Originally Posted by
sfrider
VO2max is interesting as a measure of cardiorespiratory fitness (stroke volume, gas exchange, blood O2 carrying capacity, etc), but I don't know how they can factor out muscular efficiency. They don't know your blood volume or lung volume, to begin with. If I were to guess they look at energy over some time window at LT and derive metabolic threshold energy, and from that infer how much O2 is consumed at LT. I doubt that's better than a rough fitness estimate. For training it's not very useful (other than you want to keep your cardiorespiratory fitness pegged, or training will suffer - but that's easy, and beyond that... meh).
VO2Max measurement pre-dates power meters, and is a summary measure of overall aerobic production of energy that could be measured without getting into the weeds of exactly how that energy was produced. What Firstbeat (and others) are trying to do is to find a way to approximate VO2Max from power data