Take the kicks and shouts of martial arts, the punches of boxing, the continuous movement of aerobic dance and set it all to a pulsating hip-hop beat. The result is America's latest fitness craze, known generically as "cardio kickboxing," but more popularly associated with the mega-hit exercise video "Billy Blanks' Tae-Bo."

Combat-style workouts have been evolving throughout the last decade, but the concept didn't become mainstream until the phenomenal success of Tae-Bo. Blanks, a charismatic, seventh-degree black belt, had been teaching Tae-Bo at his studio in Sherman Oaks, Calif., for nearly 10 years before launching an infomercial last year that kicked him to worldwide prominence.

Buoyed, in part, by its star-studded list of proponents -- from singer Paula Abdul to basketball giant Shaquille O'Neal--Tae-Bo has been the nation's number one health and fitness video since January and was the top-selling video overall for five weeks this spring. Blanks has appeared on a host of TV shows, from "Oprah" to "ER," and is expanding his Tae-Bo empire to include a new series of videos, a clothing line and a book, "The Tae-Bo Way," just released by Bantam Books.

As a result, cardio kickboxing has exploded. Classes are now offered in most health clubs and many martial arts studios, going by names such as Kardio Kick, Cardio Karate and Tae Box. The boom hit so fast and so strong that the International Health, Racquet and Sportsclub Association didn't even keep statistics on cardio kickboxing until this year, says spokeswoman Maeve McCaffrey, who estimates that nearly 78 percent of member clubs now offer the classes.

"Many fitness trends gradually take hold," McCaffrey notes, "whereas kickboxing really came in and just blew everyone away."

Too Much, Too Soon

As the fitness industry scrambles to meet the demand for qualified instructors, top groups of exercise professionals have begun sounding an alarm about kickboxing's potential risks. An intense, high-energy workout, kickboxing relies on movements that are unfamiliar to most people--such as kicking and punching--and requires a certain level of conditioning. As a result, injuries are beginning to surface, especially surrounding two problems: poor instruction and novices doing too much too soon.

"Safety is the big concern with kickboxing right now because we're seeing a lot of injuries, from the feet on up" to the knees, hips, lower back, elbows and shoulders, says Keli Roberts, a Los Angeles personal trainer who is a spokeswoman for the American Council on Exercise (ACE). "The movements are very forceful, very explosive and very fast. Yet people who've never done this sort of thing before are jumping into a class and doing 50 roundhouse kicks."

Kickboxing's fast-paced music and crowded classes also "can exert a lot of pressure on people to keep up even if they're exhausted," says Peg Jordan, an Oakland, Calif., registered nurse and editor of American Fitness Magazine, the official publication of the Aerobics and Fitness Association of America (AFAA). "Fatigue ratchets up the risk of getting hurt."

In addition, Jordan says, the "repetitive, ballistic" nature of Tae-Bo can make it hazardous. While she says that Blanks has improved the safety focus in his newer tapes, she warns that a rash of unqualified "Tae-Bo imitators" have contributed to what she calls "the injury fallout from the kickboxing craze." To keep the exercise a positive experience, AFAA and several other professional groups have created kickboxing instructor certification programs. Launched in May, AFAA's program is already its "hottest certification," notes Jordan, who says the organization has trained several thousand kickboxing instructors. "The goal is to teach instructors how to create a safe environment for students," says Jordan. This includes giving participants important cues about doing each technique safely, letting them go at their own pace and avoiding doing more than 12 repetitions of a movement before changing it.

Kickboxing teachers should be trained in both martial arts and group fitness instruction, many exercise experts contend. A martial arts background gives teachers an understanding of proper punching and kicking technique, while a certification in group fitness instruction typically ensures competency in CPR, plus an understanding of how to use music, choreography and cueing safely and effectively.

"An instructor taking a weekend boxing training class is not going to cut it," says Kathie Davis, executive director of IDEA, a San Diego-based group that represents more than 23,000 health and fitness professionals. "Like yoga, martial arts is a discipline that takes people years to master, and a good instructor needs to be a student of this art first."

