Aerobic Endurance Measures and Analyses

The following is an exclusive excerpt from the book Developing Endurance, published by Human Kinetics.All text and images provided by Human Kinetics.

During most endurance competitions, athletes rely heavily on energy developed through the aerobic system. For this reason, the training plans for many endurance athletes place almost exclusive focus on the development of aerobic fitness and endurance. The VO2max test is the most effective measurement of the body’s ability to deliver and use oxygen for producing energy that can be used by the muscles. VO2max (also known as maximum aerobic power) simply stands for the maximal volume of oxygen (O2) that can be used.

This measurement is important because the more oxygen an individual can consume, the more energy (ATP) can be produced for the muscles to use to contract. For the athlete to move faster, more energy must be available to enable a muscle to contract more rapidly, contract with more force, or some combination of these two. Therefore, the more energy an athlete can liberate through the aerobic system, the faster the athlete can move.

Measurement of this variable requires the use of devices that measure oxygen and carbon dioxide along with a monitor of breathing volume and rate. The process involves measuring the amount of oxygen consumed with each breath, the amount of carbon dioxide produced by the athlete, and the amount of air that the athlete is breathing in and out. All of these measures are used to calculate the actual extraction of oxygen from ambient air for use by the muscles to generate usable energy.

Keep in mind the basic calculations that are used to derive oxygen consumption values. Basically, oxygen consumption (VO2) is equal to cardiac output multiplied by the difference in arterial and venous blood oxygen (A-V O2 difference) concentrations.

This is expressed by the following equation:

VO2 = cardiac output (Q) × (A-V) O2 difference

Cardiac output is determined by the following:

Cardiac output = heart rate × stroke volume

Therefore, oxygen consumption can also be expressed this way:

VO2 = heart rate × stroke volume × (A-V) O2 difference

As exercise intensity increases, cardiac output rises, and extraction of oxygen from the blood by the muscles increases. This results in increasing VO2. At some point, maximal heart rate (and therefore, cardiac output) is achieved, and VO2 plateaus with increasing work rate. The highest rate of oxygen consumption measured is typically defined as VO2max.

A typical VO2max test begins with a warm-up of 10 to 15 minutes of relatively easy effort followed by a progressive test to exhaustion. The progressive portion of the test should take between 6 and 12 minutes depending on when the athlete reaches fatigue. Typical cutoff points for a VO2max test include reaching volitional fatigue, reaching a plateau in VO2 with increasing work rate, and reaching a respiratory quotient (VCO2/VO2) greater than 1.10. The test data gathered during a VO2max test may include the measurement of the ventilatory threshold (VT) as identified by characteristic changes in respiration rate (VE) versus VO2. The ventilator equivalents of O2 and CO2 (VCO2 and VO2) versus VE may also be plotted to identify the ventilatory threshold.

The ventilatory threshold is typically used to identify the maximum sustainable effort that an athlete can maintain. It is often correlated to the lactate threshold, which is identified by measuring blood lactate concentrations during progressive exercise testing. The definition of the lactate threshold point varies from lab to lab and from one physiologist or coach to another; however, as long as consistent methods are used to identify the threshold, comparisons from one test to another can be made. Other factors to consider when comparing test data from one lab to another include the elevation of the test facility, the length of the stages used, the increases in work rate from one stage to another, the warm-up protocol, the pretest nutrition and hydration instructions, and the amount of rest before the testing.

Measurement of oxygen consumption via indirect calorimetry (oxygen uptake) can also be used to calculate energy expenditure during exercise. These calculations can determine the total calories of energy oxidized as well as the percentages and amount of carbohydrate and fat being used at each workload. This kind of analysis is very helpful for athletes who want to calculate actual energy expenditure in order to monitor nutrition intake (e.g., athletes with goals for body composition). These data are also helpful for athletes competing in long-distance events where glycogen stores are likely to be depleted during competition; these athletes can use the information to devise appropriate fueling and pacing strategies. The best scenario is when an exercise physiologist and a registered dietitian (one who has sport nutrition experience) can work together to present these data to athletes and help them devise proper nutrition goals.

Lactate profile testing can be performed with or without oxygen consumption or indirect calorimetry measurements. As previously mentioned, the method used to determine the threshold point is less important than using a consistent testing protocol. The lab at the Boulder Center for Sports Medicine (where Neal Henderson works) has performed thousands of lactate profile and VO2max tests on all levels of athletes. These athletes have included NHL ice hockey teams, Olympic cyclists and triathletes, recreational runners, and individuals with cardiovascular and pulmonary disease. The researchers at this facility have found that good results are provided when the testing method involves using 4-minute stages, starting at a level that allows for seven to nine stages of progressive intensity, and monitoring heart rate, rating of perceived exertion (RPE), and blood lactate at each stage. In addition, the quality of the lactate-measuring device cannot be overlooked, and daily calibration and proper maintenance help ensure consistent results.

Figure 2.3 provides the lactate profile of a professional cyclist tested at the start of the season. All stages were 4 minutes long. The top line being plotted is heart rate (right vertical axis), and the bottom line is lactate level (left vertical axis). Heart rate is linear as workload increases. In other words, there is a direct relationship between heart rate and workload. Lactate level increases in a linear manner as workload increases until the seventh stage; at that point, a sharp upturn or increase in lactate occurs, indicating lactate threshold. The goal of training is to move the upturn to the right on the graph—that is, the goal is for lactate threshold to occur at a higher workload.

Figure 2.3: Lactate Profile of a Professional Cyclist

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