Definition and Intro
VO2 is a term that describes our body’s ability to deliver and utilize oxygen. It is a term that is thrown around amongst athletes and doctors alike. To doctors, VO2 can serve as a predictor of mortality in cystic fibrosis or in patients with heart failure. To an athlete, the maximum VO2 uptake is important as a predictor of performance and a measure of the efficacy and progression of training.
Elements of VO2
Our VO2 at any given time is dependent on three elements controlled by our body: heart rate, stroke volume per beat, and extraction of oxygen from the blood. Our body has the amazing capacity to increase our VO2 from 3 ml/kg/min at rest to over 80 ml/kg/min in athletes at peak exercise.
Mechanisms of Increase
Our body used both central (heart) and peripheral (muscle, blood) mechanisms to increase our VO2 on demand. Here is how:
Our heart has the amazing capacity to beat on its own without any outside input from our body…you heard that right. If our heart were left to beat at its own rate with no input, it would typically beat much faster than it does at rest, but our body slows it down. Thus, the first step in increasing heart rate is removal of the input that slows the heart, and the heart rate increases. This is especially important early in exercise and is almost instantaneous.
The second component to increasing heart rate is epinephrine (adrenaline) released by our adrenal gland as our muscles’ oxygen demand continues to increase with higher intensity exercise. This typically takes a little bit longer, and is important in the completion of all-out efforts.
Our body also extracts more oxygen from the blood during exercise. At rest we extract 25% of this oxygen content in our blood, but during exercise this can increase to 85%.
This happens because our red blood cells release more oxygen as the temperature increases and as lactic acid builds up. Also, our blood vessels dilate and more blood is delivered to the area of work under the influence of metabolites produced in the muscle.
This is probably the element most influenced by training over time. Our heart becomes more efficient as it becomes trained, and it can pump more blood with each beat. This is also the explanation for the decreased heart rates you see in athletes. If our heart pumps more with each beat, it doesn’t have to pump as many times to meet our demands.
This decreased heart rate in a trained athlete continues into the first stages of exercise, and it stays below that of an untrained person at most exercise intensities. Only near maximum effort does the heart rate finally rise to levels that would meet that of an untrained person, and when it does, that athlete’s cardiac output is much higher (equal rates with more volume per beat = higher cardiac output).
This is the protein in our red blood cells that actually caries oxygen. It is also highly affected by training and takes time to change. With increased hemoglobin, our blood can carry more oxygen.
Considering that the usual limitation in reaching VO2 max is oxygen delivery, not oxygen utilization by our muscles, one can see the importance of increasing our blood’s capacity to deliver, all in addition to our heart’s ability to increase delivery.
Distribution of Blood
At rest our body sends nearly 20% of our blood to our skeletal muscle. During exercise, as much as 80% of our cardiac output goes to muscle, with the remaining flow giving our other organs just the bare minimum to keep them functioning.
This is where one of the major difficulties with hot temperatures comes in. When it is hot, our body needs to send more blood to the skin to keep us cool. Where is that blood going to come from? Our organs are already getting a minimal amount, so that means we have to take from the blood flow to our working muscle and send it to our skin, and our performance decreases. Remember, our usual VO2 max limitation is delivery of oxygen, and hot temperatures further reduce delivery to muscles.
Associations with VO2 Max
Here are some markers that correlate well, either positively or negatively, with one’s VO2 max:
HDL cholesterol – this is our “good” cholesterol, and it has a positive association with VO2 max. That is, as HDL cholesterol goes up, VO2 max goes up.
LDL cholesterol, Body Mass Index (BMI), Smoking, and long-term Blood Glucose Levels – quite a long list, no doubt, and all of these factors, from “bad” cholesterol to BMI have a negative impact on VO2 max. As they go up, VO2 max goes down.
A beneficial off season activity is to go to your doctor and have all of those markers checked. As the season goes on and you get them checked again, you have a virtual window into how your VO2 max is coming progressing.
Obviously, these markers are players in much more important things than VO2 max, so it is a good idea to get them checked anyway. This way, you can use them to monitor your training to boot.
How to Measure
Measuring VO2 is a rather difficult process that requires a great deal of sophisticated instrumentation, in addition to time and effort. This is just another reason why utilizing the aforementioned markers may be a more realistic practice.
In a laboratory, the test can be performed while running, cycling, or rowing; the more muscle mass utilized, the better. The athlete breathes through a machine that senses the difference in oxygen between inspired and expired air, and it measures the volume of expired air. This information, plus a little math, and voila, you’ve got your calculation.
The test is carried out with increasing intensity over 6 to 12 minutes. This time frame is necessary to let the cardiac response reach a maximum (remember that adrenaline takes some time). The athlete stops the test when at maximum exertion, or it is stopped when VO2 levels out with increasing intensity, and you’ve got your number.
Another predictor that requires no equipment is the Cooper test. In this test you run 12 minutes all out, and you take the distance you ran (in meters) and subtract 505, then divide that number by 45, and you have a predicted value in ml/kg/min. Again, this is a gross estimation and not as personal as even tracking the progress of your markers.
Get those marker numbers now from your doctor and watch them improve throughout the year.
Cameron Chesnut is a Triathlon Coaching Associate with Trismarter.com.
Hill DW, Rowell AL. Running velocity at VO2max. Med Sci Sports Exerc. 1996 Jan;28(1):114-9.
Leger LA, Lambert J. A maximal multistage 20-m shuttle run test to predict VO2 max. Eur J Appl Physiol Occup Physiol. 1982;49(1):1-12.
Sandals LE, Wood DM, Draper SB, James DV. Influence of pacing strategy on oxygen uptake during treadmill middle-distance running. Int J Sports Med. 2006 Jan;27(1):37-42.
Unnithan VB, Timmons JA, Paton JY, Rowland TW. Physiologic correlates to running performance in pre-pubertal distance runners. Int J Sports Med. 1995 Nov;16(8):528-33.