How does oxygen reach the muscles?

In order for oxygen to be used by our muscles it must first be transported from the air to the mitochondria. The oxygen in the air we inhale is transported deep into the lungs where it is transferred into our blood through fine blood vessels, or capillaries. The red blood cells arriving in the lungs via these capillaries have no oxygen left, but are loaded with carbon dioxide.

As the blood passes through the lungs, this carbon dioxide is released and exhaled. At the same time, the oxygen we have inhaled passes into the blood in the capillaries. A small proportion is dissolved directly in the fluid portion of the blood, but most is taken up by the haemoglobin molecule, which has a high affinity for oxygen, and is the main form of oxygen transport in the body. This oxygen-rich blood is then transported throughout the body via the blood stream.

As we exercise the oxygen required by the active muscle can increase, almost instantaneously, by as much as 20 times. The requirements of inactive muscles remain unchanged. This is achieved through dilatation of the blood vessel in the active muscles. Simultaneously, the rate at which fuel enters the mitochondria increases. This results in a selective increase in the energy and oxygen supply to active muscles.

Another mechanism which increases the blood supply to active muscles is called the "splanchnic shunt". In this process, blood is shunted away from less active tissues, particularly those in the splanchnic (gut) circulation, and redirected to the active muscles.

VO2 max: what is it?

Oxygen consumption increases with increasing work rate. How does this happen? As exercise becomes harder, the muscles bring in more myofibrils to produce more and more powerful muscle contractions. This requires increasing amounts of energy, which in turn requires a greater supply of oxygen.

Early research into the relationship between oxygen consumption and exercise suggested that just before an individual reaches his or her maximum work rate, the rate of oxygen consumption reaches a plateau and does not increase further, ie the individual can not take up any more oxygen. This is regarded as the maximum rate of oxygen consumption or VO2 max, usually expressed in millilitres of oxygen per kilogram of body weight per minute.

VO2 max is measured during exercise on a treadmill or stationary cycle. You start off at a low work rate which is then increased in small steps at regular intervals, until you are so exhausted that you can't carry on. The highest rate of oxygen consumption measured is the VO2 max. So, VO2 max is the maximum rate of oxygen consumption an athlete reaches when running or cycling at the maximum work rate achieved during exercise where the work rate is progressively increased over a period of 8 to 15 minutes.

What is the significance of this concept to fitness? Initially, it was believed that an athletes performance could be predicted by his or her VO2 max. This relied on the idea of the postulated plateau in oxygen consumption suggesting that the muscles had developed oxygen deficiency, which would explain the exhaustion. If this is so, then, for example, the fastest runners would be those with the greatest capacity to take up and use oxygen, ie those with the highest VO2 max. Then the limitations in exercise performance could be explained in terms of oxygen delivery to the muscles and/or oxygen use by the muscles. So, the high VO2 max of the best athletes would be either because they had superior hearts which could pump more blood to the muscles, or they had superior muscles, which could take up large quantities of oxygen. So, anything increasing the delivery of oxygen to the muscles, or uptake of oxygen by them, so delaying the development of this plateau, would increase performance.

However, this plateau in oxygen consumption is only found in less than 50 percent of people tested. Also, recent work suggests that no true oxygen deficiency develops in muscles with exercise. The absence of this plateau does not suggest that the athletes can continue to take up oxygen for ever, only that there is plenty of oxygen available to the muscles when they become exhausted.

Tim Noakes has now suggested that it is a muscle factor, not a cardiovascular (heart and blood vessels) factor which limits maximum work rate. He believes that the rate of oxygen transport is not the critical factor determining exercise performance. Rather, the best athletes have muscles with superior ability to contract, either because of better myosin ATPase activity, or increased capacity to bind calcium. They can achieve higher work rates and so higher rates of oxygen consumption during exercise. As a result their VO2 max will be high, leading to the false impression that it is their high VO2 max in itself which predicts performance. However, it is in fact the better muscle contractility which will lead to a high VO2 max.

It seems that it is actually the maximum work rate achieved which is the best predictor of performance, and this depends on the ability of the muscles to contract. This may have little or nothing to do with the rate of delivery of oxygen to the muscles.

However, VO2 max is still a popular talking point among those who exercise, particularly among runners. Use the term with caution, recognising that it is only an indirect equivalent of maximum work rate. The measurement of VO2 max does give something which allows comparison of athletes in different sports in which maximum work rate may be difficult to measure.

Is VO2 max a good measure of fitness, allowing for the limitations discussed above? Healthy people who start a running program, gradually increasing in distance and intensity, will only see an increase in VO2 max of about 5 to 15 percent.

As people progress through a well-put-together running programme their ability to cover distance increases by more than this, and also differences in running ability between individuals varies by more than 15 percent. This suggests that VO2 max, in itself, is not a good measure of fitness. It also suggests that if there are large differences (more than 15 percent) in VO2 max between individuals, then this probably has more to do with heredity than with training.

(Source: The Lore of Running. Third Edition, Oxford University Press. 1992. Tim Noakes.)