WHAT IS PHYSICAL FITNESS?

There are all sorts of aspects of physical fitness – muscle strength, balance, flexibility to mention a few. The aspect which interests me and is the subject of my blogs is aerobic fitness otherwise known as cardio-respiratory fitness (CRF).   It is the measure of our ability to move about comfortably and accomplish physical tasks.

Fitness and Physical Training

Your level of physical fitness can be described as exercise capacity, aerobic capacity.or maximal oxygen uptake  VO_2max (see Glossary & previous blog about exercise and oxygen). Physical fitness is influenced by general activity and by exercise training. Inactive people are usually unfit, the very active are fit. The increase in fitness, as measured by VO_2max., which can be attained by physical training is inversely proportional to the starting VO_2max – the less active you have been, the less fit you are and the more you have to gain from training. An inactive unfit person can be improved by a greater percentage than an out-of-training athlete, though not to such a high level. The increase may exceed 30% in young, unfit individuals who then train intensively. Older subjects show a smaller response to training than younger ones.

The graph at the head of this blog shows the effects of fitness level on heart rate and oxygen uptake – VO₂ which reflects how hard you are working._. The fitter you are the more exercise you can do, i.e. the more oxygen you can absorb and use, for any given heart rate. Since it is the maximum heart rate which limits your ability to exercise maximally, the fitter you are the greater your maximal effort capacity. For most of us for most of the time, however, it is submaximal exercise which is more important because that is what we do most of. In this case any exercise level for a fit person expends a lower proportion of what is possible – all effort becomes easier.

If you take up a fitness regime you can get an idea of changes in your fitness level by observing the fall in your heart rate (pulse rate) at rest and during exercise.

How does Physical Training Work

The increased aerobic capacity produced by training is brought about by a mixture of “central” and “peripheral” effects.

Central effects include:

1. Improved cardiac performance.
   After training, the heart is able to contract more efficiently, pumping out a larger volume of blood with each beat (the stroke volume).At rest the body’s need for oxygen and therefore cardiac output is the same for the fit as for the unfit person. Since the fit individual pumps out more blood with each beat (the stroke volume) than the unfit, his or her resting heart rate is lower, perhaps as low as 50 beats per minute in the super-fit.During submaximal exercise (exercise at a defined level which is lower than the peak which can be reached) again the need for circulating blood and oxygen delivery is the same in trained and untrained people. So at any given level of exercise the cardiac output is the same for the fit as for the unfit – but the stroke volume is higher and the heart rate is lower for the fit. Submaximal exercise is easier for the fit than for the unfit and achieved with a lower heart rate.Maximal oxygen uptake, however, is increased by physical training. The maximal heart rate does not increase but cardiac output is greater because stroke volume is greater. More blood and oxygen can be pumped to the working muscles which can therefore do more work. That is why the fit long-distance runner can run faster than the unfit.
2. Reduced sympathetic tone. The resistance to flow in the peripheral blood vessels is partly determined by the “sympathetic” nervous system – the network of nerves which supply the tiny muscles which can contract and narrow small arteries. Physical training reduces this tone and opens up peripheral arteries allowing blood to flow more freely through them and reducing blood pressure at rest and during exercise. The heart has to work less hard to pump blood round the system.
3. Changes in blood. The trained individual has a greater total blood volume and more oxygen-bearing red cells. The beneficial effects of “blood doping”* relies on this effect.

Peripheral effects include:

1. Increased muscle performance. Physical training produces a number of changes in the trained muscles. They become larger, they develop more mitochondria (the enzyme packages within muscle cells which are much concerned with energy production) and they have a better ability to extract oxygen and glucose from the blood.
2. Increased muscle blood flow. The trained muscles have a greater network of capillaries feeding them and the distribution of blood flow favours working muscles over inactive muscles.
3. Increased arteriovenous oxygen difference. Better muscle performance and more efficient blood flow results in an increase in the overall extraction of oxygen from the blood – the venous blood carrying blood back to the heart has less oxygen left in it. This is most marked at maximal effort.

Next time I will tell you how physical fitness is measured – and how to do it for yourself

PS. A worrying finding from a recent large study from Finland has been that people who are prescribed medication for raised blood pressure or raised cholesterol change their healthy life style. Starting statins was associated with reduced physical activity and greater weight gain (but a greater chance of stopping smoking and reducing alcohol). It is as though the prescription of preventive medication gives patients permission to stop looking after their own health – oh dear.