In an ever-changing world, there are some things you can count on: seasons will change, rich people will complain about taxes and Geri Halliwell will change shape. Unlike her last transformation, though - when she shrunk to near-emaciation - Halliwell is now looking healthy and strong and has been parading her six-pack around the south of France in a selection of fetching bikinis. And, if you believe the hype, her new look is all down to the latest fitness craze: kettlebells.

On reading about Halliwell, I was sceptical that any single form of exercise could effect such a transformation. Ten minutes into an "extreme kettlebell" session with instructor Tommy Matthews, though, and my scepticism has melted. My heart is hammering and my legs feel like jelly.

"Using a kettlebell works far more muscle groups than almost any other exercise, so you get a true all-over workout, and it improves your posture much more quickly," explains Matthews. "Because you are moving the weight around or swinging it, the whole body has to work hard to move and to stabilise it."

Kettlebells are nothing new - the word girya, meaning kettlebell, first appeared in a Russian dictionary in 1704 - and they have been used for centuries by strongmen. They became popular in the west a few years ago, and in my household shortly afterwards. My other half is often to be found sweating and grunting like a farmyard pig along to DVDs with titles such as Full Kontact Kettlebells and Art of Strength.

I, meanwhile, remained immune to their charms. Like most women, I don't know much about weightlifting and usually avoid it at the gym, where it seems to be the preserve of posturing men in peculiar belts. Lifting weights seems technical, and therefore an easy way to hurt yourself without proper guidance. It's also an activity that summons up images of 1980s female athletes from behind the iron curtain, with Popeye-style biceps.

Matthews, however, is having none of it. "Women just don't produce enough testosterone to bulk out and build large muscles," he says. "Kettlebells will get rid of body fat and replace it with lean muscle, which will give you a more toned look. You can also tailor it specifically - whether your goal is weight loss, toning or improving your fitness."

A kettlebell workout can also use up a phenomenal number of calories - a well- conditioned person can burn up to 1,500 calories an hour. That compares with roughly 600 running on a treadmill and 300 for Halliwell's previous favourite exercise, yoga.

Matthews runs me through a series of the basic kettlebell drills. First, the dead lift - which involves lifting the bell from the ground, using your hips and bum to provide the oomph. "Your arms shouldn't feel like they're taking the strain," says Matthews.

Other exercises soon follow - including the front squat, military presses (lifting the bell from chest level to above the head) and finally the swing and snatch. The swing is fairly self-explanatory - with your arms acting as pendulums, you swing the kettlebell from between your legs to around head height. It's a fluid movement; the only difficulty is trying to stop, as the weight threatens to bring your arms right through your legs on the way down.

Matthews next demonstrates the snatch - an extended swing, followed by rotating the kettlebell over your wrist through nearly 360 degrees. It looks like it could snap your wrist like a twig, but it's actually quite easy.

Many other exercises are possible - anything you can do with dumbbells you can do with kettlebells. However Matthews says that because your muscles also have to balance the weight, kettlebells are superior to dumbbells.

The bells also come in a variety of weights. "A female beginner would probably start with an 8kg weight and move up through 12kg to 16kg, " explains Matthews, "while a man will typically start with 16kg and go from there."

Once I have got to grips with the basic exercises, Matthews has me doing circuits - sets of kettlebell drills interspersed with jogging to a nearby tree. A few sets in and I can feel my heart rate reaching an alarming rate. A small boy wanders up and asks Matthews: "Can I have a go?" Alas, I'm allowed no reprieve.

So how soon before you get to see some results? "As with all exercise, it really depends on how your body adapts," says Matthews. "Generally about six weeks for a noticeable difference, but it could happen sooner. Two or three sessions a week should get you looking great pretty quickly."

My aches the next day certainly seem testament to the all-over workout the kettlebell provides - although, strangely, my arms feel fine. I resolve to retrieve my own unused kettlebell from its current role as a doorstop.

The other great advantage of kettlebells is that you can practise the moves anywhere. Matthews runs classes in local parks, but once you have an idea of what you are doing, you can merrily swing away wherever you like.

If you need a refresher course, you can also view Matthews's videos on YouTube, where he and his colleagues talk you through the various techniques. But don't go thinking you can opt out if it's a bit nippy.

"We just put gloves on if it's freezing cold," he says casually. "Otherwise your hands can stick to the kettlebell."

· Further information: outdoor-extreme.com