Hey Fatty, get off your derrière and get down the gym! Your continued employment depends on it. That was the message I took away from a conversation with Mark Jenkins, marketing director of LA Fitness, who told me: "Gym usage appears to be increasing. People who are afraid of losing their jobs are linking their fitness and how good they look with their competitiveness in the market place."

That scared me. My idea of exercise is levering myself off the sofa to collect another glass of white from the fridge and eat that last Happy Face mini pizza that the kids rejected. Yet the modern workplace may shortly resemble the video for the aerobics-inspired pop song Physical. This featured a Spandex-clad Olivia Newton-John bullying hapless wobble bottoms. I would be the inevitable Untermensch in that Nietzschean struggle for employment.

However, the surge in exercise discerned by the optimistic Mr Jenkins may be short-lived. The UK's private health club industry has gone through a rapid but little-remarked renaissance over the past six years. Now, as the recession deepens, it is set to run out of puff faster than a 22-stone man on a cross-trainer. The nastiness is likely to start next month. In January those citizens who have indulged in the sin of gluttony over Christmas traditionally seek the absolution of weight loss at their local temple of health. But with disposable incomes shrinking, far fewer would-be gym bunnies will sign those vital direct debit mandates.

Fitness clubs were well-established in the US when Ms Newton-John released her chubby-chiding hit in 1981. They were a later arrival in the UK. David Lloyd, an English tennis star (if that is not an oxymoron), was one pioneer, setting up his first club in 1982. He floated his expanding business in 1993, triggering the start of a stock market love affair with fitness clubs. Membership growth was propelled by the interlinked modern fears of heart disease and looking fat. The clubs were reputedly good places for business networking, too. Men who joined for this purpose could usually be found in the café, eating muffins and watching Sky Sports.

"By 2000 growth was explosive and we were opening 40-50 new clubs a year," recalled Colin Waggett, chief executive of Fitness First, the UK's largest fitness chain, "We cannot return to that, nor would we want to." A stock market slump that year hit membership sales in 2001. One company, Lady in Leisure, went into administration. Several others warned on profits. Their shares languishing, the dominant businesses swapped the hurly-burly of the stock exchange for the peaceful obscurity of private ownership.

Statistics from The Leisure Databank, a data company, show that the number of private fitness clubs has risen 74 per cent to 3,100 since 2002. The income of an industry that also includes council gyms has climbed by a similar percentage to £3.7bn. Affordable fitness clubs have led the expansion.

Little of the increase happened this year. Leisure entrepreneur Duncan Bannatyne, whose Bannatyne Fitness manages 61 clubs, believes the UK market is approaching saturation. "Most large towns now have health clubs," he told me, "We are not building any new [Bannatyne] clubs as a result." Fitness First sees scope to install more of its small, flexible clubs, which rarely boast swimming pools. But it is hampered by the uncertainties that falling property prices create.

"Sales are level and attrition is picking up," warned David Stalker, chief operating officer of the Fitness Industry Association. About 35 per cent of new gym members already drop out within a year. Many January joiners backslide to a life of takeaways and television by February 1. To some commentators that looks like good business for fitness clubs. But operators bemoan the cost of membership drives and the lost opportunities for incremental sales of Pilates classes.

This January, the challenge will be to sign people up at all. The industry has been infuriated by a NatWest Bank advertisement that showed a financial adviser telling a cash-strapped customer to cancel his health club subscription. Its prescience was probably what rankled most. A monthly charge of £30 to £60 is a cost some will decide they can live without. "To my customers, membership is a necessity not a luxury," riposted Mr Bannatyne, as any entrepreneur who believes in his own business would.

My guess is that there will be a shake-out. Companies House figures for 2007 do not depict an industry that is in strikingly robust health. Some big chains made impressive operating profits. But most ran into the red after paying interest on their considerable debts. They will have limited room for manoeuvre in a serious sales downturn

Health club promoters claim that members need the endorphin rush of funk-a-robics or the deltoid crunch deck more than ever as recessionary gloom intensifies. The problem is that you can achieve the same buzz by digging potatoes or cycling to work on a boneshaker with Sturmey-Archer gears. These activities are more in tune with a zeitgeist that requires Britons to stay in their underheated dwellings, bottling unpalatable foodstuffs and knitting unwearable jumpers. Austerity chic will wear off in a couple of years. More people back in work will mean more people working out. Meanwhile, the only burn many fitness club owners are set to experience is the financial kind.

jonathan.guthrie@ft.com

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