health
Studies comparing wrist fitness trackers to lab equipment found one of their popular numbers is far less reliable than their heart rate readings.
Calories burned is the figure wrist trackers estimate least accurately. Trackers measure heart rate fairly directly using optical sensors that detect blood flow through the skin, but calorie burn cannot be measured directly at all; it has to be estimated using formulas that factor in body characteristics, activity type, and heart rate, and those estimates can be significantly less reliable than the heart rate readings that feed into them.
The current time and the day of the week are simply displayed by the device's clock and calendar functions rather than measured or estimated from the body at all, making them clearly unrelated to the accuracy question being asked here.
Energy expenditure depends on individual factors like body composition and fitness level, factors a wrist sensor cannot directly observe, which helps explain why calorie estimates can be off by a substantial margin compared to lab-based measurement methods.
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