Criterion Validity of Smartwatches for Measuring Heart Rate and Energy Expenditure During Resistance Exercise

Commercial smartwatches are increasingly used to monitor physiological responses during exercise; however, their validity for resistance exercise remains uncertain. This study evaluated the criterion validity of four commercial smartwatches (Apple Watch Series 8, Samsung Galaxy Watch 5, Fitbit Charge 5, and Garmin Vivosmart 5) for measuring heart rate (HR) and energy expenditure (EE) during resistance exercise. Seventy healthy men aged 20–39 years performed two sets each of the bench press, T-bar row, squat, and deadlift at individualized 10-repetition maximum loads. Smartwatch-derived HR was evaluated against electrocardiography, whereas smartwatch-derived workout-session EE was evaluated against indirect calorimetry. The Apple Watch showed the lowest measurement error for mean HR across the protocol (bias = −0.36 bpm; MAE = 1.81 bpm; ICC = 0.969) and for overall dynamic HR (bias = −0.72 bpm; MAE = 4.95 bpm; ICC = 0.904). HR agreement deteriorated markedly immediately after each set, particularly for the Samsung and Fitbit devices, which underestimated ECG-derived HR by 25.60 and 19.59 bpm, respectively. In contrast, EE estimates showed poor agreement with indirect calorimetry across all devices (ICC = 0.021–0.033). These findings demonstrate that the agreement of smartwatch-derived HR with electrocardiography measurements varies across time points during resistance exercise and indicate that smartwatch-derived workout-session EE should be interpreted cautiously when accurate metabolic assessment is required.