I have discovered the StairMaster from hell, and it is the steps of the Philadelphia Museum of Art — 99 of them, which I ran up and down about 10 times.

Think that doesn't sound like a lot? Try it just after you've done about 100 lunges at the bottom of the stairs. And, oh yeah, let's not forget the 30 "knuckle squats." That's when you stop lunging for a minute and squat and rise, squat and rise, over and over again until your legs start shaking worse than a drunk on a 30-day bender.

Welcome to one of the hottest fitness trends in years: extreme outdoor workouts, or "boot camps." Not for the faint at heart, these programs — held rain or shine, freezing cold or suffocating heat — are modeled after military physical training and are geared to the exercise aficionado who craves structure, discipline and variety.

"The idea of military fitness has been around for a long time," says Andrew Flach, author of "The Official Five Star Fitness Boot Camp Workout" (Hatherleigh, 1999). "It just lost itself in the Lycra and spandex of the modern gym."

Yet, just as the Marines differ from the Army, every boot camp program is individualized, says Ken Alan, a Los Angeles-based personal trainer and spokesman for the American Council on Exercise. "But when we use the term, generally we're referring to the non-music, or nonrhythmic based exercise."

Classes can be held inside, although most are outdoors, and they can involve anything from basic calisthenics modeled after military physical exercises to sports drills involving football, tennis or basketball. "It's as individual as the teacher leading it," Alan says.

Which is why, on a brilliant, cool fall evening, as a full moon rises over the twinkling lights of downtown Philadelphia, I am stepping up and down a concrete ledge until my thighs burn. My instructor, active-duty Army Capt. Lane Turner, 32, who runs these classes for Philadelphia-based Platoon Fitness, is counting off in a singsong cadence, and I'm panting to keep up because I'm too embarrassed to quit. Embarrassed. Not scared.

For, unlike some boot camp programs, in which instructors yell and berate participants as if they were snotty-nosed recruits, Platoon Fitness espouses a softer, gentler approach.

"We tried that method," says Director of Operations Mike Smaltz, himself a former Navy man. "But we found that people got fed up with being yelled at and pushed until they got sick."

So now Platoon Fitness uses gentle yet insistent persuasion to keep its members going. The approach seems to be working. Platoon Fitness runs 11 classes a day, five days a week in and around the Philadelphia area and has more than 900 active participants ranging in age from 12 to 78.

But they're not above a bit of boot camp interplay, as when Dori Cowan, 28, a Philadelphia research analyst, turns up five minutes after class starts. "You're late," Turner says mildly. Then, "Drop and give me 10." Which she does.

"I like the motivational factor," Cowan says, explaining why she willingly puts up with the (good-hearted) abuse and industrial-strength workouts. "I'm not real good at disciplining myself."

The instructors know that. So when a Platoon participant skips a class, he or she gets a phone call from one of Platoon's 25 instructors. Skip several classes, Smaltz says, and the platoon leaders may turn up on your doorstep, perhaps with the entire class, and run a few drills right in your hallway, chanting your name.

"It's just our way of showing we care," he says with a grin.

One of the unique aspects of this type of workout is how it turns the outdoors into one giant gym.

In Chicago, for instance, participants in Bulldog Boot Camp use the sand on the shores of Lake Michigan for drills and the benches in Lincoln Park for body dips.

Despite the weather extremes in the Windy City, classes have been canceled only three times in 18 months, owner Greg Major says, and that was because of lightning.

The weather is an integral part of these programs, says Alan. "One of the objectives of boot camp is to toughen up both your body and spirit and discipline," he says. "If you can do it under these miserable conditions, then you're a better human being."

Still, what kind of person gets up at 4 a.m. to attend a 5 a.m. class held outside in the bitter cold and snow?

"The kind of person who wants to get their workout in, who's bored with the gym, and who knows that for one hour they don't have to think about what they have to do, they just have to follow along," says Major.

Keith Devoe, a 38-year-old salesman from Chicago, is one of "those" people.

Although he played hockey two or three times a week and was already in pretty good shape , he was looking for something more challenging. "I find health clubs tend to be boring," he says. "The same things day in and day out."

Bulldog and its instructors push Devoe harder. "They get in your face a little bit to give you that nudge," he says. During a run, for instance, they come up and run next to him, shouting "Why are you running so slow?" and then take off, challenging him to keep up. If he's struggling during push-ups, Devoe says, they get down and do them with him, egging him on with their ability to complete the exercise.

"They give you a pat on the back and a kick in the ass," he says, "but it's all in good faith, all positive, never negative."

"The whole purpose is to create a fun environment," Major says. To that end, he says, his instructors are motivating and even in some cases nurturing, but not in any way soft. "We ask that every day [class members] do their absolute best, and every person's best is based on what they're capable of doing. The only measurement is against themselves."

Participants in both programs range from the out-of-shape, never-run-a-mile-in-her-life homemaker to the gung ho ex-Marine trying to keep in shape.

Which is why most programs structure workouts to the individual, not the group. So when I showed up and told Turner that my idea of vigorous exercise was walking the dog at a brisk pace and getting up to change the channel instead of using the remote, he took that into consideration. While the rest of the group did knuckle squats after every 10 lunges, I had to do them only toward the end of my lunges. While the rest of the group ran at full speed up the stairs, he let me walk the stairs and run the landings.

And when he and Smaltz saw my endurance flagging 45 minutes into the 60-minute class, they took pity and let me stop early.

And you always have your fellow "recruits" to keep you going. In fact, the group spirit that evolves during these programs is one reason participants like Richard Grobman, a 39-year-old stockbroker from Villanova, Pa., keep coming back. The classes, he says, remind him of his days as a college athlete, when everyone worked together toward the same goal. "The camaraderie is just amazing."

That's an important part of any boot camp-type program, Alan says. "There's almost an unwritten responsibility on the part of the participants that other people depend on you. When you're told to get down and do 10 push-ups, other people are depending upon you to help them do their push-ups by you doing your push-ups, so there's a synergy between the people in that workout."

Many of the exercises, such as relay races, build on this synergy. The night I took his class, Turner had us all link arms and use a foot-high concrete ledge as a modified stair stepper. Over and over and over again. By the end, the fear of dragging the whole group down with me kept me going, despite the pain in my legs. Had I been doing it alone, I would have stopped long before it was over.

Platoon Fitness also sponsors monthly get-togethers for participants at a local restaurant, and early morning workout groups often head out together for breakfast after their session.

Even I felt included in the group, especially when Grobman passed me on the steps and leaned in close to give me some advice. "Take lots of ibuprofen when you get home. You're going to need it."