A system made with two inexpensive sensors proves to be more accurate than smartwatches for measuring calories burned during activity – and the instructions for making the system yourself are available for free online.
Calorie counter component information: https://www.nature.com/articles/s41467-021-24173-x
Read the story: https://news.stanford.edu/2021/07/13/accurate-wearable-calorie-burn-counter/
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Stanford University: http://www.stanford.edu/