One can actually measure the work done in cardio - chest HRM or pod can measure running through acceleration, and bikes and rowing machines can have power meters, measuring work applied to the drive axle. It's not the consumed energy but the aerobic COP does not change much so it's easy to estimate the consumed energy from the work done. Because the power output during an activity can vary by orders of magnitude, the time in activity is a very poor estimate IMHO.
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