Wrist-worn motion sensors alone underestimate the increase in activity-induced energy expenditure during resistance and endurance training by approximately 50% compared to doubly labeled water measurements in healthy young men.

When people lift weights or do intense cardio, their muscles grow bigger and burn more energy even at rest. Wrist sensors only track arm movement, so they miss the extra calories burned by the whole body’s muscles working harder and growing. This causes the sensors to show much less energy use than what actually happens.