However, before you dig out your plimsolls and leotards and hightail it to the nearest workout centre, a word of warning: check exactly what the class entails, and that you are wearing the correct footwear.
I speak from experience. Three years ago an exercise class changed my life.
It was early in the New Year that I booked a weekend with my husband at a reputable health farm, with the intention of getting his business- lunch paunch into his ski-suit, and his office legs strong enough to tackle the Swiss Alps by February. I wasn't too worried about myself: I played tennis or worked out in my local gym at least four times a week. In fact, although the dreaded 40 was looming, I felt quite smug about my youthful physical condition and the fact that, unlike my friends, I had never suffered any sports injuries.
Following his first morning's activities at the farm, my husband put himself to bed to recover and I went off in search of an exercise class. I found one, Beginners' Step, which I had never even considered before; I was told that it was quite safe and would do a great job of strengthening my legs for the ski slopes.
While the rest of the class stacked up their step like a Lego block, I floundered around on the lowest board, trying to look as though I was enjoying myself as I stepped up and I stepped down, felt bored and thought about supper.
Afterwards my knees ached, but I was used to post-exercise pain and considered that a healthy omen. I wasn't even alerted by the fact that the dull discomfort increased by the day. Back home, I continued my normal activities, including some gentle exercise. But towards the end of the week, my knees were so swollen that I could hardly bend them and I was walking with a slight limp. By the following weekend I was in such pain that at night I couldn't even bear the weight of the duvet on my knees. Not only had they swollen to the size of a rugby ball, but they had also turned a deep shade of purple.
It was at that point, it seemed to me, that my "youth", as well as my tennis days, ended: intense, constant pain sobers and ages you quickly. The phone soon stopped ringing asking me to make up a fourth; as friends dropped away I realised how important sport had been to my social life.
Over the next two years, in the hope that someone would be able to help, I was to visit three top knee specialists - one in Switzerland - who all confirmed that step classes were known to cause stress and injury to backs and knee joints. One even joked that business had been booming since the craze took off. All agreed that, because of the impact on the joints, it could be an extremely dangerous form of exercise, and that I was one of its many casualties. I had incurred patella and soft-tissue damage through the step class; the good news was that with plenty of rest the injuries should gradually heal.
Had I injured just one knee, I could have used crutches, relying on the uninjured side for support. However, with both knees causing pain I could hardly walk. I spent weeks in bed to give my joints time to heal, only to trigger the swelling again when I eventually put weight on them. It was a vicious circle: rest and try again.
I felt so bored that when a physiotherapist told me I could go swimming, I headed straight for the pool - with the result that the pain and swelling increased. No one had thought to mention that breast stroke rotates the knees and can aggravate injury. I did swim after this, but made sure I only kicked my legs up and down without rotating the knees.
Whenever I did try to walk, I moved about like an old lady. And my knees had become so sensitive to cold - even the slightest draught - that most of the time I needed to cover them with a blanket.
One year after my visit to the health farm I bore little resemblance to my former active self. In that time, I had tried various remedies without success: physiotherapy, reflexology and acupuncture. X-rays showed that nothing was actually displaced and each knee specialist I saw felt that an operation was unnecessary and advised me simply to wait.
But, 18 months later, I was still limping; I had put on several pounds through lack of exercise. I looked and thought of myself as an invalid. By this time I had given up work as a personal clothes consultant: it involved too much running around to clients wielding large bags of garments. And along with tennis, my social life had completely disappeared, with only a few close friends visiting. I even took up bridge: it was one activity I could do that didn't involve using my legs. My form of travel was a wheelchair.
It is now three years since I did that step class. Finally, after being able to do little but sit on the sofa, I can walk unaided. I have been forced to admit that my tennis days are finally over, and I wouldn't even consider running for a bus, but at least, if I am careful, I can work out in the gym. I have learnt to predict the weather by the twinges in my knees.
When I spoke to the health farm about the injury, I was informed that I should have been wearing stronger, impact-resistant trainers and that I was obviously not performing the steps correctly. Why had I not been warned at the time?
I have since met others who have suffered a similar fate after doing step, among them a 24-year-old instructor. My advice to anyone interested in step is to approach it with extreme caution. I have learnt to be humble and accept that the body has its limitations, especially after a certain age. I for one won't be rushing to try out any new form of callisthenics, however fashionable, without checking it out thoroughly firstn
Anyone who has suffered similar problems is invited to contact the author via `The Independent'.
Step aerobics: how to avoid injury
Step is a high-impact aerobic exercise that aims to tone up muscles in the legs and thighs. But specialists say that the repetitive movements, with the increased impact on knee and ankle joints, can strain the knee and damage the joint surfaces. Step exercises "should be carefully monitored and should start slowly for short periods, building up as the knee becomes adapted", says John Browett, the consultant orthopaedic surgeon who treated Paul Gascoigne's shattered knee. Instructors and participants are equally at risk, with those exercising more than four to five times a week liable to suffer injuries in the knees from overuse. The risk increases with bad training shoes or a poor surface.
To reduce the risk:
l always do a five-minute warm-up to stretch muscles, especially quadriceps, hamstring, anterior shin and calves
l always wear good, supportive, shock-absorbing trainers
l do not "train through pain", even if large releases of endorphin dull discomfort
l wear special exercise supports to reduce stress on joints
l eat a nutritionally balanced diet, including calcium-rich products to strengthen bones