Tuesday, December 29, 2009

VO2max Testing

One way of assessing fitness is with a number called VO2max.  The higher your VO2max, the more intensely you can exercise, and in general the VO2max of someone that is well conditioned is higher than someone who isn't in such great shape.

Ok, but what is VO2max? 

VO2max is the maximum amount (in milliliters) of oxygen that a person can use in one minute, per kilogram of body weight.  Brian Mac explains it in depth on his website, so if you want to know more that's where I'd suggest you go. 

Why do I care what my VO2max is?

The primary purpose for the fitness assessments I've been doing in this year's preseason is the establishment of a baseline of fitness.  I want to be able to repeat this tests at later dates and refer back to them so that I can evaluate how well my training is going.  Quantifiable metrics are the name of the game here.

Additionally, a more accurate VO2max will allow me to more accurately estimate my calories burnt in gym sessions with my Polar HRM, and that should hopefully help with my nutrition.

Polar HRMs use a secret sauce for calorie burn estimation, but it is thought to have once resembled this polynomial (although now Polar uses resting HR as a variable and that isn't expressed below):

Let C = KCals and weight = the person's weight in Kgs.

Men: C/min = (-59.3954 + (-36.3781 + 0.271 x age + 0.394 x weight + 0.404 x VO2max + 0.634 x HR))/4.184
Women: C/min = (-59.3954 + (0.274 x age + 0.103 x weight + 0.380 x VO2max + 0.450 x HR)) / 4.184

So clearly, the amount of calories your HRM reports burnt is rather dependant on VO2max.  

Estimating VO2max

So, outside of lab testing, how can you accurately estimate your VO2max?

There are quite a few ways, however the one I chose based on the environment I had available was the Balke VO2max test.

You can read about the testing protocol in detail at the above link, but essentially it's very simple.  Run around a track as fast as you can for 15 minutes and record your distance.  You're supposed to use a 400m track, and my track was 9 laps/mile (roughly 179m/lap).  In 15 minutes, running as fast as I could, I covered 3168 meters. 

Using this formula:  (((Total distance covered ÷ 15) - 133) × 0.172) + 33.3 gives me a result of 46.75 mls/kg/min VO2max

How accurate is this number?  Well it's probably good enough to set my heart rate to, and additionally as long as I repeat this exact test in the future it will serve as a measure of improvement to some degree.  According to Mac's website the correlation between this number and  actual VO2max is high as well.  

Estimating Caloric Burn

What does this mean for my energy expenditure estimations?  Well, the default setting on my HRM is a VO2max of 36, which would be about the norm for a non athletic 30-39 female, which is perhaps a safe bet for the typical Polar F6 consumer, but I believe it was causing my HRM to grossly under report calories burnt for me.  

For example, I recently did a bike ride averaging 150 W for 60 minutes.  Assuming 22% efficiency, which is conservative at best, that would indicate a calorie burn of:

150W = 150 J/s
3600 seconds * 150 J = 540 KJ / 4.184 (conversion rate between KJ and KCals) = 129 KCals

129 KCals / .22 (22% efficient, yeah I wish I was that good...) = 586 Calories burnt.  However in this same time frame my Polar F6 was reporting about 286 Calories burnt.  

I'm expecting that my new and more accurate VO2max will greatly improve the estimated calories burnt, but we will find out when I bike tonight...

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