Getting fitter and losing weight are popular new year's resolutions – but what's the best way of doing so, asks Susie Weldon?

As thousands of people sign up to gyms and vow to lose weight, what is the most efficient way of getting fit?

The exercise industry has fads and fancies like any other. Remember Jane Fonda's aerobics frenzy in the early 1980s when it seemed everyone was donning leotards and "going for the burn"? That was before we realised that high-impact workouts could leave us with dodgy backs and painful knees.

And what about those vibrating straps that were supposed to wobble away the weight? You don't see many of those any more - probably because they didn't actually work.

Current fitness trends include Spinning, core training, bodypump classes, yoga, Pilates, personal trainers, celebrity exercise DVDs, Swiss balls and kettlebells.

And Mr Motivator has returned to our screens. Now 56, the fitness instructor is back on GMTV for three weeks – and can still get into the same clothes he wore 15 years ago.

Gyms have changed from being the exclusive preserve of weightlifters and boxers in the pre-Jane Fonda days to trendy hangouts with scary-looking hi-tech equipment.

The problem is that many of us use gyms enthusiastically when we first sign up but then drop out. So what's the latest thinking on getting fit – and losing weight?

Personal trainer Alex Poole used to play rugby for Bristol and was also lead condition specialist for Gloucestershire County Cricket Club. He now has his own gym, Kinetic Fitness, in Bristol.

He says the business of fitness has become increasingly scientific as we've learned more about how the body reacts to exercise. This means spending less time working out – but doing so more effectively.

"The old thought was you could just go for a few runs and you'd lose weight and, yes, running has a health benefit," says Alex. "But the body adapts to exercise. The fitter you get, the less energy you use, so over time you burn fewer calories."

To try and counteract this, Alex varies his clients' workouts every four weeks to keep challenging the body and uses techniques such as interval training – short bursts of intense activity interspersed with slower-paced workouts.

"The idea is to change the pace of a workout to challenge the body to do more than it's comfortable doing," he says. "That way, you burn more calories and continue doing so for up 36 hours afterwards depending on how intensive your workout is." There's also been a move away from hi-tech kit towards a back-to-basics approach, with training that mainly uses the body and some weights.

Machines strengthen muscles in a "non-functional way", says Alex: "They're about fixing people in place and isolating a particular muscle.

"Now we're more concerned about health and fitness and there's been a big move in the last 10 years towards functional training – training that mimics daily life.

"For example, one of our core exercises is the lunge. This works all of the stability muscles around the ankle, calf, front and back of the thigh and stomach. To get that workout on a machine, you'd have to use several different ones."

This kind of training is just as effective in helping you lose weight, says Alex, but it also strengthens the muscles we use in daily life.

■ Kinetic is offering a fitness MOT for £10. Call 0117 377 9265.

* Alex Poole has launched a website to encourage people to exercise more and eat healthily.

Health Secretary Alan Johnson has warned that nine out of 10 British adults and two-thirds of children will be overweight or obese by the middle of this century unless we change our lifestyle.

This year in Manchester, the Government will pilot its £30 million Change4Life scheme encouraging people to be more active and make healthy food choices. If successful, it could spread nationwide.

"Most people know they have to do some exercise to keep fit and healthy," says Alex, "but sometimes they don't know how to go about it. The website www.getbristolfit.com gives clear information and contains videos and audio instruction, nutritional advice, plus general hints and tips."