Oxygen Deficit and Excess-Postexercise Oxygen Consumption

Oxygen Consumption (VO₂)

Oxygen Consumption (VO₂) | Maximal Oxygen Consumption (VO_2max)

Oxygen Deficit (O₂D) and Excess Post-Exercise Oxygen Consumption (EPOC)

Oxygen Consumption (VO₂)

One characteristic of living animals is that they all give off heat. As a result of cellular respiration and cellular work, heat is produced. An operational definition of metabolism is the rate of heat production which describes the metabolic rate.

	respiration		work
foodstuffs + O₂	———————>	heat + ATP	———————>	heat

The direct measure of heat production, called direct calorimetry, is a technically difficult problem so an alternative method is to measure the volume of oxygen consumed or utilized by the body for ATP production. The determination of the metabolic rate from the measure of oxygen consumption is called indirect calorimetry.

Measuring oxygen consumption (VO₂) under submaximal and steady-state exercise allows for estimate of the energy cost, and indirect calorimetry is the method most frequently used. This method assumes all of the energy expended for the exercise is reflected by the magnitude of VO₂.

Indirect Calorimetry

Amount of VO₂ reflects total energy expenditure (if measured under steady-state conditions).
Most common expression of VO₂ is (mL/kg/min) [mL of O₂ consumed per kg body weight per min]

Maximal Oxygen Consumption (VO_2max)

Oxygen consumption (VO₂) is linearly related to the workload and as exercise intensity increases, VO₂ increases proportionally. However, there comes a point at which the VO₂ ceases to rise even though the exercise intensity continues to rise. This point is referred to as the maximal oxygen consumption (VO_2max) and is considered to be the benchmark of maximal aerobic power. It represents that maximal amount of ATP that the aerobic system can produce.

The VO_2max assesses the maximal ability of the body to deliver and utilize oxygen and is related to the ability to perform prolonged exercise. Genetic factors and training regulate the various physiological factors that contribute to the body's ability to transport oxygen.

By far, the best method to assess aerobic capacity is to measure VO₂ directly in the laboratory while maximally exercising the subject. Various protocols can be used which usually yield similar results. However, treadmill tests generally give higher values than cycle protocols. This is probably due to the fact that most individuals are accustomed to walking or running but not to cycling. While laboratory testing using indirect calorimetry is the most accurate method to determine maximal aerobic capacity, the procedure is expensive and time-consuming. Field tests were developed in order to test large numbers of subjects more quickly and easily and were based on their correlation with laboratory data. Cooper's 12-minute and 1.5-mile runs are two of the most widely known and used field tests. However, these tests also require a highly motivated subject exercising to voluntary exhaustion in order to maximize their predication ability. Not all individuals have the motivation to perform a maximal test and certain contraindications prohibit maximal testing of an individual. Consequently, tests to estimate VO_2max were devised based on the heart rate (HR) response at a submaximal workload. These methods commonly utilize bench stepping, cycle ergometry, and walking/running protocols, and are able to quickly test large groups of individuals. Some of the more well-known prediction tests include the Harvard Step Test and the Åstrand-Rhyming nomogram.

Oxygen Deficit (O₂D) and Excess Post-Exercise Oxygen Consumption (EPOC)

Knowledge that PCr and anaerobic glycolysis also contribute ATP to muscles without using oxygen makes the above statement confusing. Oxygen can be viewed as the "currency" the body uses in order to purchase ATP. In other words, oxygen must be used in order for ATP to be produced. The aerobic energy system operates on a "pay-as-you-go" principle as it must always have oxygen when it forms ATP.

However, even though the other two systems don't use oxygen, they should not be viewed as producing "free" ATP. After PCr rephosphorylates ADP, it requires energy (ATP) in order for it to be rephosphorylated back to PCr. Also, lactate must be metabolized, and while some of it is converted to glycogen, most of it during exercise is converted back to pyruvate which then enters the aerobic pathway and produces ATP. Moreover, these processes occur during exercise which ultimately uses oxygen and contribute to the total amount of oxygen consumed. Thus, these two energy systems only "borrow oxygen on credit" and must eventually pay back the deficit.

