Like most fitness trends, it started in California. Movie stars who needed to reshape their bodies for a film employed coaches to develop an exercise prescription geared to the star's goals, then worked out with them over weeks and months until those goals were achieved.

Today, these exercise coaches are better known as personal trainers and are one of the hottest new trends in the fitness industry. High-powered executives with more money than time hire them to come to their office and take them through a tailor-made workout. Out-of-shape people employ trainers to get them started in the privacy of their homes, sophisticated exercisers use trainers to boost them over a fitness plateau, and post-trauma patients hire trainers to help them get moving again.

"In some ways, it's the new Yuppie Puppie Lollipop," says physician Bob Goldman, founder and director of the National Academy of Sports Medicine (NASM) in Chicago, one of several fitness organizations that recently launched a certification program for personal trainers. "Some executives will announce, 'I can't make that meeting because I've got a session with my personal trainer.' Some people relate to their trainer almost like a therapist, since you can really vent when you're working out."

But personal trainers are much more than simply a status symbol or fad, Goldman notes. "You get really good results a lot quicker when you're working with a trained professional who is focusing on you and your needs," he says. "It's like a private lesson in fitness designed to give you maximum results." According to Peter Francis, associate professor of physical education at San Diego State University, "A good personal trainer can help define your goals, assess your current fitness level and effectively help you achieve what you want with a reduced risk of injury."

Top personal trainers are educated in exercise physiology, behavior modification, health screening, nutrition and weight control. They can custom-design a program for people in good health as well as those with physical limitations who have their physician's approval to exercise. And good trainers can be great motivators, particularly for people who are just starting out.

"More than 50 percent of people who begin an exercise program drop out within six months," notes Mitch Sudy, director of education at the San Diego-based IDEA Foundation, an association for fitness professionals, which started certifying personal trainers this year. "A good trainer helps you get results, which is great motivation. They can offer a safe, varied program that keeps you from getting bored or injured, two reasons why people become fitness drop-outs."

Also, paying anywhere from $20 to $100 per hour, with an average of about $30 to $35 per session, can be an effective motivator for sticking with a fitness program.

"Since the American philosophy seems to be that if you pay for something, it's really valuable, people are more likely to adhere to a fitness program when they employ a personal trainer," says William Herbert, director of the exercise research lab at Virginia Tech and head of the American College of Sports Medicine's committee on certification for health and fitness instructors. ACSM is in Indianapolis.

Many people locate with personal trainers through gyms that permit trainers to work with clients in their facility. An emerging trend in California is the "one-on-one" gym designed solely for use by personal trainers and their clients. Some trainers bring equipment to people's homes or offices, often in a van in which the client works out.

Arrangements vary, but clients frequently contract with a trainer for a short period, say one to three times a week for six weeks. Then, once they've got the program down, they'll meet with the trainer for one refresher session each month to have their program modified or updated. Others, with the money and the desire, hire personal trainers to take them through every workout, indefinitely.

The main problem with personal trainers, says San Diego State's Francis, is that "the good ones are extremely good, and the bad ones are atrocious." In fact, "there have been suits filed by people who feel they've been injured due to negligence on the part of the trainer," notes NASM's Goldman. "There are people calling themselves personal trainers who don't know what they're doing."

To find a qualified personal trainer, contact university physical education departments, reputable gyms or hospital-based sports medicine facilities. Before hiring a trainer, experts suggest you ask about:

Educational background and certification. A bachelor's degree in a health-related field is a good sign, and certification in CPR is essential. Several organizations including IDEA, NASM and ACSM certify trainers. (For referral to an IDEA-certified trainer call 1-800-825-3636. For an NASM trainer, call 1-800-999-5101.)

Practical experience. Have they run a corporate fitness program? Worked in a hospital rehabilitation program?

Fees. Ask what they charge, the length of the session and what their policy is on missed appointments. Will they give you a written outline of your program with goals to be met at specified periods?

Professional liability insurance. The good ones should have it.

References. Get at least two names and call them. Remember, you're putting your body in this person's hands.

Sources. Be wary of trainers who base their claims solely on personal research. Says San Diego's Francis: "I'd feel more confident in someone who tells you what the American College of Sports Medicine recommends than someone who relies almost exclusively on the first person singular."

Also, "make sure that person listens to you. If they simply start telling you what you need before they ask anything about you, forget it."

Bodyworks appears on alternate Tuesdays.