Ilaria Montagnani is peaking.

Across Manhattan, students of all ages line up in gyms angling for a spot in one of the 38-year-old Italian instructor's martial-arts fitness classes. Ms. Montagnani, who has appeared on the "Today" show and "Good Morning America," is in such demand that latecomers sometimes stand in the hall trying to follow through the glass. Class sizes got so out of hand at one Equinox club, managers had to lecture attendees on proper etiquette -- for instance, not saving spots in the front row.

"For a small hour, I'm a rock star," says Ms. Montagnani. "Then I come back to earth." That's because as the chiseled 125-pound teacher approaches 40, she knows her rock-star days are numbered if she can't create a vehicle to sustain her work when she physically no longer can. Currently she teaches 20 hours a week and can command $100 an hour or more; the industry average is $22. "I'm at my highest earnings level, and there's only one way to go from here," Ms. Montagnani says. "And that's downward."

It's a quandary entrepreneurs of many ilk face: When the business is you, how can it stay viable once your personal limits are reached? It's true whether you're a consultant or hairstylist, a model or pro athlete. Sometimes the limitation is an aging face and body; other times it's simply the number of hours one person can work in a day. Parlaying celebrity, however temporary or localized, into an enterprise with longevity requires some universal steps -- from finding other mediums beyond yourself to deliver the brand to having the confidence to tap others who can lead where you can't.

"Regardless of the strengths of the celebrity founder, the key to successful business development is a strong management team that can remove the actual product from the personality," says Deborah Larrison, head of Citigroup Capital Strategies, a unit of Smith Barney serving owners of privately held businesses. "Another key is that the personality not be the actual product. For example, as celebrity fades a useless product is exactly that -- useless."

The challenges can be seen vividly among instructors in the $14.8 billion health-club industry, where aching knees and torn ligaments can shorten classroom careers and keep teaching the domain of the young. So rather than milk her current celebrity status by cramming in more of her popular classes or giving lucrative private lessons, Ms. Montagnani instead devotes equal time to building Powerstrike Inc., the company via which she trains other instructors in her methods, produces videos and attempts to create new branded instruction.

While the downside can be painful -- especially the loss of immediate revenue -- the hope is that Powerstrike will perpetuate her fitness legacy, and income, once Ms. Montagnani must slow down. "I don't want to be a pathetic 50-year-old jumping around trying to keep classes with seven people," she says. "If you want to stay in the fitness industry, the question is, how do you create a continuation of what you do?"

UNLIKE MOST CHANNELS OF COMMERCE, there are few clear long-term entrepreneurial paths for those such as Ms. Montagnani. That's partly because new ideas in her field don't have a natural path to market the way, say, consumer goods do. Health clubs shy away from paying for proprietary class content -- often preferring to develop programs they own in-house -- and selling workout products is tough unless you're already a brand name. Some teachers open their own studios. But that's increasingly difficult with industry consolidation into the hands of big names with one-stop fitness and spa shopping, such as Crunch, Equinox and Sports Club/LA.

"Not a lot of young people are choosing this industry as a career anymore," says Carol Espel, the national director for group fitness at Equinox Fitness Clubs who oversees nearly 1,000 instructors nationally. "The ones who are really serious and organized and smart do what Ilaria is trying to do. To be successful at it, there are very, very few."

The "few" are now household names -- among the most prominent, Richard Simmons, Jack La Lanne and Billy Blanks, the founder of Tae Bo. In each case, these instructors carved out a specific fitness niche and then used various means to leverage their personalities out of a local market and onto a national and international platform. That, in turn, has allowed them to keep teaching well past their prime.

"It's the same philosophy as selling Avon: We are selling our services," says the 58-year-old Mr. Simmons. "You have to figure out what you have to offer in the area where you work." For Mr. Simmons, the breakout medium was video. He has sold more than 20 million copies of his 50 fitness tapes and DVDs, including "Sweatin' to the Oldies." That has given him cachet in nonfitness areas; for instance, he has a new line of kitchenware with Salton Inc. due out later this year. He's also expanding the "Richard Simmons Method" through a $195 weekend of coaching called "Hoot Camp" for fitness instructors, trainers and others.

With this diversification, says Mr. Simmons, "I think there will be these people who will continue to teach and have my same philosophies. When I'm long dead and gone, it will still be, 'Love yourself, watch your portions and move your buns.' "

There is still a long road between Mr. Simmons and Ms. Montagnani, who currently runs Powerstrike out of her one-bedroom Manhattan apartment and answers all her own email. But her journey thus far offers a window into the kinds of sacrifices required when creating an enduring business whose core brand is, at the end of the day, you.

"To be sustainable, there has to be a process and a system," says Doug Hall, one of the judges on the ABC reality series "American Inventor," and the 47-year-old founder of Eureka Ranch, an invention and research firm. "The challenge is the ego. You have to make the shift from being a doer to a teacher."

The story of Ilaria Montagnani as "doer" begins in Florence, Italy, where hard-core exercise among women was rare. The daughter of a banker and a mother who worked as a treasurer at the local university, Ms. Montagnani swam until her parents made her stop because they believed her shoulders were getting too big. She then focused on ballet but with her allowance bought instructional books on judo and karate and secretly practiced poses in her bedroom mirror.

