To understand how much, or little, exercise you need to do to meet your own health or fitness goals, it may be helpful to consider the following scientific principles behind most tailored exercise programmes.

Frequency

This relates to how many times a week you need to exercise in order to become fitter or improve or maintain your health. To be healthy, experts recommend being physically active on at least five days out of seven.

This might sound like a lot, but don't forget that our bodies have evolved over four million years to hunt. With our biological inheritance, it's not surprising that to keep in shape we need to be active on most days.

Exercise intensity can be measured in a number of different ways. In a laboratory it can be measured through gas analysis, looking at how much oxygen the exerciser is taking into the body and delivering to the working muscles. In the gym it can be measured using a heart rate monitor, which records the heart rate at different work loads. It can also be estimated using the Borg scale, which asks the exerciser how hard they perceive they're working.

The intensity at which you work can be described as either strenuous, moderate or mild. What constitutes a strenuous, moderate or mild exercise workload for you will depend on your current state of health and fitness. If you're an Olympic 10,000-metre runner, jogging one mile in nine minutes would count as mild activity; for most people, though, it would be strenuous or impossible!

As exercise scientists have discovered more about how the body responds to exercise programmes, they've discovered that for health purposes, mild to moderate levels of physical activity are all that's required.

For many of us, this means brisk or purposeful walking. Again, what brisk means will depend on your current state of health or fitness, but it definitely doesn't mean race walking as you'd see at the Olympics. It's a pace at which you feel you're making good progress while still being able to hold a conversation.

This is the length of time you need to spend being physically active in any one session. According to much of the research conducted over the past 20 years, you need to be active for up to 30 minutes, five days a week in order to benefit your health.

This might sound like a lot at first, but don't forget that you're only working at mild to moderate intensity. You don't have to run or do strenuous aerobics for half an hour; you simply have to walk.

If you haven't done much activity for some time, it's important to build up to this level over a period of weeks. This might mean starting with a walk of just five minutes.

If you're not sure how hard you can work or are worried about any health problems you may have, talk to your GP or practice nurse for help and advice. They might even refer you to a local leisure centre to work with an exercise professional - just be sure to check their qualifications first.

The recommended physical activity guidelines from a range of expert bodies (including the Health Development Agency in the UK and the American College of Sports Medicine) suggest that to improve your health you should build up to being physically active at a moderate intensity for 30 minutes, five days a week.

A little goes a long way. The key message to take home is that any physical activity, no matter how small, is better than none.

We're increasingly living in a world where physical activity has been taken out of our lives. We have domestic appliances to wash and dry for us, cars to transport us and desks at which to sit and work or study. One of the first lessons we learn at school is to 'sit still' - and we see nothing unusual in spending hours sitting or lying down while pictures move on a box in front of us.

Given these huge societal and cultural influences that have stopped us from moving, any physical activity is a health gain. There isn't a magic amount of exercise you need to do to get a benefit, the key is to make an effort and persevere. As soon as you move, you win!

This article was last medically reviewed by Dr Rob Hicks in September 2005.
First published in May 2001.