Fabio Comana, an exercise physiologist and spokesman for the American Council on Exercise (ACE), says CrossFit has a 'cult-like following' among current and former athletes, and those who enjoy competition and tackling an intense challenge. In the US Army, CrossFit is replacing or augmenting traditional physical training methods.

Created by former gymnast Greg Glassman, CrossFit is a strength and conditioning programme that combines gymnastics, Olympic weightlifting and cardiovascular exercise. The goal is to develop a broad and general fitness level through constantly varied, high-intensity, functional movements.

As Glassman says on his website, the workouts are designed to prepare trainees 'for any physical contingency - not only for the unknown but for the unknowable, as well'.

In a sense, the regimen is about being a jack of all trades and master of none. The aspects of fitness worked on are: cardiovascular and respiratory endurance, stamina, strength, flexibility, power, speed, co-ordination, agility, balance and accuracy.

The core of CrossFit is the workout of the day, posted daily on crossfit.com. One staple workout, for example, is the 'Murph', named after a US Navy lieutenant who was killed in Afghanistan in 2005. Done against the clock, it includes: a 1.6-kilometre run, 100 pull-ups, 200 push-ups, 300 squats, and another 1.6-kilometre run to finish.

Before my session at CrossFit Asphodel, I do some research into the phenomenon. It's hardly encouraging and leaves me a bit shaken: there are many sceptics decrying the workout as dangerous and irresponsible, and an even larger number of bright-eyed and burly converts showing off their CrossFit tattoos.

Pukey, the unofficial mascot, comes up repeatedly, pictured crawling away from a barbell, vomiting all over himself. He is joined by a more disturbing clown, Uncle Rhabdo, who is portrayed hooked up to a dialysis machine and standing in a pile of his own bloody organs.

Rhabdo is named after rhabdomyolysis, a condition induced when the body is pushed beyond its limit. Skeletal muscle breaks down and enters the bloodstream, where it can cause kidney failure. Glassman has written about the risks of rhabdo and warned participants about the potential problems linked with the unforgiving CrossFit workout.

'You'll be fine, Charley. I think you might even like it,' says my friend Adam Martin, director of lifetime fitness at a weight-reduction spa in South Carolina, on the phone, although he is aware of my sedentary nature. Martin has been doing CrossFit for three years, up to five days a week, and recommends the workout to everyone.

'You will definitely survive,' says another friend, Sarah Gillio, from Boston. 'Pain is temporary; glory is forever.'

They are right. My experience at the gym is challenging but rewarding. Asphodel is a spartan warehouse space furnished with concrete flooring and old-school gym equipment including free weights, hanging rings, pull-up bars and jump ropes. Rowing machines are as fancy as it gets; there are none of the treadmills and ellipticals that you find at other fitness facilities.

The day I visit, the workout consists of shoulder presses with a barbell, followed by three bouts of 500 metres on the rowing machine with the goal of improving your time at each go. I manage to complete the workout and keep my lunch down.

James, who has a degree in sports science, understands my limitations and encourages me to push myself. I feel consistently challenged but supported; exhausted but certainly never in danger.

I think I do rather well with the shoulder presses. James points out that I am lifting the same weight as one of the regulars, a petite and very fit-looking woman next to me. I am elated with what I think is a compliment, until he says: 'So, obviously you have a lot to work on in terms of strength.'

Members write their times on a board to track their improvement and standing within the group. (You can also post it for the world to see on crossfit.com.) Competition is a big part of the CrossFit experience - not a macho rivalry, but a supportive and fun one. 'Everyone is humble,' James says.

In fact, what I really like about CrossFit is the sense of community. 'At other gyms you go with your headphones in and pay little attention to anyone else,' says Martin. 'At CrossFit you interact with dozens of individuals who help push you to your max.'

Because of the intensity of the exercises, there are many benefits, says Pete McCall, an exercise physiologist with ACE.

CrossFit burns an impressive number of calories in a short time frame. According to Comana, women can burn 13 to 15 kilocalories per minute and men 15 to 18 kilocalories per minute - plus extra bonus calories during recovery following such an intense workout. That's about 50 per cent more calories than traditional machine-based weight training.

McCall says it also improves aerobic fitness while promoting the anabolic hormones that are responsible for muscular growth and may have an anti-ageing effect.

'However, the intensity of the exercises which deliver the benefits can also increase the risk of injury if not done correctly,' he says.

Comana disagrees with CrossFit's concept of universal scalability - where the same exercises are used for everyone regardless of experience, but scaled by load and intensity rather than by programme. 'Although CrossFit does offer beginner workouts, pushing someone to complete a 500-metre distance on a rowing ergometer as fast as possible on day one may be a little excessive.'

The biggest weakness of CrossFit seems to be its lack of consistency. One's experience depends on the specific gym and individual trainer - and the competence of CrossFit trainers can vary enormously.

CrossFit instructors only have a few days of training before being certified. But good strength coaches usually spend years attaining this level of expertise, says Comana. 'The reality is that many of these [CrossFit] trainers do not possess the appropriate levels of knowledge commensurate with the complexity of many of the exercises performed.'

Before beginning a CrossFit programme, McCall advises to work with a personal trainer to learn how to perform the movements required, and to develop the necessary mobility, stability and movement skills required.

CrossFit Asphodel requires all new members to take four one-hour 'on ramp' classes that teach basic movements and core philosophy, safety tips and nutritional advice.

According to the CrossFit webpage, the ideal diet is 30 per cent lean and varied protein, 40 per cent predominantly low-glycaemic (complex) carbohydrates, and 30 per cent predominantly monosaturated fats. The basic rule to 'base your diet on garden vegetables, especially greens, lean meats, nuts and seeds, little starch, and no sugar' is fairly non-controversial and in line with the opinions of many nutritionists.

The fact that CrossFit encourages practitioners to embrace the 'paleo diet', or the 'caveman diet' - based on the presumed eating habits of early humans - is more divisive.

The idea of the caveman diet is that 'evolution has not kept pace with advances in agriculture and food processing resulting in a plague of health problems for modern man'.

The actual CrossFit session lasts about an hour, and includes a dynamic warm-up, strength/movement practice, the WOD, and a cool down. Each class usually has between six and 15 participants.

Steve Wilson, a locally-based construction firm project manager, started at Asphodel 10 months ago after injuries forced him to retire from high-level rugby. He finds the workouts 'tough, competitive and fun'.

But is there a down side? 'Sometimes it's frustrating when you think you are fit, but then get shown a new CrossFit movement and suck at it and have to work hard just to get competent at it,' he says.

But the great thing is, you won't be short of people egging you on. Says Gillio: 'Everyone really supports each other, cheers each other on, shares stories and revels in the challenges. We're all in it together.'