I'm really straining now, pushing up the bars with clenched hands, exhaling in tortured puffs. A stinging trickle of sweat runs into my right eye as the bars extend over my head. Two more reps, if I can just squeeze out two more for the chest press.

"I'll count the reps," says Chester, my coach, in his halting, metallic voice. "You do the work." This guy is a slave driver.

I pull down the bars and then gulp for air as I force them up again, pushing so hard my biceps feel as if they'll burst.

"I've got an 80-year-old grandmother who works out harder than you do!" says Chester. I see Chester's weird face displayed on the video screen in front of me. "Listen, you pile of cybernetic junk," I hiss. But cursing at Chester isn't worth the breath, so I put everything I have behind the last rep, forcing the bars up with my eyes closed, grunting like an . . .

"You're an animal!" Chester exults.

Done with the chest press and rowing exercises, I move to the shoulder-press unit. Coach Shroeder's face appears on the screen. "I see Coach Chester has given you a mark of 100," he teases, "but I'm not going to be that easy."

Maybe you think your leather- lunged high school or college coach was merciless, not even human. But these coaches, Chester, Shroeder and their colleagues, really are inhuman. They are humanoids (or at least that's what their makers call them), syntho- voiced personas that exist only in the electrical gateways of integrated circuits. They are denizens of a new system of exercise machines called Powercise, soon to be installed at a Washington-area health club chain.

The inspiration for this revolutionary concept came from Rick Dyer, the San Diego electrical engineer who invented the popular video arcade game "Dragon's Lair." Dyer hooked up a crude prototype machine using an array of microprocessors and an electromagnetic brake for resistance (instead of the traditional weights, cams and hydraulics found on most exercise machines). He wanted to create a computerized coach that could monitor, analyze and direct a user's performance, making subtle adjustments in resistance during the workout while remembering and factoring in past workouts.

Richard Keelor was skeptical about the idea when Dyer approached him. But as an exercise physiologist, coach and longtime member of the President's Council on Physical Fitness, Keelor knew that most people just don't get enough exercise to stay healthy. Experts say only 15 to 20 percent of the population gets enough regular exercise to work up a respectable sweat. Many give up after getting bored or hurting themselves.

So Keelor, Dyer and other fitness and electronics kahunas worked for three years to come up with Powercise, a fascinating marriage of computer technology and exercise physiology. The $58,000 system (roughly comparable to what a complete Nautilus setup costs) consists of six exercise machines plus Wally the Scale and Peter the Printer. Before you condemn this as sounding just a little too cute or far out, think for a moment about getting fit.

Getting started and sticking to a serious fitness program is frustrating, even more so if you lack basic knowledge of exercise physiology. Even when you make the commitment, you may not get the benefits you seek. Jogging and cycling, for example, are incomplete exercises, contributing nothing to the development of upper-body strength. So which exercises are best for you? How much and how often? A trained coach could answer your questions and safely guide you through a program, but personal exercise instructors charge about $75 per hour.

Powercise offers silicon coaches with artificial intelligence who can, in synthetic but helpful voices, guide and inform exercisers. Here's how the system works: Powercise uses your vital statistics (age, sex, weight, physical limitations) and goals to create an exercise-diet plan tailored for you, drawing from 50 such plans in its data bank. (Any of these programs can be modified to fit specific needs.)

Because Powercise builds muscular strength, not aerobic fitness, each program also includes a cardiovascular workout (jogging, swimming, cycling, etc.). Each time you work out, you weigh in on Wally and have your weight added to your other statistics and performance data stored in the system's 20-megabyte memory. Powercise then automatically makes any necessary adjustments to your program.

After that, the routine at each workout is the same. You sit at a station and tap in your identifying code. The machine, uh, coach then greets you by name and asks you to take a strength test. When the result is entered and compared with your last strength test, you're given a goal -- 10 reps of leg extensions, let's say, at 75 pounds. As you do the exercises, your coach -- Harry, for instance -- cajoles or encourages your participation, drawing from Powercise's 1,000-word vocabulary.

If you can't handle the reps, Harry will say something like "Stay with me while I reduce the weight," and "he" will automatically make adjustments while you're exercising. All stations employ a "double-positive" action (you exercise while pushing and pulling), so you work more muscle groups in less time than with free weights. When you're finished, Peter the Printer pops out a report card of your performance with tips on training and diet.

Is a humanoid coach as good as a real one? Keelor says yes, and emphasizes that computerized coaches never get tired or irritable. Critics say no because they can't physically touch you or really relate to your fitness problems. Whatever you think, the fact remains that the future that sci-fi writers fantasized is here: Powercise is among the first wave of interactive talking computers that will, like it or not, grow in sophistication as they entertain, protect and teach us.

Sure, I admit to some uneasiness about making chitchat with a sassy computer, but when it comes to exercise and other inherently boring activities, a humanoid is better than no coach at all. At least he won't snap a towel at you or make fun of your physique. Now excuse me. I've got to go pump some megabytes! ::