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Eating alone will not keep a man [or woman] well; he [or she] must also take exercise.

Hippocrates (460–370 BCE)

Physical activity is good for us. Well before the age of double-blind randomised placebo-controlled trials and peer-reviewed journals, Hippocrates and others espoused the benefits of exercise on the body and mind. Herodicus (400 BCE), a former teacher of Hippocrates and regarded as the pioneer of sport and exercise medicine, devoted his time to recommending exercise to help recovery from athletic and gymnastic injuries. Later, Galen (131–201 BCE), a Greek physician to the gladiators, proclaimed that ‘the form of exercise deserving our attention is therefore that which has the capacity to provide health of the body, harmony of the part and virtue in the soul and these things are true of the exercise with the small ball’.

Fast forward a few thousand years and we have proof of many benefits of physical activity and reduced sedentary behaviour. Empirical evidence that physical activity was associated with health came in the 1950s. Dr Jerry Morris, a Scottish epidemiologist credited as ‘the man who invented exercise’, established the importance of physical activity in preventing cardiovascular disease after noticing that sedentary drivers of London’s double-decker buses had higher rates of cardiovascular diseases than did the conductors who climbed the stairs. ‘Is this chance a phenomenon?’ asked Morris in his 1953 Lancet paper.1 He answered his own question by reproducing similar findings when extending the study to London postmen and less active postal clerks.

Today, systematic reviews conclude that physical inactivity is a key risk factor for the leading non-communicable diseases and, conversely, that regular physical activity has a fundamental role in the primary and secondary prevention of many diseases and injuries. We have an incontrovertible evidence base for the millennia-old conclusion: physical activity is medicine.

Simon Sinek, a successful British-American author and motivational speaker, encourages everybody to ‘start with why’,2 and this is relevant if we expect people to undertake physical activity. Epidemiological data provide part of a compelling reason to exercise; the mechanistic ‘why’—asking what does exercise do at the cellular/tissue level?—complements the epidemiological data. Why does physical activity confer so many health benefits unmatched by any medication?

Galen believed that physical activity ‘thins the body, hardens and strengthens muscles, increases flesh, and elevates blood volume’.3 Was he wrong? Here we explore Galen’s hypothesis by delving into the physiological mechanisms whereby physical activity influences many (perhaps all) tissues and organ systems for health.


The following describes the basic physiology of physical activity, one of the most extreme stresses to which the body can be exposed (Fig. 2.1).

Figure 2.1

A summary of the physiological response to physical activity


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