Exercise experts acknowledge that kickboxing is an effective cardiovascular workout, but warn that some claims about its benefits may be inflated. "Consumers should be cautious about the testimonials of how many calories kickboxing burns," says Len Kravitz, an assistant professor of exercise science at the University of New Mexico, who was commissioned by ACE to study the effectiveness of cardio kickboxing. While original estimates suggested that kickboxing can burn 500 or even 800 calories per hour, Kravitz's study of 15 female kickboxing enthusiasts showed that an hour-long workout burned about 400 calories--roughly the same as an hour of brisk walking or light jogging.

Cardio kickboxing does provide additional benefits not associated with walking or jogging, Kravitz notes. These include increased strength and flexibility as well as improved coordination and sharper reflexes. The safety concerns surrounding kickboxing are a flashback to the early days of aerobics, when Jane Fonda's first workout videotape was both praised and panned by fitness professionals. While Fonda won kudos for getting Americans motivated to move, she was criticized for using unsafe exercises and urging viewers to "go for the burn."

Similarly, Blanks is applauded for motivating Americans to move, but is chastised by some industry experts for using repetitive, high-risk techniques. Home workout videos hold an extra measure of risk, since exercising without supervision can increase the chance of injury--even for someone in good shape. For example, Cincinnati Reds third baseman Aaron Boone bought a Tae-Bo tape during the off-season and hurt his hip attempting a high kick, although the injury was minor and he went on to have his best season in the major leagues.

Blanks says the main reason some exercisers get hurt doing Tae-Bo is that they're "too gung-ho and go right to the advanced workout . . . . The most important thing about kickboxing or Tae-Bo is to learn the fundamentals. Then, once you've got a good foundation, everything else can blossom."

Another reason for kickboxing injuries is unqualified instructors, contends Blanks, who has certified 12 instructors to teach at the Billy Blanks World Training Center in Los Angeles, and 15 more who teach around the country.

The key element for kickboxing safety--which many programs neglect--is to start slowly and offer several workout levels, says Tim Rochford, a fourth-degree black belt and an ACE-certified personal trainer from Sandwich, Ill. "New people should begin with an orientation, not a workout, where they simply learn the movements without speed or power." For novices, Rochford recommends an orientation program that meets twice weekly for a month. After acquiring appropriate skills in a subsequent beginner's class, they can move to an intermediate level.

Equipment that adds resistance to a workout, such as punching bags, should be introduced gradually as people become more experienced, Rochford says. "You wouldn't let someone walk into a gym and do a curl with a 40-pound weight," he says. "And you shouldn't let someone walk into a studio and kick or punch a bag in their first class."

"One of the programs I teach is in its third year and we're just now introducing an advanced level," says Rochford, who has written a kickboxing certification manual for ACE.

Often, people get injured in this--and any other trendy workout--because "they think, 'It's the rage now, so I'll try it to get in shape,' " says Eric Guidi, an orthopedic surgeon at the Nirschl Orthopedic and Sports Medicine Clinic in Arlington. "But if you're not in shape before you take up a strenuous activity like this, you're going to get hurt." Typically, he says, these injuries are to muscles, joints and bones and are related to three main problems: poor technique, poor equipment and poor conditioning. "So many of the injuries we see are from deficiencies in strength, endurance and flexibility," says Guidi, who advises exercisers to warm up first, then stretch, then do their activity, then cool down, then--ideally--stretch again. He also recommends a pre-participation physical, which will screen for orthopedic and cardiovascular problems. Men over 40 and women over 50, and people of any age with risk factors for heart disease, such as smoking or obesity, should check with a physician before beginning any vigorous exercise program.

Certain kickboxing techniques, such as back kicks, put too much pressure on the lower back to be safely used in general classes, says Gus Gates, a fourth-degree black belt and AFAA member who runs a certification and training program called Kardio Kick, in Westlake Village, Calif. "A lot of people are doing things I don't agree with," notes Gates, who says two of the biggest safety problems are "too many repetitions" and "trying to kick too high."

