartwatch-Derived Heart Rate and Energy Expenditure Measurements at Rest and Across Exercise Intensities: A Pilot Study
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Smartwatches are widely used to monitor heart rate (HR) and energy expenditure (EE) in daily life and exercise settings. However, their measurement accuracy compared to criterion devices such as electrocardiography (ECG) and gas analyzers has not been clearly established. The purpose of this study was to validate HR and EE measurements from four commercial smartwatches (Apple, Galaxy, Fitbit, Garmin) against ECG and a gas analyzer. Fifty healthy adult men participated, and HR was simultaneously measured using ECG and the four smartwatches during rest, low-, moderate-, and high-intensity exercise, as well as recovery after high-intensity exercise. EE during exercise was compared using a gas analyzer as the reference. Data were analyzed using repeated-measures ANOVA, Pearson correlation, intraclass correlation coefficient (ICC), and Bland–Altman analysis. HR significantly increased with exercise intensity (p < 0.001), but no differences were observed between devices. Correlations between ECG and Apple, Galaxy, and Garmin were generally very high (r = 0.92–0.97), whereas Fitbit showed lower correlations, particularly at high intensity. ICC analysis demonstrated excellent reliability for HR (ICC = 0.94). In contrast, EE showed significant differences across devices (p < 0.001), with weak correlations relative to the gas analyzer (r = 0.17–0.37; ICC = 0.25). Bland–Altman analysis demonstrated wide limits of agreement (LoA ±85–120 kcal), indicating considerable individual-level variability. These findings suggest that while smartwatch-derived HR values may be used reliably across exercise intensities up to moderate levels, smartwatch-based EE estimates are not interchangeable with criterion measurements at the individual level and should be interpreted with caution.
Abstract
Smartwatches are widely used to monitor heart rate (HR) and energy expenditure (EE) in daily life and exercise settings. However, their measurement accuracy compared to criterion devices such as electrocardiography (ECG) and gas analyzers has not been clearly established. The purpose of this study was to validate HR and EE measurements from four commercial smartwatches (Apple, Galaxy, Fitbit, Garmin) against ECG and a gas analyzer. Fifty healthy adult men participated, and HR was simultaneously measured using ECG and the four smartwatches during rest, low-, moderate-, and high-intensity exercise, as well as recovery after high-intensity exercise. EE during exercise was compared using a gas analyzer as the reference. Data were analyzed using repeated-measures ANOVA, Pearson correlation, intraclass correlation coefficient (ICC), and Bland–Altman analysis. HR significantly increased with exercise intensity (p < 0.001), but no differences were observed between devices. Correlations between ECG and Apple, Galaxy, and Garmin were generally very high (r = 0.92–0.97), whereas Fitbit showed lower correlations, particularly at high intensity. ICC analysis demonstrated excellent reliability for HR (ICC = 0.94). In contrast, EE showed significant differences across devices (p < 0.001), with weak correlations relative to the gas analyzer (r = 0.17–0.37; ICC = 0.25). Bland–Altman analysis demonstrated wide limits of agreement (LoA ±85–120 kcal), indicating considerable individual-level variability. These findings suggest that while smartwatch-derived HR values may be used reliably across exercise intensities up to moderate levels, smartwatch-based EE estimates are not interchangeable with criterion measurements at the individual level and should be interpreted with caution.