Exercise Metabolic Testing: More Than Just Bragging Rights – Part 2
Posted Jan 13 2013 8:56am
The EMT and VO2max test use similar equipment and test 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, a test subject is only taken to approximately 85% of peak aerobic capacity, since higher exercise intensities do not yield much useful information acquired for the endurance athlete. But prior to the 85% intensity mark, several important variables are collected during the EMT, including Energy expenditure. In an exercise physiology lab, the actual amount of energy, or calories, that the body uses during exercise can be measured via a calorie measurement that is either “direct” or “indirect”. A direct calorie measurement would require exploding the body inside a closed chamber and measuring the amount of heat that is released. For most triathletes, this does not conjure up a pleasant image.
However, the mask, tubes and gas analyzer used during an EMT offer a far more humane and indirect method of calorie measurement. This measurement is based upon the principle that the ratio of carbon dioxide produced to oxygen consumed will allow an accurate calculation of the total number of calories used for energy, as well as the percent contribution from both carbohydrate and fat.
Thus for any given intensity, including perceived exertion, heart rate, wattage, speed or incline, an athlete can attain a precise calculation of exactly how many carbohydrate and fat calories that are burnt for energy! This can be highly valuable for determining the number of post-workout calories necessary to replenish fuel stores. In addition, several exercise physiology studies have indicated that most athletes can replace approximately 30-40% of the calories that they burn. This principle can allow an individual who knows total caloric expenditure to generate a customized race-fueling plan.
Aerobic threshold. At a specific exertion level during the EMT, the body reaches a point of maximum fat burning, which is called the aerobic threshold. For every endurance athlete, and especially for Half Iron and Iron distance triathletes, the aerobic threshold value signifies the point of maximum endurance efficiency. This is because the human body can only store approximately 1500-2000 calories of carbohydrate, but can store over 30,000 calories of fat! Therefore, at the aerobic threshold, there is little risk of “bonking”, “hitting the wall”, or running out of carbohydrate energy.
By working at aerobic threshold heart rate, pace, or power for long, slow distance workouts and races, an athlete can be confident that the body can last for hour after hour with limited fatigue. In most individuals, the aerobic threshold is reached at 50-60% of maximum intensity, or about 20 heartbeats below anaerobic threshold.
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 at a rate faster than it can be removed. As the lactic acid builds, the body’s production of hydrogen ions begins to increase. The only way to buffer these acidic hydrogen ions is via formation of carbon dioxide, which is then “blown off” 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% intensity, which is the rationale for only bringing an EMT test subject to this point. Above AT, the body begins to consume large amounts of oxygen, which results in rapid fatigue and drainage of valuable carbohydrate stores. In training or racing, most endurance athletes spend very little time at such 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.
In the final part of this series next week, you’re going to learn exactly how you can use the data you get from these three values to become a better triathlete.
Ben Greenfield is the Renaissance man of the sport of triathlon.
He's a fast triathlete, a coach, a personal trainer, and much more more.