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Metabolic Testing: The VO2max

  • By Paul Moore
  • Published August 21, 2009

Photo: John Segesta

A triathlete’s VO2max number can be a good basis for bragging rights in the endurance sports arena. If you know this number, which defines your maximum aerobic capacity, and it’s a good number, you can talk trash while hanging at the pool edge, chatting in spin class or stretching on the track.

But as a measurement that originated in the second century with Greek physicians blowing into sheep bladders, is the VO2max really that valuable?

Think about it this way: Having a big VO2max number is like owning a Ferrari engine. Imagine that this Ferrari engine is dumped into an old pickup truck chassis. It is useless. Unless the engine is perfectly installed in an actual Ferrari automobile, it will never perform to its full potential.

Likewise, unless you have the ultimate combination of economy, efficiency and experience, your high VO2max is worthless.

The process of determining VO2max is demanding and unpleasant. The test begins with the attachment of a mask or other breathing device to your face. Through this mask, a computer measures your exhaled and inhaled gases during a graded exercise protocol on the bike or treadmill. The test concludes when you reach an extreme state of fatigue, at which point an exercise physiologist excitedly scribbles down some numbers that represent your maximum aerobic capacity, typically expressed in millilitres of oxygen per kilogram per minute.

So if the process is so uncomfortable, and a high VO2max number is useless without a Ferrari body, why even bother to test? Indeed, if all you’re looking for is the bragging rights that come with a high VO2max, you may want to save your cash for a couple of good racing tubulars.

But before you abandon the idea, consider the following: Many athletes do not realize that a significant amount of important test data is collected before reaching the terminal point of maximum pace, peak power and puke factor in a VO2max test. Some of this other data is the focus of the exercise metabolic test, or EMT, a close cousin of the VO2max test protocol that has been used in nutrition physiology for many years.

The EMT and VO2max test use similar equipment and protocols, but the focus of the EMT is caloric utilization and specific physiological data points rather than peak aerobic capacity. In fact, during an EMT, the test subject is taken to only approximately 85 percent of peak aerobic capacity, since higher exercise intensities do not yield much useful information for an endurance athlete. But prior to the 85 percent intensity mark, several important variables are collected during the EMT, including:

1) Energy expenditure. In an exercise physiology lab, the amount of energy, or calories, that the body uses during exercise can be measured using an indirect calorie measurement. A direct calorie measurement would require exploding the body inside a closed chamber and measuring the amount of heat released. For most people, this obviously does not conjure up a pleasant image.

However, the mask, tubes and gas analyzer used during an EMT offer a far more humane if less direct method of calorie measurement. It is based on the principle that the ratio of carbon dioxide produced to oxygen consumed will yield an accurate estimate of the total

number of calories used for energy, as well as the percent contribution from both carbohydrate and fat.

Photo: John Segesta

Therefore, for any given intensity, whether measured by perceived exertion, heart rate, wattage, speed or incline, an athlete can obtain a precise calculation of exactly how many carbohydrate and fat calories are burned for energy.

This information is valuable in determining the number of post-workout calories necessary to replenish fuel stores. In addition, several exercise physiology studies indicate that most athletes can replace approximately 30 percent to 40 percent of the calories they burn during exercise. This principle can allow an individual who knows total caloric expenditure to generate a customized race-day fueling plan.

2) Aerobic threshold. At a specific exertion level during the EMT, the body reaches a point of maximum fat burning called the aerobic threshold. For every endurance athlete, and especially for half-Ironman and Ironman distance triathletes, the aerobic threshold value signifies the point of maximum endurance efficiency. This is because the human body can only store approximately 1,500 to 2,000 calories of carbohydrate but can store more than 30,000 calories of fat. Therefore, at the aerobic threshold, there is little risk of “bonking” or “hitting the wall”—that is, running out of carbohydrate energy.

By working at aerobic threshold heart rate, pace, or powering through long, slow distance

workouts, an athlete can build confidence that his or her body can work for hour after hour with limited fatigue. In most individuals, the aerobic threshold is reached at 50 percent to 60 percent of maximum intensity, or about 20 heartbeats below anaerobic threshold.

3) Anaerobic threshold (AT). The AT is nearly synonymous with the lactate threshold (LT). This is because at a certain point during exercise, blood lactate begins to accumulate in the muscles faster than it can be removed. As the lactate builds, the body’s production of hydrogen ions begins to increase. The only way to buffer these acidic hydrogen ions is through the formation of carbon dioxide, which is then exhaled by the lungs and can be measured via a gas analyzer. During the EMT, a significant increase in carbon dioxide production signifies a physiological point very close to LT.

In most individuals, AT occurs at about 85 percent intensity, which is the rationale for bringing an EMT test subject only to this point. Above AT, the body begins to consume large amounts of oxygen, resulting in rapid fatigue and drainage of valuable carbohydrate stores.

In training or racing, most endurance athletes spend very little time at such high exercise intensities. Thus, knowledge of AT or LT is paramount for any endurance athlete who is concerned about proper pacing and avoiding the dreaded bonk. Once these three EMT data points are collected, a customized nutrition, training and racing profile can be created.

Ben Greenfield is director of sports performance at Champions Sports Medicine (Champsportsmed.com) in Spokane, Wash., where he manages bicycle fitting, run gait analysis, swim stroke analysis, blood lactate testing and exercise/resting metabolic analysis. As the head coach for Pacific Elite Fitness (Pacificfit.net), he trains all levels of triathletes from around the world and hosts a free nutrition and performance podcast at bengreenfieldfitness.com.

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Paul Moore

Paul Moore

Paul Moore is the Online Editor for Triathlete Europe. When not glued to a computer he can be found writing books - most recently Ultra Performance: The Psychology of Endurance Sports and The World's Toughest Endurance Challenges. Both are available on Amazon. Paul has also written Ultimate Triathlon: A complete training guide for long-distance triathletes which is also available on Amazon.