As I understand it (and hopefully an expert will correct me if I'm wrong) they measure the composition of the air you breathe in and the air you breathe out. Fr

As I understand it (and hopefully an expert will correct me if I'm wrong) they measure the composition of the air you breathe in and the air you breathe out. From that they figure out how much O2 you're converting into CO2 and hence how much energy you've consumed.