WHAT good is having the sexiest biceps in town if you can't scramble up subway steps with ease, run for a bus without knee pain or lift a toddler without wrenching your back?
That's the premise behind a new school of thought called functional fitness, an approach that is transforming the techniques of many trainers.
Functional fitness means that the goal of working out is preparing your body so it can perform daily activities -- walking, bending, lifting, climbing stairs -- without pain, injury or discomfort.
''It's training for life, not events,'' said Jarrod Jordan, an advocate. At the Sports Center at Chelsea Piers in Manhattan, Mr. Jordan, 27, a fitness trainer, regularly puts clients through workouts that have them kneeling on wobbly, oversized rubber balls, racing up and down stairs and balancing precariously on multicolored yoga blocks -- all in pursuit of core strength, flexibility, coordination and balance.
This approach, which borrows liberally from disciplines as varied as yoga, Pilates, dance and physical therapy, is ''very much the direction of the fitness industry,'' said Micheal A. Clark, a physical therapist and the chief executive of the National Academy of Sports Medicine in Calabasas, Calif., which trains fitness instructors.
''The average person today has goals other than, 'Can I tone up my abs or my butt,' '' Mr. Clark said. ''More people are coming in overweight, with injuries, knee pain, back pain. Even people who are gym-fit and lean have postural and muscle imbalances.''
The whole-body regimen may be particularly well suited to women. According to Mr. Clark, ''Ninety percent of women want to tone their buttocks, stomach and the back of their arms, but unless they straighten out the front of their hips, strengthen their abdomens and learn how to use the glute muscles, they won't get the muscle tone they're after.''
The remedy, functional fitness advocates say, is learning to use multiple muscle groups in an integrated way. This runs counter to the idea behind machine-based weight training, which was developed to allow bodybuilders to isolate single muscle groups.
''Your muscles may get stronger working on machines, but you're not creating synergy in the body,'' Mr. Jordan said. ''With seated bench press curls, while you're working your arms, the rest of your body remains inactive.''
By contrast, Mr. Jordan said, ''functional fitness workouts challenge the body to work collectively as a whole, firing up the muscles in a sequential pattern.''
In putting together each workout, trainers can choose from thousands of exercises, including more than 25 ways to perform a simple forward-facing lunge. The equipment includes physical-therapy staples like rubber fitness balls and yoga basics like foam blocks and balancing cushions. Mixing it up this way helps mitigate boredom and the exercise dropout factor that so often follows. ''Yes it's more difficult,'' Mr. Clark said, ''but it's also more fun.''
Trish Talerico, 53, of Clark, N.J., a hospital administrator, began functional training at Chelsea Piers with the goal of strengthening her abdominal muscles for better back support. ''You think it's going to be easy when you start but it's not; it's difficult,'' she said. ''I can see each week that I've made progress. Things I couldn't balance on before, now I can. I think it's because I'm now using the correct muscles in the correct way.''
Because of the integration of more muscles into the workout, functional fitness can be an effective alternative to conventional training for those trying to lose weight. ''When you train on a stability ball,'' Mr. Clark said, ''every time the ball moves, to stay on it you have to activate muscles deep in your back, your stomach and hips. Because of that, you're going to burn more calories and build more muscle.''
Ilicia Silverman, 33, of New York, a software executive, said she lost 22 pounds in the year since she began training to get back in shape after back surgery. ''My clothes fit differently,'' she said, ''my midsection has shrunk and I have more definition in my arms and thighs.''
Group exercise classes often incorporate functional fitness techniques. At the Minerals Sports Club at the Crystal Springs Spa and Golf Resort in Vernon, N.J., Caite Fitzgerald, the fitness director, said she had replaced 1980's-style body sculpting classes with functional training workouts using stability balls and light weights. ''Rather than the old below-the-belt abs-and-buns classes of the 80's and 90's, now we're talking about head-to-toe fitness,'' Ms. Fitzgerald said. ''It's about how the whole body moves, not just the inner thigh or abs.''
All the workouts in the world won't mean anything if you don't change the way you move on a daily basis, said Dr. Elisabeth A. Lachmann, a professor of rehabilitation medicine at the Weill Medical College of Cornell University in New York. ''While it's important to strengthen the core,'' she said. ''you also need to sit, stand and get out of bed in ways that don't strain the body, and change the parts of the environment that contribute to any aches or pains.''
Among Dr. Lachmann's tips: ''At the computer, sit on an adjustable chair, knees higher than your hips, feet on a footrest to allow your back to maintain a natural arch. If driving for long periods of time, sit close to the steering wheel, and get out and walk from time to time, if possible. Vacuum or mop with your knees bent. When you reach for an object, use a stepstool. When you move furniture, try to push rather than pull.''
In other words, try combining functional training with changes in the way you function. The results could stay with you for a lifetime.