I used this 12 Minute Run Calculation and it gave me a VO2 Max of 33 and said Average for my age (75). My fitness program (Fitbit) says I am Excellent and records a VO2 Max of 44. I run around 5 miles nearly every day and don't see many 75 year olds able to do this. Is there a reason for the discrepancy?
First let me congratulate you on keeping up your active lifestyle. Most people your age have the mistaken notion that they're too old for vigorous exercises, not to mention a five mile a day routine.
I understand your confusion. The most accurate method of measuring your VO2 will be on a treadmill where you run until exhaustion with a mask strapped to your nose and mouth to measure the oxygen you consume. You can contact Universities with an exercise physiology laboratory in your area to check availability and pricing.
For certain diagnoses, your doctor may order a maximal exercise test on the treadmill using the Bruce or Balke protocols (See Cardiorespiratory Endurance Tests). Even if they do not measure oxygen directly, you can also estimate your VO2 indirectly: Maximum Treadmill Cardiovascular Test. With an indirect measurement a formula is derived from a set of correlational data using curve fitting techniques. So with this formula, the calculator can predict VO2 from time to exhaustion with an acceptable degree of accuracy for men (R=0.90).
Both the Fitbit and 12 minute run also estimate VO2 indirectly. For the 12 minute mile we've provided the references to the study so you can see how they've created a formula based on correlational data. You would have to refer to Fitbit documentation to see how they have estimated VO2. However, I would not be surprised if they do not fully disclose their methods based on trade secrets.
There are several potential sources of inaccuracies for indirect measurements of VO2. For the 12 minute run for example, your ability to push yourself to your maximum overall speed may play a role (potentially underestimating your value). Another factor that causes potential inaccuracies include measurements that use heart rate to estimate VO2 max, which are likely used in the Fitbit algorithm. Even if heart rate is measured accurately at a submaximal intensity, accuracy is based on predicting maximal heart rate so Heart Rate / VO2 correlational values can be extrapolated to derive VO2 Max. If they use 220-age, for example, SEE is ± 12 bpm for younger individuals, SEE and ± 15 bpm for older individuals, which could potentially result in a substantial error of estimate (over or under estimation of VO2).
In addition, if you happen to be on any medication which affects heart rate (such as heart medications, or even certain allergy medications), algorithms that utilize heart rate to calculate VO2 can be inaccurate (over estimation for medications that lower HR and under estimation for medications that increase HR). Here are some commonly used medications (but not all inclusive) showing their effect on heart rate (under medications column): Pharmacology
I suspect that in your case, that the 12 mile run test has underestimated your cardiovascular fitness, for one reason or another, based on your reported activity level. However this does not mean that the 12 mile run is not a reasonably accurate and valid test for most people, given its relatively high relationship to actual VO2 values. The bottom line is that both calculations you are comparing are not direct values, but are estimates and those values will likely be different for some of the reasons explained. To monitor your progress simply use the method that makes most sense for your situation and be aware of what factors can influence test results. And if you are really curious to see what your MaxO2 is by a direct measurement, see if you can schedule an Max VO2 treadmill assessment with a nearby University exercise physiology lab. But even gold standards have their own issues of practicality (eg: expense and convenience), if not of accuracy, reliability, or validity (depending upon your objectives). Also see 'Developing a Fitness Testing Battery'.
Hope that sheds some light on considerations in estimating your VO2 Max.
Thank you very much for your detailed reply. I will certainly look into the areas you suggest. I had my doubts about Fitbit and I do believe it probably relies heavily on a set heart rate formula. My RPR is around 42. Although in my youth I was a sprinter and long jumper rather than a distance runner. I have done around 20 marathons the last around 25 years ago. I am now going to do more 12 minute runs As you say I don't think I actually push myself enough on such runs. Thanks again for your advice. Best wishes.