"Some people want to be dancers," says Ms. Montagnani. "My sister wanted to be a mother. I was intrigued by the mind component of the martial arts and realized that the ideal body was one that could come from that. It would be strong, and you could take care of yourself."

In 1986, she traveled to New York to visit a friend for six months. Unable to speak English, she watched soap operas and "The Price Is Right," where, she says, "they spoke slowly and announced the words."

During that stay, the 5-foot-7-inch Ms. Montagnani gained weight, eventually reaching 135 pounds, but she wasn't in good shape, she says. One foggy November afternoon near the end of her trip, she passed underneath a studio where an aerobics class was in session. Intrigued by the pounding music and shadows of moving bodies, she walked in and joined for one month. That moment was the beginning of Powerstrike. "It wasn't elegant, and there was no room and people were sweating all over each other," Ms. Montagnani says. "But it gave me the foundation of realizing how exciting and beautiful and fun for the soul it can be to be with people moving together with music."

Ms. Montagnani was 23 when she figured out the next piece: martial arts. She had returned to the U.S. and was working with a Manhattan-based wholesale jeweler. On her off time, she lifted weights and eventually pursued, and obtained, her black belt in karate. Around that time, Ms. Montagnani took an aerobics class with Patricia Moreno, one of the top instructors in Manhattan. The two began exploring ways to combine martial arts and aerobics, with Ms. Montagnani showing a kickboxing move and Ms. Moreno helping her to incorporate that with a musical beat and eight-counts.

"It showed a level of strength that I hadn't seen before in aerobics and a new way of moving," Ms. Moreno says. "The idea was to make martial arts accessible to everyone, especially women for whom this was a completely new way of moving."

Over the next seven years, the pair took Powerstrike from a no-name program to one of the most recognized classes on the New York fitness scene. The time was right: Jane Fonda had whet America's appetite for group fitness, and Mr. Blanks's Tae Bo program was fueling interest in martial arts. In 1999 and 2001, Powerstrike was named best exercise class in New York magazine.

As class popularity soared, Ms. Moreno and Ms. Montagnani began teaching separately -- something that helped Ms. Montagnani establish her own loyal following. Over time, she got a marketing boost from strong female characters boasting martial-arts prowess in films and TV shows such as "Charlie's Angels," "Kill Bill: Vol. 1" and "Alias" with Jennifer Garner.

"I think she is the pied piper of fitness," says Sue Carswell, a reporter/researcher for Vanity Fair magazine. Ms. Carswell says she dropped from a size 16 to a size eight in about five months due mostly to taking some 10 classes a week from the instructor. Partly, the lure was the workout's high-octane structure; partly, it was Ms. Montagnani. "She started kicking and punching," Ms. Carswell says. "I thought, 'Cool, I'm in the middle of a super action flick.' "

Teaching every day was heady, but it soon taught Ms. Montagnani a hard business reality: With limited hours in a day, earnings potential was limited. "Doing this every day of the week, it would only take us so far," Ms. Montagnani recalls thinking.

At the time, the notion of having "certified" instructors in gyms was fast gaining traction as students clamored for instruction in the likes of step aerobics, spinning and kickboxing. The American Council on Exercise was formed in 1985 in an attempt to set some competency standards; the group currently has 45,000 certified instructors who've paid a fee averaging about $200 to take an exam to earn the ACE seal of approval.

To Ms. Montagnani, this seemed a good model for Powerstrike. If she could create a certification program, that would both drive revenue and give the program legs outside of New York. In other words, she would have a system. "That's one of the hardest routes to go," says Graham Melstrand, ACE's director of educational services. "It's also, I would think, one of the most profitable."

Over time, the question of Powerstrike's survival fell firmly into Ms. Montagnani's lap, as she and Ms. Moreno drifted apart with the latter pushing more into yoga and meditation. Ms. Montagnani eventually bought Ms. Moreno out of her stake in Powerstrike and trademarked the name, taking full control of the business. Says Ms. Moreno: "Anywhere it goes from here is truly Ilaria's doing."

For Ms. Montagnani, that has meant getting others to "do" Powerstrike for her. She currently has six types of Powerstrike classes and trains instructors in three of those: Powerstrike Kickboxing (her signature class), Powerstrike Impact (kickboxing with a bag) and Powerstrike Forza, which uses a weighted wooden and plastic fitness sword to replicate Japanese sword-fighting techniques. Often, she travels to fitness conventions or holds open certifications throughout the U.S. and abroad where she teaches her methods, usually over the course of a weekend. Attendees pay a fee that typically ranges from $200 to $300 and receive a certificate of completion at the end. The smallest class Ms. Montagnani will teach is 10 people; in Russia she has had a group as big as 350. To date she has issued about 6,500 certificates of completion for her various disciplines -- though not all were at the same fee level.