Like many fitness professionals whose kickboxing careers preceded the Tae-Bo craze, Gates has mixed feelings about Blanks's success. While it irks him when people assume his program was inspired by Tae-Bo--in fact, he's been teaching Kardio Kick since 1990--Gates acknowledges that "Tae-Bo has been very positive over all, and has brought people into my program. Its popularity has made everybody profit."

Even martial arts schools--which previously didn't offer programs designed solely for a physical workout--have benefited from America's fascination with Tae-Bo. "Enrollments have doubled in many martial arts schools because of the trend," notes John Graden, executive director of the National Association of Professional Martial Artists (NAPMA). Nearly 2,000 martial arts schools around the country have sent instructors to certification classes in NAPMA's fitness kickboxing program, called Cardio Karate, which began in 1996. "Many people who come to a martial arts school for Cardio Karate become intrigued by traditional classes and sign up," Graden says.

What a Relief

The real secret of kickboxing's success is that "it's an incredible stress release," says Kardio Kick's Gates. "We do visualizations, where you 'see' an attacker, then punch him in the chest. The fact that people can punch and kick and shout lets them release their emotions and express themselves. There's a tremendous amount of benefit to yelling and to feeling strong. People are forced to let their hair down and their makeup drip--it's not something you can fake. There's an energy and an excitement that makes it a passion for me and for many other people."

Tae-Bo creator Blanks says people love the workout because it goes far beyond the physical body. "It's about being spiritual and having the will to want to do things," he says. "Discipline, concentration and dedication are essential, and physical exercise is simply a tool to reach that inner state of grace." Blanks's story reads like a modern version of the American Dream. One of 15 children from a poor family, Blanks started karate at age 13 because he was inspired by the movies of martial arts legend Bruce Lee. He overcame dyslexia and a bad hip on his way to becoming a successful amateur boxer--he won two Golden Gloves titles--and martial artist.

While Blanks's credentials are impressive--he holds black belts in six different styles of martial arts and has won numerous medals in international competition--he has been criticized for claiming to have captained the U.S. Olympic karate team in 1980. Karate is not yet an Olympic sport and the United States boycotted the 1980 Olympics.

"That was a miscalculation," says Blanks about the statement, which is still on his Web site. "It should say the U.S. Amateur Karate Team."

Blanks was a "phenomenal competitor" who dominated the international martial arts circuit in the early 1980s, says Joe Mirza, the Amateur Athletic Union's U.S. national chairman for karate. "He beat everyone up."

Blanks created Tae-Bo in his basement as a way to train himself and his family. "Tae" means leg in Korean and relates to the kicks and the lower-body segment of the workout, he says. "Bo" comes from boxing and the upper body punches. Blanks says his success "reflects the fact that God has a purpose for everyone. I thank God that I can help people with their health and with their life."

Achieving any goal--including getting in shape--takes discipline and hard work, Blanks says. "I tell people it's not easy," he notes. But he says that almost anyone can do his workout and get results if they put in the time and effort.

Despite the huge popularity of kickboxing and its many benefits, fitness professionals caution that it's not a workout for everyone. "Kickboxers represent a small segment of the exercising population who love killer classes," says AFAA's Peg Jordan. "They think that 'If I'm not dropping dead, I'm not working out.' "

Other vigorous workouts in recent history also had this "injury backlash," she notes, pointing to step aerobics and group stationary cycling. While they're all effective workouts, their vigorous nature makes them riskier than more moderate exercise, especially for people with orthopedic problems.

In contrast, Jordan says, the other "huge trend" in fitness is the boom in gentle exercise such as yoga, tai chi and Pilates, a stretching and strengthening technique initially embraced by dancers.

As a growing body of scientific evidence supports the numerous health benefits of moderate exercise, Jordan and other fitness experts warn against any claims that kickboxing is "the best" form of exercise. The "best" form, they agree, is one that you enjoy and will do regularly, whether it's kickboxing or simply a brisk, 30-minute walk.

RESOURCES:

* The American Council on Exercise (ACE) is a "workout watchdog" organization that certifies and sets standards for fitness professionals. For information on exercise safety or for referral to an ACE-certified trainer, call 1-800-825-3636 or visit the group's Web site at www.acefitness.org.

