At what percentage of the maximum oxygen uptake is there a plateau in stroke volume?

Learn about VO2 max and maximal oxygen uptake in athletes and the relationships between cardiac output, pulmonary, muscle, oxygen, and exercise intensity characteristics in athletes.

Exercise Physiology | Muscle Contraction | Muscle Fibers | Muscle Adaptations | Exercise Fuels | CHO Metabolism | Fat Metabolism | Oxygen Uptake | Cardiovascular Exercise | Respiratory Responses | VO2 Max | Temperature Regulation | Heat | Fluid Balance | Fatigue | Sprinting | Endurance | Genes | Practical Case Example

Maximal Oxygen Uptake – VO2 Max 

In the final lecture of this module, we’ll focus on the maximal oxygen uptake or VO2 max. The most widely accepted measure of cardiorespiratory fitness. VO2 max is really the reflection of the combined abilities of respiratory and cardiovascular systems to deliver oxygen to the contracting muscle and of those muscles to consume oxygen. If we look at the relationship between oxygen uptake and power output, we see this linear relationship. However, depending on the level of training, the level of fitness, and indeed the genetic endowment, the VO2 max level is attained at varying levels.

VO2 Max 

Learn about VO2 max as defined by the maximum volume of oxygen that can be recruited during high-intensity exercise expressed as a value in milliliters of oxygen expended within one minute per kilogram of your body weight. VO2 max testing is used as a tool to determine your capacity to perform mild through highly intense exercise expressed within a defining value. This test is used, among others, to estimate your cardiorespiratory, aerobic, fitness and measure of endurance. Values as a result of VO2 testing range by age, gender, and fitness level. The norms range from very poor, less than 20, to very highly trained elite endurance athletes scoring 90 or slightly above.

In sedentary individuals, it’s relatively low. The value here, 30, is the VO2 max expressed in milliliters of oxygen, a kilogram of body mass per minute. If we look at normally active individuals they might be in the range of the 40s to the 50s. If we take those individuals and put them on a training program we can expect to see an increase of anywhere from 10 to 20% depending on the starting level. If we look at highly trained endurance athletes we see very high values up here in the 70s and 80s and in some cases they’re very low 90s. These very high values probably represent genetic endowment and hard physical training. Both are required to really reach these very high levels.

Sections 

1. Maximal Oxygen Uptake in Athletes
2. Physiological Determinants of VO2 Max
3. Oxygen Delivery – Mitochondria & VO2 Max
4. Oxygen Delivery & VO2 Max
5. Pulmonary Limitation to VO2 Max
6. Systemic O2 Delivery, Cardiac Output & VO2 Max
7. Limb Muscle Blood Flow, O2 Delivery & VO2 Max
8. Cardiac Output Limits Leg Muscle Blood Flow During Maximal Exercise
9. Central Circulation & VO2 Max
10. Muscle Capillary Density
11. Muscle Diffusion Capacity
12. O2 Transport Variables & VO2 Max

Maximal Oxygen Uptake in Athletes 

In cross-sectional studies that have been taken on various athletic groups, you can see that the VO2 max is generally higher in those groups where there’s a heavy reliance on the oxidative energy systems and the cardiorespiratory systems. So in cross-country skiers, in distance runners, speed skaters, these are not the speed track speed skaters but more the endurance type speed skating, orienteering and running, cycling, all very high values in the 70s, 80s. I think the highest values that have been reported in the literature are 92 for a very well trained cross country skier. In the less aerobically dominated events you can see lower values, and in the untrained values, as I said, 40 to 50. With a period of endurance training, we could expect an increase in that VO2 max value for nontrained individuals.

Physiological Determinants of VO2 Max 

If we think about the factors which determine the VO2 max, most of the attention is focused on the oxygen delivery pathways and factors within the muscle. In recent years it’s been suggested that such an approach ignores the important role of the central nervous system in driving people to exercise recruiting the muscles. And there’s no question that the brain has an important role in motivation during a max test can drive subjects or athletes to a high level. But in terms of limiting the VO2 max, a lot of attention, as I said, is focused on the oxygen delivery pathway, the respiration and the ability to maintain arterial oxygen saturation, the central circulation, the maximal cardiac output, and the oxygen-carrying capacity of the blood, the ability to distribute that blood into the contracting muscles, and finally, the ability of the muscles themselves to consume that oxygen, which is going to be determined by the amount, the extent, of mitochondria, the oxidized fuels, and the fiber type, and the capillary density.

Oxygen Delivery – Mitochondria & VO2 Max 

If you look at the relationship between oxygen delivery and the oxidative capacity of the muscles in a small muscle group exercise where the maximal cardiac output is not challenged, the oxygen delivery is more than adequate for the oxidative capacity of those arm muscles. In contrast, if you look at exercise involving the legs and a larger muscle mass, and you begin to approach the maximal limits of cardiac output, the oxygen delivery is somewhat less than the oxidative capacity of the muscle, and this implies that during large muscle mass exercise it’s the oxygen delivery which is really right limiting for the maximal oxygen uptake.

Oxygen Delivery & VO2 Max 

Here’s the relationship between the VO2 max and oxygen delivery derived from the earlier slide that I showed you have redrawn with VO2 max now as the dependent variable and oxygen delivery. We define oxygen delivery as the arterial oxygen content times the cardiac output. And we know that cardiac output is the product of heart rate and stroke volume. And you can see for those various groups that were listed earlier, sedentary, active, trained, and endurance athlete, with an additional group here, studied after a period of bed rest, or de-conditioning, you can see the relationship between VO2 max and oxygen delivery, and it’s for this reason that there’s been so much attention focused on the ability to deliver oxygen, particularly, the arterial oxygen content of the blood that’s being delivered to the contracting muscles, I’ll talk about that a little bit later.

