Indirect calorimeters, like those used for VO2 max testing with a mask, utilize the same technology as larger metabolic chambers to calculate energy utilization and substrate source.

“I use an indirect calorimeter all the time measuring V2 Max which is another type of it's it's basically the exact same technology but it's easier because you're strapping The Mask to your face so you have perfect access to exactly the uh O2 consumption and the CO2 production but it's doing the exact same calculation it's going to tell you exactly how much energy you're utilizing and um where the substrate comes from”