The Reebok Club is a six-storey, $55m (pounds 34m), state-of-the-art exercise emporium that looks more like a marble-finished cruise liner than a gym. Ordinary members pay around $2,500 for their first year, while "executive members" pay twice that for added exclusivity and luxury - plus the chance to rub shoulders with assorted stars and sports personalities. As well as basketball's Magic Johnson, Bruce Willis, Demi Moore and Morgan Fairchild are reputed to be members.
Apart from the usual mirrored aerobics studios, the Reebok has an entire floor devoted to cardiovascular work-out machines; a three-storey climbing wall; a swimming-pool wired for underwater sound; skiing, windsurfing and golf machines; plus an all-weather outdoor running-track around the top of the building. It also boasts all the services of a five-star hotel, from an executive business centre to the trendy Reebok Grill.
But the hottest new exercise craze at the Reebok, fresh from California, is "spinning". Invented by Johnny G Goldberg, former LA racing cyclist and millionaire, spinning (which is just hitting one or two of the smarter London health clubs), is technically no more than pedalling an exercise bike with a narrow seat, low handlebars and a 30lb iron flywheel. The main thing about spinning, though, is that it is a group activity, led by a "motivational" leader; devotees describe it as both emotionally inspiring and a great way to burn up calories (up to 800 a session, it is claimed).
So popular is spinning that at the Reebok that you have to sign up for classes 45 minutes beforehand. Which is what I did on a recent trip to the Big Apple - and in the interim, I took in the rest of the Reebok empire.
On the cardiovascular floor, there were waiting lists for many of the 130 different treadmills, stair-climbers and cycling and rowing machines, as New Yorkers fresh from the office pounded and sweated their way to nowhere. There was little chance to make conversation. Most were listening to personal stereos while reading documents or newspapers propped on stands at eye level. Large televisions high in every corner kept them abreast of the news.
I asked a staff member to explain the "microfit" computerised fitness assessment available to all members, and she showed me the print-outs that tell them their weight, fat ratios, strength, endurance and heart/lung capacity. But what did these mean in terms of health? I asked. She looked at me blankly.
On another floor, members in white robes moved trance-like through t'ai chi routines, while on one of the two full-sized basketball courts, very tall men were leaping and shooting at the net (although I didn't see Magic Johnson). At the top of the building I found the outdoor running-track just as the lights of downtown Manhattan were sparking into life. "You're never going jogging in this weather?" admonished a forty-something man in a track suit, leaning against the wall.
I noticed smoke curling gently from behind his back into the night air. Cigarette smoke! "Oh promise you won't tell them!" he begged. "I'd be barred for life!"
Reassured that I was not exclusively in the company of health fanatics, I went along to my spinning class to find serried rows of exercise bikes arranged in a semicircle around the instructor. She was called Lindsay and she helped me make the half-dozen adjustments to saddle, handles and speed that were necessary to match my height. She also explained that once in motion, the pedals would be spinning the flywheel - which wouldn't stop suddenly, even if I wanted to.
Very soon the other bikes were all occupied by avid spinners, sweat bands, hand towels and water bottles at the ready. Then the pounding music kicked in while Lindsay shouted instructions to us over a microphone.
We began relatively slowly, but soon she was ringing the changes in pedalling speeds and posture, making us sit up, sit down, lean to left and right, "pulse" up and down with our arms, all to a vigorous count of eight.
As I was in leggings (as opposed to the regulation padded cycling shorts) I found the saddle excruciatingly uncomfortable, but despite the inspiration of Lindsay's skinny bottom vibrating in mid-air before me I had no hope of standing up to match the whizzing of her pedals. Fortunately the pace slowed as she encouraged us to close our eyes, change gear and imagine that we were cycling up a long hill, pushing ourselves to the limit, reaching the crest, finding our "champion" within ...
Obediently, the whole class embarked on this communal exercise of imagination, sweating and pushing in unison. This carried on for another half hour, long enough for me to work out how to coast in comfort while the others had their eyes shut.
After the class I talked to fellow spinner Becky St Jean, a trim 35-year- old whose immaculate make-up had survived our surreal journey intact. A friendly and charming single woman, she works a stressful 11-hour day as a senior manager for a chain of shops that includes Macy's - and then comes straight to the Reebok, exercises, eats at the grill and goes home to sleep.
"Coming to Reebok is real lifestyle," she explains: spinning enthusiasts form little cliques, follow their favourite instructors and sometimes find romance into the bargain. "Spinning is real popular here because it is a unisex sport, a group activity and an incredible sweat work-out which burns as many calories as running."
She was keen for me to go down to the second floor to try the "transporter", an elaborate treadmill that mimics the action of jogging uphill. "It's so fun, just totally cool: burns calories and isolates your glutes!" And away she bounded, the picture of American enthusiasm: another day, another dollar, another work-outn
But for improved health and stamina ...
Spinning may be coming soon to a gym near you, but is it, and other ways of working out really good for you?
The Health Education Authority says that working out is fine if you enjoy going to the gym - but there has been a major shift in recent years away from promoting high intensity exercise, and towards moderate-intensity activity. The new consensus is that half-an-hour's walking or gardening a day, five days a week, is better for health than one or two work-outs weekly - and you lose more weight that way.
You can also gain as many health benefits, and lose as much weight, from a half-hour walk as from a 15-minute run, because the walk lasts longer, and so do the benefits of such moderate exercise.
"Fitness" (strength, endurance) is one thing; health (lowered risk of heart disease, stroke and high blood pressure) is another.
In terms of health, the shift to aim for is from doing nothing to doing something: if you are a couch potato, there are immense benefits from taking up walking, gardening or houseworkn