Pulmonary Limitation to VO2 Max 

We look at the respiratory system, we saw in the previous lecture, in most healthy people, exercising at sea level, the arterial oxygen content is reasonably well maintained, the arterial oxygen saturation. In some individuals, however, as we discussed, you can see this desaturation, and this is what occurred when these subjects were breathing in the air at 21% oxygen, the normal atmospheric oxygen content. Clearly, if you reduce the inspired oxygen partial pressure, for example, if you go to altitude, you see a more pronounced desaturation and that’s not surprising. But what is surprising, to some, was this desaturation of 21%. And as I said, the most likely explanation for this is pulmonary diffusion limitation. You’d see large increases in cardiac output, the maximal cardiac output after training, there’s relatively little change in the morphology and the diffusion characteristics of the lung and this reduction in transit time challenges the ability of the lungs to fully oxygenate the pulmonary blood. If you increase the oxygen in the inspired gas so that these subjects are now breathing 26% oxygen, you can say that you prevent the saturation and you see a slight increase in the VO2 max, so for some individuals, there may be a pulmonary limitation to the VO2 max. As I said, however, most healthy people exercising at sea level, the lungs are not thought to be a limiting factor for maximal oxygen uptake.

Systemic O2 Delivery, Cardiac Output & VO2 Max 

There is a plateau in oxygen delivery, and here’s the relationship I showed you earlier between oxygen delivery and VO2, and you can see at the higher intensities here, this leveling off. This corresponds with a leveling off in cardiac output, and the reason that cardiac output levels off, is that stroke volume has leveled off or even declines slightly at maximal exercise. And although heart-rate can increase slightly up to its maximal level, it is this slight reduction in stroke volume, which means that cardiac output can no longer increase.

Limb Muscle Blood Flow, O2 Delivery & VO2 Max 

This has the consequence of limiting the leg blood flow and leg oxygen delivery. And it’s thought that this is an important factor in determining the maximal oxygen uptake.

Cardiac Output Limits Leg Muscle Blood Flow During Maximal Exercise 

Another experiment that’s been used to investigate these is using small muscle group exercises. So a single-leg exercise where just the knee extensors, of the lower leg, were used, that two to three kilo of active muscle mass, compared with two-legged cycling, either incremental exercise or very intense exercise up to maximal levels. And you can see with the single-leg knee extensor the ability to increase cardiac output and maximally perfuse the small muscle mass is really quite free and is not limited. In contrast with the two-legged cycling, the increase in cardiac output is somewhat constrained. If you then look at the increase in leg blood flow, you can see at the higher intensities with the larger muscle mass the inability to maintain the trajectory of increasing blood flow and a leveling off. These are some of the data in the literature that suggest that oxygen delivery, as determined by the maximum pumping capacity of the heart, is an important factor in determining VO2 max.

Central Circulation & VO2 Max 

The central circulation can influence the maximal cardiac output and the amount of oxygen that is in the arterial blood that can be delivered to the contracting muscle. Alterations in blood volume have an impact on end-diastolic volume, maximal stroke volume, and therefore maximal cardiac output. Plasma volume expansion acutely, in untrained subjects, has been shown to increase the VO2 max. And similarly, loss of central blood volume and dehydration reduces maximal stroke volume, reduces maximal cardiac output. Over the years there’s been some interest in interventions that are aimed at increasing the red cell mass. Blood doping, which involves the removal of a small volume of blood, storage in appropriate chemicals, and then reinfusion, to increase the blood volume and increase the red cell mass, has been associated with an increased in VO2 max, and an increase in exercise performance. And it has been outlawed by various sporting authorities as a form of doping. Erythropoietin, a hormone released from the kidney in response to low oxygen levels, stimulates the production of red cells and was a popular chemical used to try and improve endurance performance. Similarly, that’s been banned. And altitude training, one of the adaptations to exposure to altitude, is to increase red cell mass. And all of these factors are designed to increase the oxygen-carrying capacity of the arterial blood. Especially in well-trained subjects who, as part of their training, that see an increase in their blood volume and a slight reduction in the hemoglobin concentration. So these other interventions have been trialed to increase red cell mass on the back of an expanded blood volume.

Muscle Capillary Density 

Increasing cardiac output has to be distributed to the muscles and there is an association between maximal oxygen uptake and the capillary density of the muscle. So, not surprisingly, not only is there an increase in the maximum cardiac output with training as we saw in the cardiovascular lecture, but also an increase in the capillary density, so that the distribution of that cardiac output into the contracting muscles can be optimized.

Muscle Diffusion Capacity 

The diffusing capacity in the tissues is also important. The ability of the oxygen to move from the red blood cells in the capillary through the tissue fluid into the muscle. And this is influenced by a number of factors, the surface area of the capillaries, the capillary network, the surface area of the mitochondrial network, the tissue fluid levels, the distances, and all of this is rolled together into a term known as the muscle diffusion capacity. And we know that it does change as a result of exercise training and contributes significantly to the increase in VO2 max following training.

O2 Transport Variables & VO2 Max 

It’s important to understand that all of the variables involved in oxygen transport to contracting the skeletal muscle, and indeed, the abilities, the ability of the muscle to consume that oxygen all contribute to oxygen, a maximal oxygen uptake. And as is, I said at the outset, also the ability to fully activate and recruit those skeletal muscles. Here’s an overview of changes in VO2 max that could be predicted to occur with changes in various parameters. And they listed here, the cardiac output, the diffusing capacity of the lung, the diffusing capacity of the muscle, the hemoglobin concentration, and the alveolar ventilation. All of these factors contribute importantly to maximal oxygen uptake. On balance, however, most of the evidence suggests that the ability to deliver oxygen to the contracting skeletal muscles is the most important determinant of VO2 max.[11].

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