A woman performs Pilates exercises at the Performing Arts Physical Therapy and Pilates studio in West Hollywood in 1996, back when the fitness practice was starting to take off. Photo: Kirk McKoy/Los Angeles Times/Getty Images

Try something. Lie on your back and engage your abdominal muscles, lifting your head off the floor and extending your legs out at a 45-degree angle. Stiffen and lift your arms by your sides and then vigorously pump them, up and down, up and down, 100 times, breathing in for five, and out for five, in for five, and out for five.

This is the 100, one of the most iconic exercises in the century-old fitness method called Pilates. Designed to strengthen and stretch the body into its most Adonis-like form — Madonna and Gwyneth are reportedly devotees — Pilates went from an obscurity beloved of ballet dancers to a 1990s fad to a full-on mainstream trend by the mid-2000s. In doing so, it demonstrated to a whole generation of muscled entrepreneurs that Americans would pay a whole lot for the pleasure of a certain kind of pain, often coughing up $20 or $30 for a small group class.

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But Pilates has not quite managed to benefit from the boutique-fitness boom that it is in part responsible for starting. Interest in the method seems to have peaked last decade, dwindling just as spinning, barre, boot camp, CrossFit, pole dancing, and a million other niches started to bloom, and as yoga continued its Zen march to omnipresence. Across the country, attendance is down. Studios are struggling, and some are closing. Teachers are seeking additional certifications. Pilates centers are adding non-Pilates classes.

Call it the Pilatesdämmerung, or even the Pilatespocalypse. “It’s 1,000 percent true, and it’s worse than you know,” said Joan Breibart, the founder of the PhysicalMind Institute, who has been practicing Pilates for five decades. “I feel like there’s a huge future for Pilates, but we have got to bounce back from this.”

That “this” is a little bleak at the moment. In general, boutique-fitness classes like Pilates are growing at an astonishing clip. Pure Barre has gone from having 100 studios in 2012 to more than 300 studios today. SoulCycle has tripled the number of rides its devotees have taken in the past three years, too, and is preparing for an initial public offering of stock. Yoga continues to grow in terms of participants, revenue, and studio count, with 20.4 million Americans partaking as of 2012.

But a survey from the market research firm IBISWorld found that the number of people doing Pilates declined about 2 percent per year every year between 2007 and 2011, dropping down to 8.5 million. The number of “regular” participants contracted 9 percent in 2011 alone. Data on how that has played out in financial terms is hard to come by. But when I asked one research analyst if investors were interested in Pilates, she retorted, “No, they’re not. No.” And Pilates instructors themselves said that the change was palpable.

Indeed, the downturn has been so sharp that an American College of Sports Medicine survey floated the theory that Pilates was a fad, not a trend. “Without reinventing themselves (as Yoga seems to do), these forms of exercise will not survive in the fitness markets of today,” Yves Vanlandewijck of the University of Leuven wrote about Pilates, lumping it in with — gulp — Zumba.

To explain what might have happened to fitness’s onetime boutique darling, Pilates instructors and market experts identified a few factors — the biggest one being that the method became a victim of its own success, inviting competition that then firmly, flexibly, leanly muscled it out. “Pilates was way ahead of the curve in terms of recognizing the appeal of group-based fitness,” said Aarti Kapoor of Moelis & Co., the investment bank. “The new factor to consider today, however, is that there is so much more choice – and in many cases, the alternatives provide an edgier experience and a stronger community vibe to the consumer. Given these factors alongside the capital intensity of Pilates as a business, the growth in Pilates studios has meaningfully lagged the growth of other studio-based fitness alternatives.”

A decade ago, in other words, a young woman in a big city looking for an intense workout to sculpt her into one of those Madonna-type mononyms would have had just a few boutique options. Fast-forward a few years, and our prototypical young woman found herself inundated: Pilates, classes that are like Pilates but are not exactly Pilates, Pilates-yoga fusion, barre classes, barre-yoga fusion, cross-training, high-intensity interval training, fusions of fusions, and on and on. The continued growth and diversification of yoga — and its lower price point — might have drawn some Pilates devotees away as well. “More people participating in yoga limits the demand for Pilates,” said Sarah Turk of IBISWorld.

Then, there might be issues with the Pilates workout itself: Given all the options out there, it seems to be too quiet, too antisocial, too critical, and too sweat-free for the millennials blowing the boutique fitness bubble. “A lot of people will fight me on this, but correction is overdone and it is too wordy,” said Breibart, describing how instructors help their students realign and reform their bodies, sometimes a little harshly. “I remember going to classes in the 1960s where if you did an exercise and your left eyebrow raised they’d criticize you. But people don’t want to be picked on. It can come across as too negative.” And that criticism often happens in a quiet room with only a small handful of students. “We were always so narrow-minded about that,” Breibart said. “You need to include some music! It’s a motivator!”

The lack of sweat has also dimmed the practice’s appeal, said Amanda Freeman of SLT, a “heart-pumping, calorie-burning, and total-body workout” performed on a Pilates machine. “I think the problem with traditional Pilates is that you don’t sweat, so you don’t feel like you’re getting as good a workout as you do with other popular workouts,” she told me. “The reason I started SLT was that I wanted a Pilates body — those are the results that most people want — but I didn’t enjoy the Pilates workout. You kind of felt like you needed to get on a treadmill or an elliptical afterwards to get a complete workout.”

Fitness experts cited one last factor: the dudes, many of whom shy away from Pilates, despite its demonstrably excellent results among individuals with a Y chromosome. “Yoga has really appealed to a nontraditional demographic, including men,” said Turk. “That is a reason why Pilates is lagging.” Freeman noted that she did not include “Pilates” in SLT’s name precisely to avoid the athletic biases some men might have. “Pilates doesn’t have a male appeal, and we think our workout should appeal to men just as much as women,” she said.

All those concerns were echoed in a quick, scientifically unsound survey of gym bunnies that I conducted. Some typical responses: “Not as much fun as SoulCycle.” “Austere.” “Expensive.” “Dated.” And those gym bunnies have been voting with their lithe feet, the data shows.

In response, some studios have closed, as have some training programs. Many others have pivoted instead — moving away from the traditional Pilates mat and machine workouts and toward disciplines like yoga, barre, and cross-training. “You have to in order to survive! You have to do it, sure!” Breibart said, arguing that now is the moment for the whole Pilates industry to pause and re-create itself more in the image of what its customers want. And what they seem to want is more along the lines of what Freeman, Tracy Anderson, and studios like modelFIT offer — Pilates results without strict adherence to the Pilates method. (That, as you can imagine, has caused a lot of headaches among Pilates’s many purists and traditionalists, Breibart said.)

But as Pilates contemplates Pilatesdämmerung, the whole boutique-fitness industry might be facing its own kind of twilight. “Today, we have such a crowded marketplace,” said Kapoor of Moelis & Co. “Everybody is jumping on the bandwagon. Every entrepreneur who ever wanted to enter the wellness category is getting in. Given how hot the market is — particularly in influential urban hubs such as Manhattan and Los Angeles — a lot of concepts are overinflated today.” She went on: “There isn’t room for everyone in the long run.”

That whole boutique-fitness trend that Pilates helped to ignite, in other words, might start slimming down itself.