In the past I had the privilege of chatting to one of South Africa's oldest regular cyclists — a man in his 80s, who was cycling the same 100km ride that I was at the time.

Both of us have trouble climbing hills, but for different reasons. I am sure that Derek was much faster than I am now, at half his age! Both of us agreed that we would continue cycling regularly until we dropped off our bikes! And it is not only cycling which appeals to those over fifty. A friend of mine ran her first Two Oceans Marathon aged 51.

While many people decrease their exercise levels as they get older, still others continue at the same level or actually increase it. Research continues to reaffirm the notion that exercise keeps you healthy for longer. But if you have never enjoyed exercise and have avoided doing it at all costs, how do you know what exercises are beneficial and safe to start when you are older?

Why exercise as you age?

Research has shown that regular physical exercise can delay and prevent many diseases, such as diabetes, colon cancer, heart disease, stroke, high blood pressure and probably osteoporosis. It will also keep your body toned and help keep your weight down if that is a problem.

However, research from the USA shows that more than two-thirds of older adults don't exercise regularly, in spite of the fact that there is evidence that even the frail elderly can benefit from simple activity. In one study, a group of people over the age of 80 stopped using walking frames after only 10 weeks of muscle-building exercise.

What type of exercise and how much?

Four types of exercise are important to keep you healthy and fit. These are activities which increase your endurance capacity, your strength, your balance and your flexibility.

Dr Peter Snell, a past Olympic runner, has been studying how much, how frequent and how long exercise needs to be to achieve the physiological change which will keep you healthy and fit. There is a huge debate at present about the relative effectiveness of moderate versus high-intensity exercise.

Peter Snell thinks that the best way to gain the most benefits is to alternate high-intensity exercise with moderate exercise. For example, walk hard for two minutes and then spend three minutes walking slowly to recover, then speed up again. This will work the heart and lungs (cardiovascular system) and the muscles. The more vigorous the exercise the more muscle fibres are involved.

Weight training is very important for older people because of the loss of bone mineral density. It is essential that this be started with a trainer to show you the correct way of using either free weights or machines, so that you don't injure yourself.

You must enjoy your activity or you won't keep it up or exercise regularly. However, if you used to run or cycle and haven't for a long time, don't expect to dive back into it at the level at which you stopped! Start slowly and build up, and preferably get a trainer to help you.

Once you reach the point at which you can no longer cycle or run, then yoga or stretching exercises can build and maintain flexibility and muscle. Swimming is the best aerobic exercise at this stage since you are least likely to injure yourself.

How long can you continue to exercise?

This depends on you and your activity. Ideally, you should have been active when younger and never really stopped, even if you decreased the intensity of exercise to some extent or changed, for example, from running to cycling. Your body will tell you how much you can handle, but your body's limits will change.

If you are starting out again, and exercise regularly and consistently, then your fitness will increase so that you can increase the level of activity.

A local expert in medicine for the elderly has said that if you do not stay active then you will be "off your legs" by the time you are seventy. My experience with older people who have never been active confirms this. But it is never too late to start.

Take charge of your life and enjoy your exercise.