Measuring physical fitness

The value of knowing physical fitness level

The level of an individual’s aerobic or cardio-respiratory fitness (CRF) is the best single predictor  of both healthspan and lifespan. When doctors try to assess the risk for such conditions as heart attacks they include a large variety of figures such as Body Mass Index, blood pressure, smoking habit, family history and blood cholesterol but never CRF. Of course CRF is not as easy to measure as the other factors – but I believe that some attempt to include CRF level would be a really valuable clinical tool.

What is aerobic fitness?

Aerobic fitness, or CRF, describes the ability of the individual to exercise – the exercise capacity – and is best expressed as a measure of the most that can be achieved with maximal effort. Since all aerobic exercise consumes oxygen, that maximal effort can be measured as the maximum rate at which oxygen can be absorbed through the lungs, distributed through the circulatory system and then used by the working muscles.

VO_2max and its measurement.

Oxygen use is expressed as VO₂ and maximal use as VO_2max.   VO_2max can be measured directly, but it is a time-consuming business and needs specialised and expensive equipment. It is usually performed in laboratory or clinical setting or on a treadmill or on a calibrated cycle ergometer. The candidate performs the exercise while breathing through a tube connected to an oxygen analyser, which measures the rate at which oxygen is being extracted from the inhaled air. The individual is subjected to a steadily increasing workload until he or she can continue no longer. Real-time display of oxygen uptake shows a linear increase with increasing workload until the maximal oxygen uptake is reached. At this point the graph levels off at what is known as the ‘anaerobic threshold’. The exerciser is capable of a bit more effort, but very soon has to stop from exhaustion. Increasing oxygen uptake is facilitated by increasing heart rate, which follows a similar trajectory, increasing to its maximum at peak exercise.

Today’s illustration (sorry it is a bit blurry) shows the direct relationship between heart rate and oxygen uptake (VO₂₎ in both fit and unfit people. 

VO_2max – is usually between 1.5 and 3.5 litres per minute. This is converted into millilitres per minute per kilogram of body weight (ml/min/kg) by multiplying by 1,000 and dividing by the weight of the individual.

Since measurement of VO_2max  is expensive and requires specialist equipment, it is usual to estimate oxygen uptake by relying on the known oxygen cost of the exercise being undertaken.

Treadmills and bicycle ergometers

The motorized treadmill is the most popular exercise machine for use in the laboratory or clinic and is the method which we use for assessing our exercisers at Alton Cardiac Rehab. A variety of different protocols can be used, the most popular being the famous Bruce protocol, devised by the Seattle-based cardiologist Robert Bruce. The treadmill is started at 1.7mph with a 10-degree slope and is increased every 3 minutes through seven stages to the maximum of 6.0mph and a 22-degree slope. The time spent on the treadmill before exhaustion can be used to predict VO_2max. The modified Bruce protocol has two preliminary stages and is used for the elderly and the very unfit.

Cycle ergometers are much more portable than treadmills and are the most popular instruments for use in ‘field’ studies. Estimation of exercise intensity requires the work achieved on the cycle to be measured with as much precision as is possible. and then corrected for the weight of the cyclist. Workload is usually measured in watts (joules per second). The exercise capacity is adjusted to allow for body weight.

Using the International Physical Activity Questionnaire (IPAQ)

This questionnaire tries to measure how much exercise the subject performs. Several equations have been devised to translate the IPAQ results into estimates of CRF. They show fairly accurate estimates of VO2max for those in the high fitness groups but  tend to over estimate VO2max in less fit individuals – probably because those who take little exercise believe that they do more than they actually do. Social desirability bias again.

 Measuring your own physical fitness

There are a number of benefits from being able to measure your personal fitness:

• It can give you an idea of personal fitness in comparison with others of the same sex and age. People should be concerned if they find themselves to be average or less than average. This is because most people are far less fit than they should be. Individuals should not be content with anything less than ‘above average’.
• Measuring personal fitness should stimulate individuals to take up or increase exercise, especially if their levels are less than satisfactory.
• It gives a benchmark to measure future improvements and the effectiveness of whatever exercise is being undertaken.

Next time I will tell you about some of the ways in which you can measure you own fitness level.