Saturday, 9th May 2020


The natural history of cardiorespiratory fitness (CRF)

The basic facts are these. Using VO2max   as the measure of physical fitness, we increase in fitness through childhood till late teens or early twenties. At this stage VO2max lies between about 30 and 50ml/min/kg for men, averaging about 40 . In women the figures are about 20% lower. Thereafter we lose fitness by about 0.5% per annum in early adult life but getting steeper in middle age at about 1% each year and accelerating in old age to about 2% or more each year.

Determinants of fitness

Baseline fitness depends upon inherited characteristics and habitual activity levels. The range of possible fitness is determined by inherited characteristics – if no-one did any exercise at all there would still be considerable variation between individuals. The more exercise you take the nearer the top of your potential CRF you will reach. Sedentary people can increase their fitness level by up to 30% with vigorous exercise training. The fitter you are the less room there is for improvement. If you stop training or reduce your usual physical activity your fitness level declines fairly rapidly. This fact is very important for older people forced into inactivity by illness or hospital admission.

Variability in response

So far, so straightforward. Unfortunately there some wrinkles, the main one being the wide variability in the extent to which different people respond to exercise training. For some people  exercise is more effective for  increasing CRF than it is for others.  Twin studies indicate that about half the variability in response to physical training is inherited – so your parents decide not only your possible fitness but also how much this can be increased by physical training

The implications

It is well known that higher levels of cardiorespiratory fitness (CRF) are associated with reductions in risk of a wide variety of chronic diseases and in the mortality risks associated with obesity, diabetes, cardiovascular disease, hypertension and even  normal health. Moreover low levels of CRF are associated with greatly increased risk of many chronic diseases and in all-cause mortality.

CRF is a better predictor of health outcomes than physical activity. This is explained in part by the likely inaccuracies in assessing physical activity –  measurement of physical activity is far less precise and accurate than CRF.

On a scale of fitness – low, below average, above average, high, elite – the greatest benefit from increasing CRF is found in those who improve from “low” to “below average”. We do not know the relative importance of intrinsic and acquired CRF in bestowing the benefits of a high CRF. In other words if our high fitness level reduces our risks of disease and death we do not know whether this is because we were born fit or had to acquire fitness through hard work. Probably both are important.  

How often, how hard, how long?

Despite the variability in response we have enough knowledge to formulate some rules of thumb to help decide how much exercise is good for us. These were first devised in the 1960s after the results of the seminal Jerry Morris study of bus drivers and bus conductors. In brief the recommendations then were to exercise three times per week, to a heart rate of between 70% and 85% of maximum and for between 30 and 50 minutes. No, you don’t need to measure your heart rate during exercise. The suggested level is that which makes you comfortably short of breath but not gasping- you can talk but you can’t sing! (See my Blog of 18th April)

Government recommendations

Most national recommendations for the amount of exercise needed for the maintenance of good health are very similar. In brief such recommendations are for 150 minutes per week of moderate exercise or 75 minutes of vigorous exercise. They also include muscle strengthening exercises on two days per week . There is no scientific reason for choosing these particular criteria – they provide a reasonable exercise dose without being too intimidating or off-putting. The recommendations come with the information that higher levels of exercise will produce greater benefits. The recommendations are a compromise between what is known to work and what people may be persuaded to do.

Weight loss programmes are more successful for the physically fit than the unfit. A recent trial of a behavioural weight loss programme divided the subjects into “very poor fitness” and “poor or better fitness” groups1. The poor or better fitness group lost twice as much weight as the very poor group – an average of 18 pounds versus 9.5 pounds.

  1. The Endocrine Society. “Poor fitness may impede long-term success in weight loss program.” ScienceDaily. ScienceDaily, 31 March 2020. <>



  1. Susan Kirkpatrick says:

    Thank you for all your blogs. I do enjoy reading them even if they sometimes produce a sense of being overwhelmed by the technical details! This one is very straightforward!

    • Hugh Bethell says:

      Thanks Sue – and sorry about the technical stuff. Not for everyone but I hope that some folk find it interesting. Next week I will be talking about evidence and how it is needed to prove that what I say is true! Hugh

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