VO2max without mask

Maximal oxygen consumption or maximal ventilation VO2max is one of the important parameters determining your aerobic fitness. Unfortunately measuring your VO2max is costly and awkward because you have to perform a gradual increasing power test until exhaustion in a testing lab, with an unpleasant gas analysing mask on your face.
How nice would it be knowing your VO2max without these tests, only using some power data. This question was treated thoroughly in a study by Jurov Cvijic et al. “ Predicting VO2max in competitive cyclists: Is the FRIEND equation optimal choice ?” Jurov lists 5 different equations of which he concludes that indeed the equation given by The Fitness Registry and the Importance of Exercise National Database is the best. But this does not really solve our problem because in this equation we still need the maximal power that is realized in this laboratory test.

Many authors have identified Critical Power CP as the gold standard of performance analysis, and any serious cyclist knows his CP from his own field rides and his self-tested best power values. Because CP is identical to maximal aerobic power MAP we can compute VO2max from MAP using the following equation

VO2max(ml/kg/min) = 12.6 MAP(W/kg) + 7

How do we get these numbers 12.6 and 7 ?
The first important number is the energy obtained from 1 ml of oxygen. When oxidizing glucose we get 20,7 Joule from 1 ml of oxygen. When lipids are burned we get only 19,6 J/ml and from the burning of protein we get 19.2 J/ml
Because riding at near VO2max levels will burn glucose almost exclusively, we will use the value 20.7 J

Next the energy obtained from the glucose must be used to perform the desired mechanical work. The Gross Efficiency GE is equal to the ratio of the useful mechanical work to the total energy. For well trained and healthy cyclists GE is approximately equal to 0.23 which means that only 23 % of the available energy is effectively used. Thus from the burning of glucose we get only 20.7 x 0.23 = 4.76 J/ml of mechanical work. Inversely we need 1/4.76 = 0.21 ml/second to produce 1 W, or 0.21 x 60 = 12.6 ml/min to produce 1 W

How we get the extra number 7 ml/kg/min ?
This number is composed of 2 contributions.
The first contribution is the oxygen consumption we need to simply' ‘stay alive’ and at rest i.e. to keep the heart pumping, the blood circulating, the organs working properly. This oxygen uptake at rest is called the Metabolic Equivalent MET and it is rqual to 3.5 ml/kg/min.
The second contribution is for simply moving the legs. Supose you are on one of those home trainers with your back wheel on a roller. You forgot to close the roller and start pedaling without any resistance . Sure your heart rate and breathing will go up. This pedalling without performing any useful work wil cost one extra MET.

Individual variations

This equation will yield an approximate value of your VO2max but individual variations may occur between 5 and 10 %. The main source of insecurity lies in the use of a standard value for the gross efficiëncy GE = 0.23
Highly trained young athletes may have this GE but most “‘common” subjects have lower GE and thus consume more oxygen.
Also women are supposed to have somewhat lower GE, but scientific results on this subject are scarce.