This white paper gives the physiological and technical background of calorie estimation in wrist-worn wearables. It also gives thePolar approach to energy expenditure estimation, called Polar Smart Calories, and the scientific validity of the Polar Smart Calories feature.
Total energy expenditure in humans comprises energy expenditure at rest and during physical activity. In Polar Smart Calories, resting energy expenditure is based the on size of the human body, whereas the energy expenditure during physical activity is estimated by measuring wrist acceleration and heart rate.
The accelerometer-based energy expenditure is based on an empirical relationship between the processed wrist acceleration signal and energy expenditure. Heart-rate-based energy expenditure is based on the direct relationships between heart rate, oxygen consumption and energy expenditure. The validity of both acceleration and heart-rate-based estimates depends on the type and intensity of the physical activity. The best overall accuracy for energy expenditure is obtained when information from both acceleration and heart rate are combined intelligently and for this reason Polar Smart Calories uses this approach to estimate energy expenditure.
In internal measurements, Polar Smart Calories are consistent with reference energy expenditure values. In an independent external validation study, Polar Smart Calories had a bias of 3.3%, a mean absolute error of 20.6% and it was considered to perform among the best in available wrist-worn EE monitors.Full document (PDF)