It's Saturday morning and I've just finished flipping a huge lorry tyre across a field with my bare hands. Now I'm about to start on a bag of sand and, when I'm finished with that, I will allow a man to tie a rope round me and attempt to run off while he pulls me back.

No, I haven't been placed in a chain gang, rather I am taking part in a new exercise that has swept America and is catching on here. Called functional exercise, the idea is that you work out doing things your body is designed to do, rather than sitting on a machine in a gym. It's a back-to-basics approach using items that usually never come through the door of a modern healthclub: climbing ropes, large tyres, kettlebells, medicine balls, Olympic rings of the sort gymnasts dangle from, even sledgehammers. The aim is to make your body more efficient at natural activities such as pushing, pulling, running, jumping, balancing and throwing.

My class is run by Spartan Fit of Brighton, one of a growing number of physical training companies to reject the high-tech gym approach. The class got its name because it uses similar training techniques to those that were used to prepare the actors for last year's movie blockbuster 300 - a retelling of a battle between the Spartans and the Persians - for their roles as muscular ancient warriors.

Anyone thinking that they are going to get a body like King Leonidas by pounding the treadmill for hour after hour is going to be disappointed, says the chief instructor Steve Liszkaa, also a fire service physical training instructor.

Demanding, but not insane

The clue is in Spartan Fit's motto: “If you want to look like a Spartan, you have got to train like one.” That said, the training is demanding but not insane. There are about 16 people (ten men, six women) assembled in a park when I do it, ranging in age from early twenties to late forties. Only Liszkaa and a couple of the pupils look what you would call superfit, but the 75-minute class is only just establishing itself and there are plenty of beginners. Also, this sort of exercise tends to produce functional, lean muscle, not the bloated body-building look.

The class begins with kettlebell swinging. A kettlebell is like a cannon ball with a handle on it and has been a popular conditioning aid in Russia for years. It is a test of explosive power, balance, co-ordination and cardiovascular fitness. The technique is not too difficult to learn with an 8kg weight, but when Liszkaa gives me a 24kg weight I can feel my body having to work to flip the kettlebell up. After the kettlebells it's on to a circuit of sandbag carrying, pulling tyres on ropes and tyre flipping (that's flipping as in flipping exhausting). It nearly kills me, as does the wheelbarrow workout, in which your partner takes your legs and you walk on your hands for 50 yards. If you want core training, throw away the Pilates ball and try a few of these. After about 30 yards I'm chewing turf. The exhaustion is intense.

The whole thing finishes with a tug of war, which fosters the team spirit that Liszkaa is keen to promote. “We vary things week to week; sometimes we'll do more sprinting, sometimes bounding and jumping. It's about always offering a challenge,” he says.

This sort of training helps with more than appearance. Olympic athletes are seeing the benefits of functional exercise and some of them, such as sprinters, have been doing it for years anyway. The Olympic sculler Alan Campbell supplements his training by lifting sandbags, short sprints and sawing logs. Chelsea and Liverpool football clubs use kettlebells in training.

It's high-intensity stuff, and it works. It operates a much broader definition of fitness than the gym, going beyond cardiovascular and strength fitness to take in power, balance, flexibility, agility and speed.

There is solid science to back this up; sports medicine journals are brimming with articles about the benefits of high-intensity training, which is the way human beings have trained since time began for sport and for the military. In fact, as Liszkaa points out, the machine-based gym model has really been popular only since the 1970s and owes more to economics than it does to its effectiveness. Machines, such as the leg press, require less training to use than free weights, therefore less qualified and cheaper instructors can be employed.

Liszkaa sums it up neatly: “The machine is designed to look as though it's not going to be hard work, that you can have a good workout just sitting down. It's not possible. If you want rewards, you have to sweat for them.”

A greater sense of fulfilment

But is this sort of training suitable for everyone? I'm 44, not a natural sportsman, though I do fencing once a week and play football in the fat lad's position of goalkeeper. I have a dodgy arm and, on the day of the class, a mild hamstring pull. I still managed to finish with a greater sense of fulfilment than I ever have sitting watching MTV in a gym.

Liszkaa allowed everyone to go at their own pace and I didn't feel any of the exercises were beyond me. The kettlebells in particular seemed to do my injuries some good. Also, the team aspect of the training is inspirational, with your fellow Spartans urging you home on sprints and pulls.

“People tend to bond in adversity,” Liszkaa says, “so we try to provide some.” And if you're still not convinced of the benefits, consider this. It is generally agreed that when, in 480BC, 300 Spartans defended the pass at Thermopylae for three days against 240,000 Persians, they exhibited a reasonable standard of fitness. How many of them had got that on a crosstrainer?

For more information, visit www.spartanfit.co.uk; the first class is free, then it costs between £3 and £9 a lesson, depending on how often you go

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