Maximum oxygen consumption, also referred to as VO2max is one of the oldest fitness indices established for the measure of human performance. The ability to consume oxygen ultimately determines any human’s or animal’s ability for maximal work output over periods lasting greater than one minute. The higher the number, the greater the potential work rate.
Determinants of Maximal Oxygen Consumption
In essence there are three major factors determining maximal oxygen consumption:
- Cardiac output (the volume of blood pumped by the heart in one minute)
- The oxygen carrying capacity of the blood (determined by hemoglobin in red blood cells)
- The amount of exercising skeletal muscle and the ability of muscle to utilize supplied oxygen
Cardiac output is defined as the mathematical product of heart rate times stroke volume. Heart rate is nothing more than the number of times the heart contracts per minute. Stroke volume represents the amount of blood ejected by the heart with each beat.
The ventricles of the heart fill with blood during a period of time called diastole. The ventricles are maximally filled at a time just before the heart contracts called end diastole. The contraction phase is referred to as systole. When the ventricles are maximally emptied (there is still some blood remaining in them) this period is referred to as end systole.
Consequently, stroke volume equals (end diastolic volume) – (end systolic volume). Very healthy hearts empty a greater percentage of their end diastolic volume, but rarely does this ejection fraction represent more than 90% of end diastolic volume.
Cardiac output in a resting individual of average size is about 5 liters/minute. In an untrained individual heart rate is about 72 beats per minute so stroke volume is about 70 milliliters. Maximal heart rates are related to age and appear to be unrelated to the level of fitness. The rule of predicting maximum heart rate by subtracting age from 220 is good for a rough index of maximum heart rate but it is far from precise, and may differ by 20 beats or more for individuals of the same age!
Stroke volume also typically increases with exercise, and maximal cardiac output in highly trained individuals may attain 40 liters/minute. The ability to generate high maximal cardiac output is a major determinant of the ability to have a high maximal oxygen consumption.
Oxygen Carrying Capacity
Red blood cells, the principle cellular component of our blood, carry an iron containing protein called hemoglobin. Hemoglobin has binding sites for oxygen, and when blood is in the presence of high concentrations of oxygen (as in the pulmonary circulation), oxygen is avidly bound to hemoglobin. When blood reaches portions of the circulation that utilize oxygen for energy production (such as skeletal muscle during exercise) the oxygen is less avidly bound to hemoglobin and leaves the red blood cell for consumption by the tissues.
In most individuals, the amount of hemoglobin in the blood is about 15 grams/100 milliliters of blood. Each gram of hemoglobin can bind about 1.34 milliliters of oxygen so 100 milliliters of oxygen carries about 20 milliliters of oxygen after it has passed through the lungs. Depending on the speed with which blood passes through metabolically active tissues, oxygen levels in the blood can be below 3 millileters per 100 milliliters. The ability of tissues to take oxygen from the blood is referred to as extraction of oxygen.
Anemia refers to a condition where red blood cell mass is reduced and the blood carries less oxygen as a result. Anemia can lead to a reduction in maximal oxygen consumption due to a decreased supply of oxygen to the working muscle, but is usually manifest through a feeling of malaise during everyday activity and workouts.
Blood doping is a term most frequently applied to the augmentation of red blood cells through unnatural pathways. Initially this was accomplished by taking blood from an athlete some time before competition, allowing enough time for the athlete to restore the amount of removed blood cells through the body’s own mechanisms, and then reinfusing the blood shortly before competition. This allows the body to carry more red blood cells, and consequently more oxygen, than it normally would. It has been difficult to document improvements in maximal oxygen consumption by this practice, but repeated endurance performance has been rumored to improve. Some very fine young athletes have also died by this practice.
Most recently a hormone made by the kidneys called erythropoietin, has been produced through recombinant DNA techniques. The chief therapeutic use of this substance is for persons with anemias of chronic diseases such as renal failure. Nevertheless, since this substance is indistinguishable from the hormone that is naturally made, it has also been used for illegal augmentation of performance.
Living at high altitude also increases the amount of erythropoietin made by the kidneys and dwellers at high altitude (typically greater than 7000 feet) have more red blood cells per volume of blood. For these adaptive effects of altitude to take place, a period of at least 2 weeks, and more likely one month, must elapse. Short daily exposures to high altitude (low barometric pressure and resultant decreased oxygen content of the air) will not cause an increase in red blood cell mass.
Skeletal Muscle Mass
Of the three factors determining maximum oxygen consumption, the most important in terms of training adaptations is the role of skeletal muscle. The larger the mass of exercising skeletal muscle the greater the potential for increasing whole body oxygen consumption. Also, the manner in which the skeletal muscle has been trained and the muscle fiber type will influence the ability of the muscle to extract oxygen.
Endurance training, loosely defined as exercise lasting 20 minutes or more, stresses the aerobic systems of skeletal muscle. Important enzymes in aerobic metabolism are augmented by this form of training as are enzymes involved in the metabolism of free fatty acids, by far the most energy rich substrate stored by the body. Muscles trained in this manner have a greater ability to extract oxygen from the blood because they use it faster, and they typically are more richly endowed with capillaries, the portion of the circulation which brings blood to adjacent individual muscle fibers. When muscles are trained by endurance exercise, they are contracting at a small percentage of their maximal tension. High intensity contractions, like those associated with strength training, do not train the aerobic enzyme systems of skeletal muscle.
