It helps posture, eases back pain – and makes your beergut vanish. No wonder sales of fitwear for men are booming. Josh Sims puts the latest pair of 'wonder pants' through its paces

Fresh out of the box, the first impression is that no grown man can get into them. These are exceptionally small smalls – and with them an itsy bitsy, teeny weeny polyester and spandex undershirt. But, despite the pants' suggesting escapees from the babywear department, the label says "men's XL". So contortion follows.

Indeed, getting into them is not for those late for work: heaving and squeezing, it is like forcing yourself into a rubber glove. Once on, panic ascends. Can I move? Can I breathe? But then another thought. Hey, where did the paunch go?

Welcome to the world of fitwear. It is the big thing in men's underwear, designed to cajole and compress your flesh into more streamlined form – a man's girdle, or "mirdle" if you like – while also assisting better posture and, through that, easing the back strain that 80 per cent of men suffer at some time in their lives. It has been pioneered by Gavin Jones, an affable Australian with a long career in community health promotion who – and this is surely no coincidence – woke up 40 one day and decided to live it up in America for a few months while he pondered what to do with the rest of his life.

That turned out to be to help other men facing the abyss of middle age to stand tall and look trim. Some £700,000 later – after a year of research into physiotherapy and exoskeleton design, and a development programme that included Jones standing in his old pants and being wrapped with duct tape – last year his brand, Equmen, was born. Following its launch in the UK, this year sales of the undershirts, vests and boxers are expected to double to around £6m.

"I just wanted something to make me feel more like a premiership footballer than a geriatric," explains Jones. "I knew there was tummy-tucking underwear for women but I was more intrigued by the specialist underwear worn by pro athletes, which supports and warms the musculature. But it's a bit full-on and the average bloke looks pretty silly in it. So I decided to make a more everyday alternative. Now it's the first thing I put on the morning and I feel and look a better man for it."

It seems that other men are sharing the same thought. King of status undies Calvin Klein has its close-fitting XT line, using what it calls "compression features" to abate that unsightly flesh tsunami. Marks & Spencer this month also launches its own advanced take on fitwear, Bodymax Plus, which promises to "battle the bulge" and "instantly reduce up to 3.5 inches from your torso".

"It can look a bit daunting but it does work," argues Lyndsay Yeates, Marks & Spencer's menswear technologist. "You can eat well and slog your guts out in the gym but this kind of underwear still provides a finishing touch."

In fact, fitwear for men may not be as niche as we first imagined. There's definitely a growing demand for it.

The new fitwear category may the boost the flagging, if not sagging men's underwear market need – the research company Mintel's latest figures show that the UK market may be worth £679m but saw growth of just 0.9 per cent over the year to 2008. And with a third of men's underwear still bought not by men but their partners, men should perhaps show little surprise when the new delivery appears to look as though it has already gone through an exceptionally hot wash. In every carton, the hope of a David Beckham ad campaign, or a buff Daniel Craig emerging from the surf...

"But it's the men who are getting vainer. What am I saying? Men have always been vain," notes Jones, who is launching a women's line this September. "Yet this is definitely not about fostering paranoia or a sense of inadequacy. That's something women have had to contend with for decades. This is about saying to men, 'You're OK – now let's put something on your body that can diminish those aspects you're sensitive about'. Any man will look at himself in the shower and want to meet his own self-image."

Exercise and diet would be the best response, Jones concedes. But for those after a superficial fix, fitwear may fit the bill, "even if it does look as though it's made for a small pubescent boy," Jones adds. It would certainly play to the male love of gadgetry: far from being a gimmick, Equmen has so far racked up 67 global patents for its "helix body-mapping technology". This uses different tensions throughout the garments' seamless, engineered construction to create a second skin that supports the body in much the same way as a physiotherapist would strap it to help an injury.

Keeping one's gut on a leash may be one benefit, but by bolstering one's core, it also claims to promote blood flow and take the strain off the lower back and shoulders. "It stands you upright, which is good for those of us who spend our lives in front of screens – that your muffin-top then also disappears is a bonus," as Jones puts it. And, indeed, it does, almost as quickly as the edible variety tends to.

But, ah, if only the illusion lasted. Sadly, like some cut-price Superman, peeling off one's super-hero underwear lets it all hang out and that familiar figure, Joe Portly, appears in front of the mirror once again.