In some cases, Ms. Montagnani expands Powerstrike's reach by partnering with gyms -- something she's able to do because her personal instruction is in such demand. At Equinox, for instance, she is paid an annual fee, which she won't disclose, to train instructors who teach at the chain's various locations, including those in New York, Chicago, Miami and California. Instructors who get a certificate of completion can say they've had Powerstrike training -- an employment boost. But only those who pass a written test given by Ms. Montagnani and are consistently re-evaluated can teach classes under the Powerstrike name.

Those requirements are "essential to the success of the program" because they guarantee quality control and ensure that Powerstrike is taught only by the best, says Ms. Espel of Equinox. "Students love Ilaria, but they love other instructors, too." Ms. Montagnani is paid separately for classes she teaches herself.

Sharing the spotlight can be taxing on the ego, but experts say such risks come with the territory. "Eventually, you have to put yourself second and develop a system and make the system the star," says Mark Hughes, author of "Buzzmarketing" and a branding consultant. "There's no other way to do it. The smarter you are, the sooner you'll begin planning this."

Ms. Montagnani also takes care to avoid exclusive deals that might limit her expansion. Powerstrike can be licensed to any gym, for instance. "I think it's smart," says Whitney Chapman, group exercise manager for Reebok Sports Club/New York, where Ms. Montagnani also has trained instructors. "It allows her to generate an income that's not so physically driven. But it also lets her expand on a concept that's not just specifically her so the service can still be provided."

Perhaps the most critical element of Powerstrike's expansion is Ms. Montagnani's farm-team program -- whereby she designates some top-notch instructors as official Powerstrike "trainers." Those trainers then act as scouts, particularly outside New York, and find new batches of fitness instructors whom Ms. Montagnani will then certify; she gives scouts a cut of 10 percent to 25 percent as a finder's fee. Ms. Montagnani says she doesn't take a cut of Powerstrike instructors' classroom earnings because the bookkeeping would be too time-consuming and she doesn't "like the rapport you create with that."

Further, a few select trainers have become her "master trainers." Those travel and teach open certifications of Powerstrike in Ms. Montagnani's stead; she still gets up to half the fees collected. Via this structure, she has made Powerstrike -- not herself -- the product and expanded its reach to a dozen countries.

Violet Zaki, a master trainer, was able to pursue fitness instruction full time after getting involved with Powerstrike. She says some 50 percent of her income comes from Powerstrike-related activities now. "Why reinvent the wheel when something is already great?" she says. "There are so many different types of kickboxing, but students are very drawn to this. The format is very broken down."

For Debbie DiCanto, a Powerstrike trainer in New Jersey and Pennsylvania, being associated with the Powerstrike name beefs up class attendance -- a sign the brand is developing legs even when Ms. Montagnani isn't around. "I get 20 to 30 people in my classes, and outside of New York that's very good," Ms. DiCanto says. "Whenever I've introduced Powerstrike, it always becomes the most popular kickboxing class in that gym."

There are risks to what Ms. Montagnani is attempting to accomplish. For starters, any energy put toward managing the various components of Powerstrike Inc. cuts into her current revenue at a time when she's at her earnings peak. Last year, Ms. Montagnani made more than $100,000 as a teacher -- roughly twice what Powerstrike collected. But teaching six days a week doesn't leave a lot of free time.

As such, questions of time allotment persistently arise. "Do I want to invest and do videos?" Ms. Montagnani says. "Then I have to give up classes and give up $20,000" of income. But when she teaches more, she's not out certifying instructors or building other facets of Powerstrike's business. (Currently, she's making a new Powerstrike video, and last year she published a book on Forza.) Each choice is a gamble.

Further, Ms. Montagnani is quickly learning that she needs more manpower and investment to capitalize in other avenues. She once sold apparel on her Web site, www.powerstrike.com, but the expense of carrying inventory was too taxing. Likewise, she knows she needs an agent to help her with future book deals or TV infomercials, and would have to raise capital to open her own studio. Handling all of the above, of course, takes time out of the classroom.

Her training "system," meantime, has presented some pitfalls. For instance, Ms. Montagnani says she has filed a lawsuit in Rome against two of her former master trainers who she claims registered the name Powerstrike in Italy and copied her manuals. The suit is still pending.

What's more, her celebrity presents its own challenges. She receives up to 40 emails a day from students and clients and says trying to respond to each is "eating me alive in terms of time." Some students, who can tell when their email has been opened, send additional angry missives if she doesn't answer immediately.

Still, it's that very devotion that gives Powerstrike Inc. legs. Crunch Fitness, a chain of 32 health clubs, has taken the unusual step of paying Ms. Montagnani to design a workout program exclusively for Crunch and to train its instructors. "We typically don't pay people to put programming together," says Donna Cyrus, senior vice president of programming for Crunch. "But we wanted her on our team."

The class, named "Weighted Warrior Workout," had its debut on a recent Friday evening. Despite the inopportune time slot (Friday at dinnertime) and a few mishaps (the weighted vests were missing), Ms. Montagnani garnered a respectable crowd of 17. For that hour at least, the worlds of Ms. Montagnani, Powerstrike president, and Ilaria, instructor, merged as she strapped on the familiar headset microphone and cranked up the music.

"It's showtime," she said, stepping into the spotlight.