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Fueling exercise

I have described the importance of oxygen for fueling exertion, but it was a long time ago – maybe even before you were reading my blog. It is a fascinating and really important topic if we are to understand some of the mechanisms of exercise so I make no apologies for returning to it.

All exercise involves muscular contraction and muscles need energy if they are to contract – and energy demands fuel.  The biochemical processes which fuel muscle contraction are immensely complicated – much too complicated for me to understand or explain – except for the contribution of oxygen (O2). Oxygen is an essential component of the metabolic conversion of various energy substrates (sugars, fats etc) into action (muscle contraction). The supply of oxygen to the muscles and their ability to use it are the limiting factors in deciding how much work those muscles can perform during aerobic exercise.

Absorption of oxygen 

About 20% or earth’s atmosphere is made up of oxygen. Atmospheric air is breathed into the lungs and comes into contact with the blood coursing through the pulmonary blood vessels. These vessels divide into tiny capillaries which supply the small air sacs (alveoli) in the lungs. There are approximately 700 million alveoli in human lungs, covering a total surface area of about 70 square metres – approximately the size of a tennis court!  The oxygen is absorbed onto the haemoglobin molecules in the blood’s  red cells as they pass around the alveoli.

Haemoglobin in this blood supply arrives in the lungs in the form of deoxyhaemoglobin and has a bluish colour. The oxygen picked up in the lungs converts it to oxyhaemoglobin which is a much brighter red. The blood is then pumped round the body to the various organs which need oxygen to maintain life – the brain, heart, kidneys, gut, liver and muscles of respiration.  There the oxygen is extracted into the local cells.

The body’s oxygen uptake is abbreviated as VO2 and is related to body weight. It is measured as millilitres of oxygen per minute per kilogram of weight (ml/min/kg). This figure varies with the weight of the individual.  At rest it is about 3.5ml of oxygen per minute per kilogram of body weight.

The resting consumption of oxygen is also known as a metabolic equivalent or 1 MET. Remember the MET – it is an extremely important measure of exercise intensity.

Oxygen and exercise

When we exercise, our muscles need more oxygen and this is provided by breathing faster and by the heart pumping out more blood. There is a straight line relationship between muscle work and oxygen uptake (VO2) until the point at which no more oxygen can be absorbed and pumped round the body. This is the maximum oxygen uptake, or  VO2max , again measured as millilitres of oxygen used per minute for each kilogram of body weight – ml/min/kg. This is the point of exhaustion. For the unfit, exhaustion is reached at a lower oxygen uptake and therefore a lower workload than for the fit individual. The fitter you are the higher the rate at which you can take up and use oxygen and therefore the higher workload you can achieve. This is illustrated by the graph which heads this blog which shows the rate of exertion in watts on the horizontal x axis versus the oxygen uptake in litres on the vertical y axis.

When VO2max is reached, further exercise can only be continued by anaerobic (not using oxygen) metabolism. This is fueled by stored energy sources in the muscles which quickly become used up. Anaerobic exercise can therefore only be continued for a very short period.

Measurement of workload and VO2max 

The best measurement of workload uses the rate of oxygen uptake – the VO2  – and is best expressed as multiples of oxygen uptake at rest – the MET which I described above. Pottering about uses between one and three METs, mild to moderate exercise uses about three to six METs and vigorous exercise uses more than six METs.

The concept of maximum oxygen uptake, VO2max, is extremely important. It is the most precise measure of physical fitness we have since it describes the maximum work rate of which a person is capable. In healthy young people it is usually between 35 and 55 ml/min/kg body weight (10-15 METs). Ultra-fit athletes may reach levels of 70-80 ml/min/kg; heart patients tend to have much lower levels in the range of 10-30ml/min/kg (3-5 METs). As we age there is a decline in VO2max of roughly 0.5 to 1.0 ml/min/kg each year. However the variation in individual VO2max is far greater than the age variation.

Next week I will describe how the body increases oxygen uptake with increasing exercise load.