* The Aerobics and Fitness Association of America (AFAA) is an educational and certification organization for fitness professionals. For information and referral, call 1-800-YOUR-BODY or visit the group's Web site at www.afaa.com.

* IDEA, The Health and Fitness Source, is an educational organization for fitness professionals. Call the group at 1-800-999-IDEA or visit the Web site www.ideafit.com.

* The National Association of Professional Martial Artists (NAPMA) certifies instructors in their own style of kickboxing called Cardio-Karate. For information and referral, call 1-800-973-6734, or visit the group's Web site at www.mapromag.com/acma_index.html.

* Health Kick Fitness Services offers instructor training and certification in the "Martial Fitness Workout." Call 1-888-627-8348.

TAKE IT EASY AT FIRST

Cardio kickboxing offers many fitness benefits, but as an intense, complicated form of exercise it carries a risk of injury. To minimize the likelihood of getting hurt, fitness professionals offer this advice:

BEFORE YOU WORK OUT

* Select an experienced, well-qualified instructor. Look for someone with martial arts training as well as certification from a reputable organization of fitness professionals.

* Pick a program that offers various skill levels, from beginner to advanced.

* Make sure the class isn't overcrowded. Each participant should have at least a 6-foot-by-6-foot area to move and kick.

* Check with your physician first if you've been sedentary and you're a man over 40, a woman over 50 or a person of any age with risk factors for heart disease, such as smoking. Also, consult a doctor first if you have a chronic health condition such as arthritis.

WHEN YOU START

* Wear a shoe designed for kickboxing, cross-training or step aerobics that has forefoot cushioning and allows your feet to pivot and move from side to side. Avoid running shoes, which aren't made for lateral movement.

* Be sure to stay hydrated by drinking plenty of water before, during and after class.

* Begin with an orientation or introductory program designed for novices. Even if you're very fit--and especially if you're not--it's important to start slowly and progress gradually. Your first few classes should simply familiarize you with the basic techniques.

DURING YOUR WORKOUT

* Always warm up and stretch before moving into vigorous activity, and cool down and stretch at the end of your workout.

* Make good technique paramount. In general, keep your kicks low unless you have excellent strength, flexibility and skill.

* Don't try to keep up with everyone around you. Pay attention to your own body and ease up when you need to.

* Never "lock out" or hyper-extend your joints.

* Avoid doing excessive repetitions of the same technique, and alternate high-intensity intervals with recovery periods of slower moves when possible.

IN GENERAL

* Limit yourself to no more than three cardio kickboxing classes per week. If you like to exercise daily, cross-train with complementary activities that are low- or no-impact, such as swimming, walking or stationary cycling.

* Never wear weights or hold dumbbells when throwing punches.

* Wear boxing hand wraps and gloves whenever hitting a bag or pad.

A FALSE SENSE OF SECURITY?

It's a popular theme in movies and TV shows: A woman who regularly takes kickboxing classes is attacked, then fights off her assailant with the punches and kicks she's learned in her workout.

Dangerous fiction, say martial arts experts, who warn that people who think cardio kickboxing teaches self-defense are sadly mistaken.

"There are emotional and mental factors that come into play when you're attacked which aren't addressed in a kickboxing class," says Tim Rochford, a fourth-degree black belt and ACE-certified personal trainer from Sandwich, Ill. "People may learn a few striking techniques. But in a real-life situation you'd probably never use a punch or a kick. Since most attacks are in close, you'd be more likely to use a palm strike or an elbow or a knee."

A former amateur kickboxing competitor and a board member of the American Council on Martial Arts, Rochford has been teaching self-defense programs since 1982 and offers instructor certification in his own program, called "The Martial Fitness Workout."

"I'm concerned that people could get hurt," he says, "because kickboxing gives them a false sense of confidence that they've learned to defend themselves."

For shaping up body and mind, he says, kickboxing can be an exceptionally effective workout--if it's taught properly. But for self-defense, he says, the most it will do is help people develop the speed and stamina to run away from an attacker.