JMIR Publications

ABSTRACT

Background:

Smartwatches are commonly used to estimate energy expenditure, but studies have reported errors of 25% to 70%. The large range in estimation accuracy may be due to numerous factors, such as activity type, use case, and sensor data. We evaluated the differences in energy expenditure estimation among different smartwatches and models to identify the optimal use cases to achieve minimal error.

Objective:

We systematically evaluated the impact of different sensor inputs on energy expenditure estimation during outdoor activities by comparing a smartwatch operated in an activity-specific mode that incorporated GPS, IMU, and heart rate data; a smartwatch operated in a generic, non-activity-specific, mode that incorporated IMU and heart rate data; and a pre-defined heart rate model.

Methods:

We compared an activity-specific smartwatch, a non-activity specific smartwatch, and a heart rate model to isolate how different sensor inputs impact energy expenditure estimation. Thirty adults (17 men, 13 women; 34 ± 14 years) completed outdoor activities while wearing a portable respirometry system and both smartwatches. Participants performed self-paced walking and running conditions, and followed ecologically relevant audio prompts to elicit real-world walking. Energy expenditure estimates were collected from each smartwatch, and a predefined heart rate model was used to exclude wrist motion. Cumulative energy expenditure from each model was compared with ground-truth respirometry.

Results:

The activity-specific smartwatch achieved 13% error during outdoor walking with three times lower error than the non-activity specific smartwatch and heart rate model. During outdoor running, the heart rate model yielded a lower error than both the activity-specific and non-activity specific smartwatches.

Conclusions:

Energy expenditure estimation reached near laboratory-grade performance when both activity-specific information and GPS data were used for monitoring low-intensity activities, while heart-rate alone appeared better suited for high-intensity activities. Meanwhile, estimates from non-activity specific smartwatches should be interpreted cautiously by researchers, clinicians, and users when monitoring physical activity.