While heart rate is an easy thing to measure, what defines one's aerobic capaicity is really VO2 Max. It is the maximum capacity to transport and utilize oxygen during incremental exercise. (The derivation is V - volume per time, O2 - oxygen, max - maximum). It is also called maximal oxygen consumption or maximal oxygen uptake. It is also known as aerobic capacity, which reflects the physical fitness of a person. It is measured in millilitres per kilogramme of body weight per minute (ml/kg/min). Your VO2 max is to a large extent determined by your genes; but it can be increased by training.

Measuring VO2 max

Accurately measuring VO2 max involves runing or cycling with enough duration and intensity to fully tax the aerobic system. This is generaly done in an exercise lab with either a treadmill or cycle ergometer, in which exercise intensity is progressively increased while measuring ventilation and oxygen and carbon dioxide concentration of the inhaled and exhaled air. VO2 max is reached when oxygen consumption remains at steady state despite an increase in workload.

Cooper test

Dr. Kenneth H. Cooper conducted a study for the United States Air Force in the late 1960s. He develped the Cooper test in where someone runs for 12 minutes and the distance is measured. An approximate estimate for VO2 max (in ml/min/kg) is:

where d12 is distance (in metres) covered in 12 minutes.

There are several other reliable tests and VO2 max calculators (and this one at Runner's World) to estimate VO2 max if you run races. Using these type of calcuators I come up with a result for myself of around 49 ml/kg/min. If I go to this page on Running for Fitness, I see for my age I get a "superior" rating. I could even be much younger, and still get a superior rating, so it should be no surprise that I can still climb as fast as many 40 year olds. But since Lance Armstrong had a VO2 Max of around 84, I don't think they will be signing me up for the Tour de France anytime soon.

World Class Athletes

In sports where endurance is an important component in performance, such as cycling, swimming and running, world class athletes typically have high VO2 maximums exceeding 80 ml/kg/min for males, and a rare few exceeding 90 ml/kg/min for men and 70 ml/kg/min for women. Three time Tour de France winner Greg LeMond is reported to have had a VO2 max of 92.5 at his peak - one of the highest ever recorded

VO2 Max vs. Maximum Heart Rate

For most of us, it we can get a good approximation because there is a good relationship percent MHR and percent VO2 Max as shown in this chart. That will not tell us what our VO2 Max level is, but we can determine at any level what percent we are from the maximum.

Relationship Between Relative Heart Rate and VO2 Max
VO2 Max vs. Max Heart Rate
Percent MHR Percent VO2 Max
35 30
60 50
80 75
90 84
100 100

For more information read this article from VeloNews.

 
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