Originally Posted by RChung

Well, *mostly* all you need to really know is that cardiac stroke volume for the kinds of guys that would read Friel's "Fast after 50" book is ballpark about 70 mL of blood per beat, assuming average ejection fraction. Then, cardiac output is just stroke volume * beats/minute, so if you know your max heart rate you can approximate your max cardiac output. (Resting heart rate helps a tiny bit because lower resting heart rates are associated with slightly higher stroke volume, but this is a relatively small effect). If you don't have some disease that affects the oxygen-carrying capacity of your blood (and once again, guys who're reading "Fast after 50" probably don't) then the amount of O2 being carried by the blood and the amount that it can deliver to muscles for usable work is also ballpark known. So, for each 100 mL of blood at max cardiac output, you can carry around 5 mL of O2. So you can kinda get from max HR to max O2 your heart can deliver to your muscles. So you can sorta kinda get to VO2Max in terms of Liters/minute. I think most of the guys reading "Fast after 50" aren't going to be 50kg or 150kg, so maybe 70 or 75kg puts his readers in the right ballpark.

So there's some (that is, a lot) of hand-waving going on but I can sorta trace back where Friel's rule of thumb comes from, and, importantly, the error bounds on his rule of thumb. (I teach a lot of rules of thumb to my students, but I go through the derivations so they can see what and where I'm making assumptions and which ones are more critical and which are less so. Friel drives me nuts cuz he doesn't do this, and it offends my sense of pedagogical fidelity).

You can do better by measuring power, but many of the readers of Friel's book (at the time of publication) probably didn't have a power meter. [Edited to add:] At average levels of cycling economy, it takes in the ballpark of 1 L of O2 per minute to produce 75 watts for a minute, so it takes around 3 L of O2 to produce 225 watts (or 4 L to produce 300 watts). So the (slightly better) way to calc your VO2Max from power data is to figure out your max sustainable aerobic power, convert that to an approximate volume of O2, and estimate your VO2Max that way. You don't really need to know HR that way at all. That said, VO2Max isn't a particularly actionable piece of information.]