What does it mean to be fit? Fitness is not just a life long journey of improvement; nor is it something we build upon for life. We now have an increasing body of evidence that changes in fitness are dynamic. Even in the elderly it doesn’t have to decline with increasing age. It can get worse and then better again.

10,000 daily steps will not restore fitness to a depressed mind.

Whilst our statistical methods enable us to be confident that differences from one point in time to another are real, they are not able to deliver the moving picture of fitness. Humans are not inanimate objects. We are subject to inexplicable, non logical or random effects; and so, it is with fitness, even for elite athletes. Recent studies of rugby union, one of the most physically demanding ball games, have shown that changes in player fitness can be transient, especially in the second half, and not related to overall performance in a game.

Fitness is not just physical. What about other types of human fitness, for example fitness of mind? Depression is the most common unfit state of mind. By middle age, people who suffer from major depression have had at least four bouts of it.

It is unlikely that 10,000 daily steps measured on a wristband or 30 minutes exercise three times a week will restore fitness to a depressed mind. The evidence of fitness helping to maintain a healthy mind is not conclusive.

Mental health can be partially predicted by genetic and environmental factors, but like the weather it subject to inexplicable changes. In understanding the connection between physical and mental fitness, recognising individual variability over periods time is essential.

For example, exercising and exorcising unfit thoughts on a daily basis and replacing them with fit thoughts form the basis of cognitive behavioural therapy.

Even with the plethora of literature that appears daily about physical and mental fitness, the picture will never be straightforward. Seemingly counterintuitive contradictions will continue to increase. For example, the effects of well documented causes of shortened life expectancy, like obesity and smoking, can be negated by increasing heart and lung fitness.

These paradoxes are hard to explain and not fully understood – probably because our bodies constantly change and adapt to challenges. Research can only capture part of the picture at one point in time. We need to develop a whole new group of specialists to decode fitness and bring it to life across the life span of individuals.