An individual beginning to exercise needs sufficient ATP immediately in order to perform the movement. However, the aerobic energy system is slow to increase its ATP production and unable to provide the all necessary ATP for up to several minutes. This can be observed by the gradual increase of VO₂ until it reaches a steady state. That portion of the exercise in which the aerobic system is unable to provide sufficient ATP is termed the O₂ deficit. Studies of this phenomenon led to the conclusion that the body increases reliance on its energy reserves (i.e., ATP-PCr and anaerobic glycolytic systems) during this period in order to meet the energy demands of the muscles.

With the onset of a submaximal exercise, VO₂ increases gradually, as shown above, and reflects ATP contribution by the aerobic system, indicated by the red area. Within 3-5 min, the VO₂ reaches a steady state indicating that the aerobic system is supplying all the energy required by the muscles. However, at the onset of exercise, muscles require more energy than can be provided by the aerobic system. Thus, the immediate and anaerobic systems contribute ATP to the muscles, but their contribution isn't reflected by VO₂. The area indicated by O₂D in the above figure is defined as the oxygen deficit and reflects energy supplied to the muscles by non-aerobic systems.

Thus, after VO₂ has reached steady-state, the aerobic system is providing all of the ATP even though anaerobic glycolysis and the immediate energy systems are operating. Lactate, a metabolic product of anaerobic glycolysis is largely converted back into pyruvate in ST fibers which then enters the aerobic system. Also, by this time, the immediate energy system is providing essentially no ATP.

Furthermore, after cessation of exercise, VO₂ doesn't immediately return to resting levels, rather, VO₂ makes a gradual decrease even though energy demands are only at resting levels. Oxygen consumption during the exercise recovery is above what is needed to maintain the resting metabolic rate. The term excess postexercise oxygen consumption, or EPOC, is used to describe the elevated VO₂ during exercise recovery, and the amount of EPOC is usually greater than the amount of the O₂ deficit (compare the areas of the O₂ deficit and EPOC in Figure 5.12). After exercise, the body is thought to "pay back" the "O₂ debt" along with some "interest." The decrease in VO₂ is alinear and includes both a rapid and slow component.

Causes of the EPOC are numerous. The rapid component is thought to reflect energy needed to resynthesize depleted ATP and PCr stores. Also occurring during this phase is the replenishment of oxygen to the myoglobin. The role of myoglobin in the muscle is similar to that of hemoglobin in the blood. Myoglobin transports O₂ from the hemoglobin to the mitochondria as well as serving as a small oxygen storage site.

Mechanisms of the slow EPOC component are more complex. Some of the components responsible for the slow phase include elevated temperature, catecholamines, and replenishment of muscle glycogen. Another interesting hypothesis is that the mitochondria sequesters some of the Ca²⁺ released during contraction which causes an interference with, or uncoupling of, the oxidation phosphorylation process. This decreases the efficiency of the aerobic system in that more O₂ is required to produce ATP. All of these mechanisms serve to further increase the recovery VO₂.

Data from the above figure was collected during Fall 1998 on a pilot study that investigated the effects of creatine supplementation on the O₂D and EPOC. Cyclists worked at a "supramaximal" intensity, one that had energy requirements greater than what could be supplied by the aerobic system. Needless to say, subjects were working extremely hard as reflected by the high blood lactate concentrations we measured. The dotted line was an estimate of the total energy required at this cycling intensity and O₂D is the difference between the estimated total energy expenditure and VO₂ (blue line). The O₂D represents energy contributed solely by anaerobic glycolysis and the immediate energy systems. The tan area is the amount of energy subjects required for the resting metabolic rate determined by measuring VO₂ at rest. After cessation of exercise, subjects' energy requirements return back to resting levels, yet VO₂ remained elevated because of the various metabolic "disturbances" to homeostasis. EPOC represents the excess amount of oxygen consumed above that needed for rest.