ACSEP Highlights: Fitness Capacity – Born or Bred?

How much influence does genetics have on baseline fitness and trainability? Now we can map the human genome. Can genetics predict athletic success?

I’ve just returned from the fantastic Annual Scientific Conference hosted by the Australasian College of Sport and Exercise Physicians (ACSEP) in Queenstown, New Zealand. If you haven’t been to Queenstown, you must go. The combination of the breath-taking beauty of the scenery, the multitude of adventure activities available and the warmth and friendliness of the New Zealand and Maori population made this the best conference destination I’ve been to so far by some distance.


My favourite presentation at this year’s conference was delivered by the eminent Professor Claude Bouchard who has spent a career investigating exercise physiology (take a look at his publications list!) and trying to understand cardiorespiratory fitness and its influence on health outcomes.

What is Fitness?

We all recognize that some people struggle with their fitness levels whilst others somewhat fortunately appear to be ‘naturally fit’.  These lucky people seem to stay fit regardless of how little training they have done. But are these same people the most likely to improve with further training? Or is their capacity for improvement less than ‘unfit’ people due to their high baseline levels?

Fitness levels can be evaluated by a test known as VO2Max. This represents the maximum amount of oxygen the body can use over a period of time. The more oxygen your body can use, the more work you can achieve. In activity which requires running, 75% of cardiorespiratory fitness is due to 2 factors:

  1. Cardiac Output: This is how much blood the heart can pump around the body per minute. This is influenced by the size of the heart, the efficiency of the heart as a pump and the maximum heart rate that can be achieved
  2. Oxygen Transport: This is affected by red blood cells and haemoglobin concentrations and their ability to donate oxygen to tissues
Cardiac output

Is Fitness Born or Bred?

Exercise studies on groups of large families and identical twins as well as several series of experiments in rats have revealed some brilliant insights. I found one particular rat study fascinating. The rats had their baseline fitness levels (VO2Max) assessed in an untrained state and were classed as low-fit or high-fit. The low-fit group were then allowed to breed amongst themselves and the same was done for the high-fit group.  This process was repeated over multiple generations of rats. The outcome was that over progressive generations, the baseline level of fitness between the 2 groups diverged. Simply put, the high-fit group became fitter and fitter whilst the low-fit group’s fitness capacity stayed stable but low. Overall, statistical models calculated that genetic make-up (i.e. how you were born) is responsible for approximately 50% of a person’s baseline fitness levels! This means that cardiac output and oxygen transport capacity can vary considerably from individual to individual purely and simply due to the set of genes you’ve been dealt.

So does this mean that if you have naturally low fitness levels that all hope is lost and you’re chances of becoming fit with training is low?

Fitness Training

Pleasingly the answer is no. Obviously fitness can improve with training and the good news is that the ‘trainability’ of cardiorespiratory fitness does not correlate with baseline fitness at all! i.e. Just because you are fit, it doesn’t mean you are most likely to improve with training. Interestingly the capacity to improve fitness for a given training stimulus is highly variable and has once again been shown to be 50% genetic. So, if you’re unlucky you may have baseline low fitness and poor capacity to improve your fitness, but this wouldn’t be expected. Certainly your baseline level of fitness does not give any indication as to whether you will be able to improve your fitness with training or not. Amazingly baseline VO2max, age, sex, weight and ethnicity only accounted for 2-3% of trainability variability each.

CRF factors

Can Genetics Predict Athletic and Fitness Capacity?

So, 50% of your baseline fitness you have been born with. In addition, 50% of your capacity to improve your fitness you are born with. This is all determined by genes. So can we predict from assessing human genome sequences which genes are responsible for fitness and subsequently predict who is likely to be an elite performer?

We can and have identified a huge number of genes related to fitness. Unsurprisingly though there are so many genes involved and the complexity of all of their interactions makes accurate fitness predictions almost impossible.


Furthermore, genetic capacity of fitness does not directly translate into performance. We’ve all seen less gifted junior athletes thrive as they mature. Some of the important factors which influence performance include:

  • High quality sport-specific training programs combined with application to the training
  • Motivation and resilience to exercise at intensities that others can’t / won’t tolerate
  • Psychological strength
  • Optimal nutrition
  • Excellent recovery strategies and sleep
  • Efficiency of movement

Fitness, Life Expectancy and 10,000 Steps

As would be expected, there is a strong association of life expectancy and all causes of death to fitness levels. Small improvements in fitness result in large improvements in death rates. Unless you live under a rock you will be well aware of the magic number of 10,000 steps a day that is often quoted as the target for positive general health outcomes. The number seems so arbitrary on the face of it that it would be reasonable to be sceptical about this target. However, 10,000 steps is the average number of steps that a sedentary worker will achieve if they walk for 30 minutes a day. Exercise recommendations worldwide are quite consistent in recommending walking 30 minutes a day on most days of the week. But why 30 minutes of walking? Well, when you have capacity to walk for 30 minutes, you have achieved a baseline fitness level (VO2Max) that has been shown to provide large improvements in health, quality of life and reduced death rates.

So fitness is important, but what is more important, intrinsic fitness levels or acquired fitness levels? There is no definitive answer to this question but both types of fitness have been strongly and independently correlated to life expectancy. In some animal studies there is a suggestion that intrinsic fitness is the most important but no strong conclusions can be inferred from this in humans.

crf surviving

Future Research

An interesting field of study is epigenetics. Many of us have genes coding for all sorts of attributes, but they may not be switched on. Epigenetics is the description of our capacity to switch these genes on and off and modify our human capacity. So the presence of genes is not enough – they need to be switched on too. Could it be that exposure to training in childhood has a positive influence on fitness-related epigenetics and potentially influence an adult’s fitness trainability? These are questions we don’t know the answer to but are the subject of further research.

For more information on training programs and injury management come and see the team at Newcastle Sports Medicine. And for more articles on injury and exercise see our blog.

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