It would be interesting to know the particulars, but in general you hook up a cyclist up to a dynamometer and a ventilator, and you measure both the amount of w

It would be interesting to know the particulars, but in general you hook up a cyclist up to a dynamometer and a ventilator, and you measure both the amount of work done and the amount of CO2 produced. I say "cyclist" because they're the easiest ones to hook up to a dynamometer.