The energy required by muscles to contract is produced by complex biochemical reactions. A critical contributor to the process is oxygen (O2). Oxygen comprises about 20% of the air we breathe. It is absorbed from the lungs onto the haemoglobin molecules in the blood’s red cells as they pass through the very small blood vessels (capillaries) supplying the air sacs (alveoli) in the lungs. Haemoglobin arrives in the lungs in the form of deoxyhaemoglobin and has a bluish hue. The oxygen picked up in the lungs converts it to oxyhaemoglobin which is a much brighter red. The blood is pumped round the body to the various organs which need oxygen to maintain life – the brain, heart, kidneys, gut, liver and muscles for breathing. There the oxygen is extracted into the local cells.
Oxygen need at rest
At rest, oxygen is needed to fuel all those processes which keep us alive – breathing, heart beating, brain activity, the kidneys filtering their blood supply to get rid of undesirable stuff, the liver quietly working away at a variety of tasks, gut digesting food etc etc. All of this takes about 0.25 litres of oxygen per minute for the average 70kg (11 stone) man. The amount varies with the weight of the individual. When adjusted for weight, the oxygen uptake is more constant at about 3.5 ml of oxygen per minute per kilogram of body weight. This is also known as a metabolic equivalent or 1 MET. Remember the MET – it is an extremely important measure of exercise intensity and of exercise dose.
And with exercise
When we exercise, our muscles need more oxygen and this is provided by breathing faster and by the heart pumping more blood to them. There is a straight line relationship between muscle work and oxygen uptake (abbreviated as VO2) until the point at which no more oxygen can be absorbed and pumped round the body – known as the maximum oxygen uptake, or VO2max. This is measured as millilitres of oxygen used per minute for each kilogram of body weight – ml/min/kg. This is aerobic exercise and at the point of maximum uptake further exercise can only continue using 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.
As workload increases so does oxygen uptake to the point of exhaustion. For the unfit this point 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 – as illustrated in the following graph of exertion against oxygen uptake.
Workload in watts (x axis) plotted against oxygen uptake in litres per minute (y axis)
Maximum Oxygen Uptake – VO2max
The concept of VO2max is very 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. 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.
Exercise to VO2max can only be attained using the large muscle groups of the legs. Because of their smaller bulk, maximum arm exercise will only achieve about two thirds of maximum leg exercise. Also, once maximum oxygen uptake has been reached with leg exercise, bringing other muscles, like the arm muscles, into action will not increase oxygen uptake further. The limiting factor is not muscular effort but the ability of the lungs and heart to supply oxygen to the muscles.
Oxygen use during exercise
From the resting state to exercise the increase in oxygen uptake, transport and use is achieved by several changes:
- Faster, deeper breathing bringing more oxygen into contact with the blood
- Increase in cardiac output – the amount of blood pumped out by the heart. Heart rate (HR) increases by two to three times. The volume of blood pumped out with each stroke (stroke volume – SV) increases by about 80%.
- Diverting a greater proportion of blood flow to the working muscles.
- More efficient extraction of oxygen by those muscles.
For a young person of average fitness, the increase of VO2 from rest to maximum exercise is about 12 fold – mediated by a fourfold rise in cardiac output (HR up by 2.7 times and SV up by 1.4 times) and a threefold rise in oxygen extraction by the muscles.
Effect of ageing
VO2max varies with age, sex and habitual physical activity. As time goes by, maximal heart rate and stroke volume both decline as do muscle bulk and strength so that the fall in VO2max each year is between 0.5 and 1 ml/min/kg body weight. Although the older you are the less fit you become, the variation between individuals is much greater than the variation with age – mainly because of the effect of habitual activity. Women, who have smaller frames and smaller hearts than men but more fat, have about a 20% lower VO2max.
Next time I will discuss the measurement of exercise dose – how much exercise you are taking.
A recent review indicates that exercise for children with ADHD produces improvements in their characteristic symptoms, mainly attention deficit and hyperactivity1. It produces appreciable improvements in inhibitory control and cognitive and executive functions. Benefits following exercise were also seen in other aspects such as reaction times motor skills brain activity. If it works, exercise would certainly be a better treatment than the usually used drugs like Ritalin. It is a great pity that the government allows so many schools to sell off their playing fields.
- doi: 10.7334/psicothema2019.211