We've all seen the pictures of celebrities looking fabulous in their 40s, but how easy is it for us mere mortals to defy gravity? Easier than you think, says Sam Murphy

Can you remember when no part of your body sagged, drooped or wobbled? What goes wrong when we hit mid-life? 'The greatest physical decline in the human body occurs between 30-40 years of age,' says exercise physiologist John McCarthy. 'During these years, muscle mass is lost, metabolism slows down, bones lose density, flexibility and strength decline and body fat increases.' In fact, the average woman gains approximately 20lbs between the ages of 20-65.

This physical decline is partly due to the metabolism go-slow that creeps on as we leave our youth behind, but experts now believe that the problem is more a case of inactivity. Think about it: do you drive where you used to walk? Sit at a desk for long hours when you used to be constantly on the run? Do you eat our more, go dancing less and always get a taxi home?

'It's so easy to slip into habits of inactivity especially when we're busy juggling different areas of life,' says Jamie Baird, a personal trainer who recently helped Linda Gray get in shape for her role in the stage production of The Graduate. 'Then we look in the mirror one day, and wonder when it all went wrong.' But even if you've reached that stage, it's not too late to regain fitness and improve your health.

Regular aerobic exercise can aid weight loss and reduce many health risks as well as providing time-out from a hectic schedule. 'Rhythmic aerobic activities - such as dancing, cycling, swimming or running appear to work best in dispelling stress and anxiety,' says Stuart Biddle, exercise psychologist at Loughborough University. 'Such activities have also been shown in scientific studies to combat mild to moderate depression as effectively as prescription drugs.'

While cardiovascular exercise is crucial for maintaining health and well being, it's strength - or resistance - training that will make the biggest impact on your mid-life body shape. 'Basal metabolic rate - the rate at which we burn calories at rest - begins to decline from 25-30 years of age,' explains John McCarthy. 'The volume of lean muscle tissue decreases too, by about four per cent per decade. The result is an overall loss in metabolically active tissue and a gain in highly inactive adipose tissue [fat].' One pound of muscle will demand approximately 35 calories per day, simply to function, while a pound of fat needs just one or two calories.

By increasing lean body mass through resistance training, this decline can be reversed. A study by strength training expert Wayne Westcott found that 12 weeks of regular resistance training resulted in a loss of 4lbs of body fat and a gain of 3lbs of lean muscle tissue. 'The best way to achieve this is to perform a total body strength workout two to three times a week,' says McCarthy. It's not necessary to join a gym to do resistance training - you can use your own body weight for many exercises or invest in dumbbells or resistance tubing to use at home. If you don't know what type of exercises to do, consult a personal trainer or a good book, such as Strong Women Stay Slim, (£12,99 Aurum Press) by Dr Miriam Nelson.

A word of advice, though: start slowly. In the US, sports medicine professionals have coined the phrase 'boomeritis' to describe the wide variety of injuries they are seeing as a result of mid-lifers - or 'baby-boomers' - taking to exercise too enthusiastically. As a rule of thumb, increase the intensity of your workout by no more than five per cent a week.

If you're reading this in despair, wondering how you can fit yet another thing into your time-starved life, take heart. Studies are increasingly showing that lifestyle activities are just as instrumental in keeping us fit and healthy as formal exercise. In fact, one landmark study at the University of Maastricht found that people who were sedentary most of the day but went to the gym regularly burned fewer calories than people who were generally more active in their daily lives but didn't do any structured workouts.

Another study, from Loughborough University, concluded that with regard to weight loss and health, 10-minute bouts of exercise were as effective - if not more so - than prolonged sessions. The United States Centers for Disease Control and Prevention (CDC) recommends we aim to expend 200 calories a day on moderate activities - a target that's easier to achieve than you might think. For example, walk briskly to appointments, run up the stairs, carry shopping bags to the car, chase around after the children, clean the windows? The take-home message? Fit in fitness where you can.