Recently, I did a metabolic stress test (often referred to as a VO2max or a cardiopulmonary stress test) on a local elite athlete. The test results were outstanding for this individual. In fact, the results were some of the highest that I’ve seen on a test subject. The only other one higher was on a classmate of mine in grad school and he was a Division I cross country star. The results were also significantly higher than I’ve ever achieved.
The test got me thinking about the basic physiology of an aerobic athlete and what makes one faster or more powerful than another. Why are some individuals “natural” aerobic athletes while others struggle to hang on to the back of the pack?
WARNING! Geekdom at it’s finest approaching
VO2max is the ability to consume oxygen and utilize it for aerobic glycolysis. The muscles use oxygen in aerobic glycolysis to break down stored carbohydrates and eventually produce ATP (adenosine triphosphate). The high energy phosphate bond in ATP is what allows muscles to contract.
Man, I wish I had a syringe of ATP to inject into my legs during cross. Kinda like nitrous oxide for your legs. Unfortunately, that wouldn’t work and would probably kill you.
Anyways, back to the nerdy stuff, if the cardiovascular and pulmonary system cannot sustain the volume of oxygen required by aerobic glycolysis, the body will begin to call upon anaerobic glycolysis to help keep up with the muscle’s demand for ATP. Aerobic means with oxygen and anaerobic is without oxygen. Anaerobic glycolysis isn’t as efficient as its bro aerobic glycolysis and only provides a fraction of the ATP from the same amount of carbohydrate. So anaerobic glycolysis is kinda like the friend you call as a last resort to pick you up from downtown when you’ve had too much to drink. You really don’t want to go there, but you have no choice.
Anaerobic glycolysis results in lactic acid (actually lactate and hydrogen ions) as a nasty byproduct of producing those valuable ATP’s. That’s the familiar burn we’ve all felt on numerous occasions and especially during cross country races, crits and cross. So as the body is unable to provide sufficient volume of oxygen to the working muscles through aerobic means, the balance swings from aerobic to anaerobic glycolysis. As a result the muscles and lungs begin to suffer. Eventually if a person “red lines” for too long the muscles will shut down. The hydrogen ions from the lactic acid combine with bicarbonate ions and form carbonic acid which spontaneously dissociates into water and carbon dioxide. The increase in water isn’t such a bad thing, but the increase in carbon dioxide eventually has detrimental effects. A lot of folks don’t know this, but carbon dioxide is actually what triggers us to breath. Not a lack of oxygen.
Now there are several ways one athlete can be superior to another in various physiological systems which leads to a higher VO2max. Huge lungs help. Also a great diffusion capacity (ability to transfer oxygen and carbon dioxide to and from the lungs and the blood) is vital. Any kind of pulmonary disease (asthma, bronchitis, emphysema and smoking) can hinder the delivery of oxygen to the blood.
After the lungs come the heart and vascular system. Having a big strong and efficient pump (heart) is also critical, but if the plumbing is shitty the delivery of oxygen is hindered. Atherosclerosis (heart and vascular disease) will narrow those arteries and make them less compliant and impede delivery of oxygen rich blood. Pressure is also higher in small pipes which can lead to greater endothelial insult and subsequently more atherosclerosis. It’s a vicious cycle.
Now once at the starving muscles, AVO2difference (arterial venous difference) is the next critical physiological variable. The AVO2difference is simply the difference in the oxygen content of the arterial blood and venous blood. The arterial blood drops off oxygen to the working muscles and become venous blood (artery away from the heart and venous towards). The venous blood goes back to the heart and then the lungs to pick up more oxygen and start the cycle over again. Some individuals have higher AVO2differences than others. This was the great debate when I was in grad school. What is the deciding factor in a high VO2? There was the central theory (the big pump – heart) and the peripheral theory (AVO2difference). I personally think you gotta have both, but there really is no way to easily measure AVO2difference potential. The heart can be assessed rather easily with echocardiogram, CT and even catheterization.
Some folks just have better genes for dropping off oxygen. If you got Norwegian genes, consider yourself lucky. The highest ever recorded VO2max results were from Norwegian cross country skiers. Records for VO2max are 96 ml/kg/min for men (Bjorn Daehlie) and conflicting records for females, but most state 80’s. Greg Lemond was 92.5 ml/kg/min at his peak and I’ve read that Lance was near 94 ml/kg/min. Just to put it into perspective, dogs used in the Iditarod can be as high as 240 ml/kg/min. Just think how fast you would be if your Mom was a canine. Maybe Dr. Moreau was on to something.
Bottom line is the really great aerobic athletes are the ones who were given great genes from their parents. A lot of one’s ability is predetermined at birth. Training can dramatically improve initial performance, but once an athlete is trained it is very difficult to increase the VO2max.
Now once the VO2max is determined, the one thing that can be shaped to increase performance is anaerobic threshold. I’ll save that discussion for another day.