Friday, May 11, 2007

VO2max and body fat percentage

Yesterday I found a relationship between cycling performance, oxygen transfer, and body fat percentage that I have always missed.

We'll start with oxygen transfer. I think I have a solid understanding of VO2max: it's the maximum amount of oxygen your body can process expressed in milliliters per kilogram of body weight per minute. Below is a detailed description of VO2max. In a later post I'll describe how I plan to use my VO2max testing result (67 ml/kg/min) from yesterday to better structure my bike training.

Variables determining a person's VO2max include lots of body processes and physical characteristics. I'm not formally trained in any of this stuff, I'll probably leave some things out and my info might not be perfect, so take it with a grain of salt, but here are a few of the variables:

  • Lung capacity - every time you breathe air into your lungs, air comes in contact with the interior surface of your lungs, which is flooded with 'used' oxygen-deficient blood that just came from working muscles. The concentration of oxygen is higher in the air than in the blood, so some oxygen diffuses into the blood, which is pumped back to the working muscles. So the area of contact between the air and the blood is important. The greater your lung capacity (volume of air you can inhale), the greater oxygen transfer you get with every breath. An average male can breath about 6 liters of air. Miguel Indurain's lungs can hold 8 liters.
  • Health of lungs - lots of things can damage your lungs. The obvious ones are smoking or working in an asbestos factory, but others such as having pneumonia, getting lots of chest colds, living in a big city, or living downwind of a large coal-fired power plant can also have effects. Lung health deterioration is one of the primary reasons that VO2max is expected to decline by about 0.5 ml/kg/min per year after age 30.
  • Blood volume - the volume of blood in your body doesn't change much with weight changes, but does increase somewhat with extended athletic training. The more blood you have the more oxygen you can move from the lungs to the muscles.
  • Heart capacity - hearts come in different sizes and strengths. I think heart size is mostly genetic, but exercise can strengthen the heart's muscles and allow it to pump more blood. Indurain's heart could pump 50 liters per minute (13 gallons/minute) of blood. For comparison, flow from a typical residential shower head is 2 or 3 gallons per minute, so think of 20 shower heads flowing at once. That's a LOT of blood!
  • Artery size - The bigger the 'pipes' carrying blood from your heart to your muscles, the more blood that can flow and the more oxygen that can catch a ride. I've seen pictures of artery cross sections for untrained average males compared to those of highly trained athletes. The difference is quite dramatic. Arteries in trained athletes can dilate to a much greater size (it seems like I remember that it was more than double) when higher blood flow is required.
  • Capillary density - The more you exercise hard and deprive your muscles of oxygen when they need it, the more your body will respond by growing capillaries to distribute more oxygen in the right places within the muscles. This is one of the primary reasons why athletic training is so sport-specific. A world-class long distance runner might have great lung capacity, heart capacity, and arterial size; but if he hasn't developed high capillary denisty in his cycling muscles, he may be bettered by even moderately-trained cyclists due to their specific training.
  • Cellular efficiency - The efficiency with which your cells convert the oxygen to sugars that your muscles can use to contract is important and can be improved through training. Changes in the number and efficiency of cell mitochondria and the efficiency with which you can buffer lactic acid are two examples.

That's what I knew about oxygen transfer before yesterday. But there is a significant factor related to body weight and body fat percentage that I have always missed or ignored. When analyzing cycling performance, I have always looked at body weight just the way I would look at the weight of the bike - just mass to be pulled up the hill in the gravity equation. I had always assumed that where accelerations and climbing were not a part of the equation (like a long, flat time trial), that body weight was not a real factor in performance except for the relatively small effect it has on body size and increased frontal area/wind friction. My friend Jeff, who won the Georgia state TT jersey for Cat5 last year, is powerful and he's not fat by any definition, but one look at him and you'd know he'd play tackle, not flanker. The fact that he won the TT jersey on a relatively flat course didn't surprise me a bit - it make perfect sense that he could excel in a discipline where I thought his power was important but his weight didn't matter much. But now I realize that there's more to look at in a flat TT than just power production.

What I know now is that blood volume is pretty constant for a given training level. So let's look at this scenario: In the spring I weigh 147 lbs and train 15 hours per week. My body fat percentage is 9.1 percent. I reach a certain fitness level by April and I can average 268 watts for a flat April TT, riding at maybe 23.5 mph. During the summer I maintain the same training regimen, but I add a pint of Chunky Monkey to my diet every other night. By September, I'm still well-trained and fit, and I have the same equipment, but I weigh 160 pounds and have a little excess fat (maybe 14% body fat). I ride the same TT course again, but this time I only produce 245 watts and my speed falls to 22.1 mph. Why did this happen?

It happened because in the fall I have to distribute my blood volume over more body tissue. I have 13 pounds of fat in my body in the fall that wasn't there in the spring (13 pounds is equivalent to 52 sticks of butter). Fat is blood dense, so there is a lot of my blood hanging out in that 13 pounds of fat that can't be used to carry oxygen to my cycling muscles, so I can't generate as much power and I go slower.

The end result is that body weight and body fat percentage has more of an effect on cycling performance than I had previously thought.

3 Comments:

Anonymous said...

What do you weigh? I do long-distance swimming and noticed that my lung volume increased substantially from changes in my training.

I typically was doing 800m front crawl everyday in at heated pool at 81 F where I had to turn every 25 m, as fast as i could and hitting an oxygen wall where breathing every 3 strokes I ran out of oxygen and had to stop and catch my breath at some point in the run. But I upped my distance to 2.5 miles instead of 800m and did the swim in a 68 F freshwater turning every 200m instead of 25m like in pool. Within a few weeks suddenly no more oxygen problems, I could breathe every 3 strokes for hours at a pace faster than I was able to achieve on my 800 m time. Instead of a 16-17 minute 800 m I was doing an 56 sec 100 m and an 11.5 minute 800 m. Maybe hypothermia helps, when your body is cold you need less oxygen? I think doing long-distance swims in cold water (68 degrees and over 2 miles) greatly increased my lung capacity although mild hypothermia is something I've experienced doing this.

I had pushed up to swimming 2.5 - 3.5 miles a day 5-7 days a week when I tore a muscle in my chest. When I had a chest x-ray the doctor asked me if I was a professional long-distance swimmer or a commercial diver. I said a swimmer and he said you have what we call diving lungs, they are very large, expand out very far, but also taper down very far in the chest, and we see it a lot in people like that.

I'm sure as I had x-rays from before I began that training regime (2 years previously) that my lungs were not as expanded, they were healthy but otherwise unremarkable. Actually I think it's the 2+mile swim fast & daily in cold water for at least 3 months that does it.

Anonymous said...

sorry i meant not what do you weigh but how tall are you.

Anonymous said...

You have a good understanding of VO2max, however, I'm not sure if you know how to assess it. You may be interested in the research performed by Vsetulov√° E, Bunc V. "Effect of body composition on physical fitness and functional capacity in obese women". While this is specific to women, you might learn why you cannot conclude that body fat percentage has a relationship with VO2max. Also, adipose tissue is far less vascularized than other tissues, so your cardiac output wouldn't increase if you were to put on more fat mass. An increase in cardiac output would be advantageous and would help improve your VO2max, since Q x (a-vO2) equals VO2max. Keep testing, maybe you'll find the answer that many people want to know