All the Olympic athletes in Beijing will have undergone a huge amount of training and performance testing to ensure that they were on target to achieve their goals.

But how do you measure your fitness levels to see if you are meeting your goals? You could use one of several methods, from your body weight to your hip-to-waist ratio to your body-fat percentage.

Some are easier than others; body-fat measurement needs tools, whereas your weight can be subjective. Other measurements, such as testing your VO2 - your ability to take in oxygen and use it - are difficult to test by yourself as they need equipment and someone to operate it.

Work out your BMI

One of the more recently used measurements is body mass index (BMI), a formula calculated by dividing your weight in kilograms by the square of your height in metres. For example, if you're 6ft (1.83m) and 12 stone (79kg), divide 79 by 3.35 (1.83 squared) to get a BMI of 23.5.

The BMI scale has “ideal” ranges. Lower than 18.5 is underweight; 18.5 to 25 is normal; 25 to 30 is overweight; 30 to 35 is obese 1; 35 to 40 is obese 2; and 40-plus is morbidly obese.

Like many tests, it has its downsides which can cause a great deal of confusion and, in some cases, unnecessary concern. A few case studies will make it clear where potential problems lie.

Under, over and just right

Paula Radcliffe, with a weight of 53kg and a height of 1.74m scores a BMI of 17.5, which makes her worryingly underweight. At 18st and 6ft 7in, Martin Johnson, the Rugby World Cup-winning captain, is not just overweight, but obese with a BMI of 30. In his prime, the 6ft 2in sprinter Linford Christie weighed 79kg, thus checking in with an ideal BMI of 22.6.

I weigh about 12st and am 5ft 11in, which makes me borderline overweight, yet I have a body fat percentage of less than 9 per cent, which makes me extremely lean.

These numbers vary so much because the calculation uses weight only and does not account for levels of body fat. BMI, therefore, can miss out how lean, or muscular or powerful someone may be.

Paula Radcliffe is clearly very lean, but she needs less muscle mass for running marathons than Linford Christie did for sprinting. Martin Johnson was one of a new breed of well-prepared rugby players who increased their mass to cope with the demands of their game. He was not fat, just heavy and fit.

Muscle weighs more than fat

Someone who has a big structure weighs more than a more finely built person, but it doesn't necessarily mean that he or she is fat.

Last week I tested a 5ft 7in woman who weighed 8st, and yet her body fat reading came out as a high 31 per cent. This is because she carried little lean mass (muscle), had a small bone structure and a low weight.

Her resulting BMI of 18.6, though, classifies her as underweight - but a very different underweight from Paula Radcliffe. My client's muscles have little function so she is at a higher risk of gaining more fat, increasing her risk of illness.

But if you are overweight by more than a stone, BMI is a useful way to track weight loss. Women also find it more accurate than men whose greater level of muscle mass weighs more.

Measure your body fat

Use as many ways as you can to measure your fitness, although I'd recommend body fat as the one to look at. Get your reading taken at your gym and have it redone every month so that you can track any changes. Or, buy scales with an inbuilt body fat sensor. They can vary in accuracy, but will give you a good enough idea.

Being told that you are under or overweight can be stressful. But, just as it is for Olympians, precision is vital, especially if you are putting in the effort and commitment required to achieve your goals.

To find out more about Matt, go to www.mattroberts.co.uk