"Knock knock."

Who's there?

"This is your personal trainer speaking, here to motivate you in the methods of fitness, hail your quest for health, celebrate your sweaty success. And you behind the door. Who are you?"

A bodybuilder wanting more bulge in the biceps; the nouveau riche suburbanite with a hankering for a svelte form (and a dash of status); a young urban professional with big goals and little time; retired folks who want a streamlined program suited to their own needs.

People are using personal trainers like never before. These trainers come to the home or the gym. They work with their trainees daily or monthly.

They can cost little ($2 an hour if you sign up to be trained by a student enrolled in Renton Technical College's health and fitness technology program) or they can cost a lot.

Kathy Kaehler of Beverly Hills, the 29-year-old lithe Lycra lady with her own exercise video, charges between $80 and $100 an hour and trains Michelle Pfeiffer and a pack of other celebrities.

"Sometimes they need to reach a goal a lot quicker, need to be in a certain shape in a couple of weeks . . . and they don't like to just sit on an exercise bike," Kaehler said.

Bring out the Nordic track, the StairMaster and free weights, the bikes and the Nautilus.

One advantage of a personal trainer is that he or she will nudge you into doing your exercises and provide you with routines that are fun.

Because Kaehler has trained Pfeiffer five or six days a week for almost two years, she has to create a wide variety of routines. This is "to prevent boredom and so you don't end up hating each other," she said.

When you're confronted by someone bright-eyed and ready to work out at 6 a.m., it's easy to feel hate - and admiration.

Terri Casey, a 38-year-old Microsoft executive, and Kim Williams-Brinck kept journals about their training relationship.

Casey, who had been reasonably fit but did little to improve tone or her cardiovascular system, had always wanted to commit herself seriously to an exercise program. But lack of time and reluctance to change to a nutritious diet were the biggest roadblocks. With her friend Williams-Brinck specializing in training women 30 and older, it seemed a natural match. The arrangement, however, wasn't without its downside.

Call it the battle of fat.

Williams-Brinck crusaded for a low-fat daily diet, and that included Thanksgiving.

"I am sorry, but I do not do low-fat holidays," Casey wrote. "I can't get past butter."

Eventually, healthy food choices became easier.

Through sore butts and biceps, they've worked together since last August, meeting every four to six weeks. In between, Casey works out three days a week with free weights for strength training and the rest of the time does aerobic walking.

The results?

A 3 percent drop in body fat and a few inches lost around the hips and waist. Casey had such an improvement in her overall fitness level that when she went cross-country skiing, she felt she could "ski forever."

The workouts are "more exercise than I've ever done in my entire life," she said. "And I do it with Kim's help. It used to be so easy for me to blow this off, to find one excuse after another. `I'm too tired. It's too late.' "

She credits Williams-Brinck for keeping her motivated.

One of the best assets a trainer can have is motivational skills, said Mark Mitsui, instructor at the Renton Technical College's health and fitness technologist program.

"Those who benefit the most are people who have a deep desire to change their fitness level but need guidance and education. Someone who needs someone else to motivate them at a deep level, I'm not sure a trainer can help," he said.

The year-long Renton program emphasizes both hands-on and course work and prepares students to be certified by the American College of Sports Medicine, which establishes industry standards.

Today, personal training is "where the job market is," Mitsui said. "People are moving from clubs to home settings. They want to have more time with the family, and (personal training) gives them more individual attention."

Most trainers get their clients through word-of-mouth or referrals from clubs they belong to. Some trainers start as aerobics instructors and then pull students from the clubs.

But don't think the majority of people using trainers are wealthy health spa fanatics with nothing else to do but work out.

"We're starting to see more seniors, who have the income, using personal trainers. People in their 60s and 70s are now exercising to help maintain their independence and cut down health-care costs. They will be a large market for the future," Mitsui said.

And yes, spending all that time together gives trainers - like hair stylists - an edge on the juiciest gossip. Who's sleeping with whom? Who's breaking up? Who's getting together?

But the good ones will keep the state of your love life as secret as your hip measurement.

HOW TO CHOOSE A PERSONAL TRAINER

Personal trainers in this state are not licensed and can have any kind of background. How do you choose one?

Make sure they know cardiopulmonary resuscitation, first aid and have had experience as a personal trainer. Ask for the names and phone numbers of their clients.

Being a graduate of a program certified by the American College of Sports Medicine or the American Council of Exercise is a plus.

Both Lake Washington and Renton vocational colleges have programs preparing students for certification and can refer you to qualified graduates.

For a low-cost introduction into being personally trained, consider Renton Vocational College's six-session program ($25) when it's available (235-2498).