Does VO2 Max Define you as a Runner?

Two years ago I was tested for VO2 max. I blogged about it at the time and you can read all the details here. As I said in the post, VO2 max is an important measure of aerobic endurance:

VO2 max is simply the maximum volume of oxygen your body can take in:

the key to muscle performance is delivery of oxygenated blood to the muscles, and adequate fuel to produce energy. Oxygenated blood comes from the lungs, and is pumped by the heart, which is itself a muscle requiring its own supply of oxygenated blood.
VO2 max can be seen as the maximum performance of this system: the amount of oxygen that can be effectively delivered to the body in a given time period.

As your workout gets harder and harder, you can’t continue to breathe harder and harder; there is a fixed amount beyond which your body can’t keep up. That fixed limit is your VO2 max. All that’s needed to find it out is a way to measure oxygen intake, and a way to systematically make a workout harder.

World-class endurance athletes have VO2 max values in the 70s and higher. The average male in his 20s has a VO2 max of around 44. At age 45, mine came out as 56.1 -- not bad!

But of course I wondered if I might improve it further, and I've worked hard in the past couple of years, improving my 5K and 10K PRs by over a minute since I took that test. This spring I had the opportunity to participate in another research study, and, as before, I was tested for VO2 max.

The procedure was similar: Start out walking uphill on a treadmill at a slow pace (in this case, 2.7 mph and 12 percent grade). This time my speed and slope was increased every three minutes (last time it was every minute, but this shouldn't matter for this test). A facemask allowed the researchers to measure the air I breathed in and exhaled. Here's the one I wore last time:

Don't I look fetching? Though I'm not sure blue is my color.

After about 12 minutes it started to get pretty difficult, and shortly thereafter I was completely exhausted and asked to stop.

So what was my result? I was a little surprised when I saw the researcher write "50.9" on my sheet. Really? Lower than before? It didn't seem possible. I looked back at my records to reconfirm that I really had been tested at 56.1 two years ago. I just PRed this year in the rain at the Lake Norman Rotary 10K with a 39:01. Two years ago on a nearly-identical course that was a little short, in perfect conditions, I ran a 39:18. It hardly seems possible that my VO2 max has declined by 10 percent during that time.

I'm looking into why the two values might be different but I think the take-home message for now might be this: If you do get your VO2 max tested, don't think that number defines you as a runner. Not only is VO2 max not the sole determinant of your running ability, it's likely that the test itself isn't always accurate. Every measurement involves some error, but the bigger error as a runner would be placing too much faith in a single number.