The muscle fiber type also will influence both the ability of the muscle to be aerobically trained and the resultant maximum oxygen consumption. Type I, or slow twitch, fibers are naturally endowed with more oxidative (aerobic) enzymes and mitochondria, the place in the cell where aerobic metabolism takes place. They also have more capillaries per fiber area and as a result can supply more oxygen to the muscle fiber. Type II muscle fibers are less well adapted for aerobic work but can still be trained to augment key aerobic enzymes. They also have a reduced capillary to fiber area ratio. (This is not a discussion of muscle fiber types, and has been vastly oversimplified.)
Therefore, the larger the mass of exercising, trained, type I muscle, the greater will be the oxygen utilization on a whole body level.
Since larger individuals typically have more muscle mass than smaller ones, they will typically have a higher maximum oxygen consumption expressed in liters O2/minute. This figure is important in sports where movement of the body against the force of gravity is of little consequence. An example of this is in a sport like crew (rowing) where “heavyweight” crews are historically faster over a 2000 meter course than “lightweight” crews. In fact large, highly trained aerobic athletes can have a VO2max in excess of 6 liters/minute.
Because body size can have such a dramatic impact on VO2max it is often expressed after adjustment for body weight, in ml O2/kilogram/minute. Elite endurance athletes like runners, who rarely weigh over 150 pounds, often have VO2max values in excess of 80 ml O2/kg/min and we have measured values as high as 90! The average 40 year old male with no specific training experience might have a value around 35-40 mls/kg/min. Females of the same age typically have values about 5 mls/kg/minute less, for a myriad of potential reasons. Professional football players average about 50 ml/kg/min although there is a good deal of variability depending on position. Elite soccer players average around 60 ml/kg/min.
Facilities like ours at NISMAT have the ability to directly measure oxygen consumption while a person is exercising. A person wears headgear which contains a non-rebreathing valve which the person holds in the mouth, like a snorkel. Room air is inhaled through the valve and air which is exhaled goes through a tube into a metabolic measurement cart. The cart measures the amount of oxygen and carbon dioxide in the exhaled air, as well as the volume of air. Knowing that room air contains 20.93% oxygen and 0.03% carbon dioxide, the amount of oxygen consumed can be computed after correction for barometric pressure, humidity and temperature.
The test consists of a person walking or running on a treadmill, pedaling a stationary cycle ergometer, rowing on an ergometer or hand cranking an upper body ergometer. The intensity of the work increases on a regular basis, usually every one or two minutes and continues until the subject can go no further. A true maximum effort is difficult and takes a great deal of motivation from both the person administering the test and the subject; it is also important to have proper resuscitative equipment and personnel on hand in the even that the subject has a problem during the test. The best indicator of a maximal effort occurs when the work rate increases but oxygen consumption does not increase over the previous work rate. Good tests of maximum oxygen consumption take about 10-15 minutes of exercise.
Many facilities like health clubs determine oxygen consumption by estimating it. Since work rates on a treadmill or bicycle are known, and average oxygen costs for maintaining these work rates have been measured, one can apply these equations and estimate maximum oxygen consumption. As shown in the graphs below, there are well-known relationships between neart rate and VO2max and work rate and VO2max. Thus, by measuring one’s heart rate and knowing the work rate for a particular activity, VO2max for an exercise bout may be estimated. These are imprecise measurements and subject to a great deal of subject variability with respect to how efficiently a person exercises as well as measurement error associated with inaccurate calibration of the exercise equipment.
Questions About Maximum Oxygen Consumption
- Is maximum oxygen consumption affected by blood donation?
We examined this question and were able to show that although maximal work capacity is reduced for about two weeks following donation of 450 cc’s (1 unit), maximal oxygen consumption is not consistently reduced. This apparent paradox may be due to the fact that anaerobic performance is influenced by blood donation, and has some influence on maximal work capacity, but we can’t be sure.
- How much can an untrained individual expect to improve maximum oxygen consumption with three months of steady training?
With three weeks of endurance training, three times per week for 30 minutes/day at heart rates about 70% of maximum, increases of 15% are typical. Exercising harder, more frequently, and longer will boost that increase up a little as well.
- Is maximum oxygen consumption genetically determined?
Yes, even highly motivated individuals who train diligently are unlikely to attain the high values mentioned above in elite runners. However, few people ever train hard enough to achieve their genetic potential.
- Does maximum oxygen consumption decrease with age?
Unfortunately, the answer to this is also yes. In longitudinal studies of elite runners who continued to train, oxygen consumption declined with time. Certainly consistent training throughout life will lessen the effect of aging on maximal oxygen consumption.
- Are there health benefits to having a high maximum oxygen consumption ?
In the broad sense of health benefits, the answer to this is yes. Epidemiological studies have shown that individuals with the lowest maximum oxygen consumption are more likely to experience a greater likelihood of all cause mortality. Contrary to this, people at the highest end of the spectrum do not necessarily live longer than people with average values.
A higher maximal oxygen consumption translates into a greater submaximal work capacity, which in turn means greater caloric expenditure in the same time while you are exercising or doing physical labor. It also means that you will feel less tired after doing the same amount as a less fit individual.