Generally, the books debunk the hours-in-the-gym approach to fitness -- yahoo! But page after page also make big promises of maximal benefits in minimal time.

Just how? Resistance training, resistance training and more resistance training.

Some authors advocate complete and utter devotion to weights, while others recommend a small side order of cardio to go with a main course of reps.

The idea is to build muscle so you burn oodles of fat and, ultimately, reshape your saggy, sorry body. Traditional aerobic activity doesn't appear to be an integral part of their fitness equation.

But, tell me, just where's the balance? And will you actually get thinner, or just your wallet?

Read on and see what you think:

Jim Karas, personal trainer to news anchor Diane Sawyer and other celebrities, says the key to losing weight is to do his strength and resistance exercises for just a few minutes each day.

"My premise is simple: The more muscle you have, the more calories you burn while exercising or at rest," says Karas, author of the new book Flip the Switch (Random House).

Cardiovascular exercise is not the key to weight loss, Karas tells The Sun. "Lean muscle tissue boosts the body's metabolism 24-hours a day."





FREE WEIGHTS

He recommends a ratio of 25% cardiovascular exercise to warm up, and 75% strength/resistance training using tubing or small free weights. "There's no gimmick," says Karas, owner of weight-loss management firms in Chicago and New York. "Calorie is king, so use it to lose it."

What's author Fredrick Hahn got in his gym bag? Probably a set of weights.

His new book The Slow Burn Fitness Revolution (Random House) advocates abandoning conventional aerobics altogether for his slow-speed strength-training exercises that he says not only build muscle but also provide cardiovascular benefits.

Turn your body into "a powerful fat-burning machine" by doing his routine just 30 minutes a week, says Hahn in a Sun interview.




SLOW BURN

The New York City personal trainer charges that all the jogging and aerobics we've been doing for years has probably done us more harm than good. "My program builds you up without beating you up." According to Hahn, "every pound of muscle you add on your body increases your resting metabolic rate by about 40 calories a day.

"Add five pounds of muscle, and that's an extra 200 calories a day -- that's an extra 6,000 calories a month you're burning," exclaims Hahn, who also recommends following a protein-rich diet as outlined in his book.

Surprise, surprise! Power-of-10 (HarperCollins) offers no resistance to resistance training.

This author claims "eliminating aerobic exercise from your workouts, except for fun, will actually make you healthier."

Author Adam Zickerman asks you to give him just 20 minutes, just once a week, to do his slow-strength exercises.

And there's rest galore in between workouts -- "five to seven days if you want it."

The fitness trainer wants to morph you into a fat-burning furnace -- but he offers different rates than author Hahn. "Three extra pounds of lean muscle burns about 10,000 extra calories a month."

It burns "as many calories as running 25 miles per week, or doing 25 aerobic workouts per month without leaving your couch," says the owner of InForm Fitness Centres in New York. "You'll be in buff land."

Meanwhile, keep those reps coming, says on-line weight specialist and author Jorge Cruise.

Actually, do it 8 Minutes in the Morning (HarperCollins) and your flab will turn to fab.

All it takes, declares the California fitness trainer, is eight minutes of super-quick strength-training moves, along with his eating plan and his weight-loss Web site.

In less time than it takes to shower, those who Cruise will drop two pounds, on average, of fat per week, firm up muscles, develop curves, rev up their metabolism and boost their confidence. There will be no more of what he calls "emotional eating."

In his book, Cruise does mention incorporating some aerobic activity into your life.

'No shortcuts'

Dr. Sender Deutsch, a doctor of chiropractic, a personal trainer, and strength and conditioning specialist, says:

"Ideally, there should be an equal balance of both -- aerobic and resistance training are equally important in attaining a healthier, leaner and stronger body.

"If you are going to choose to do only aerobic or resistance training, then it should be cardiovascular exercise; your heart is the most important muscle in your body," says Deutsch, director of SHAPE Health and Wellness Centre in Toronto.

"There are no shortcuts to good health. Often followers of exercise fads and fitness gurus find themselves in worse shape than when they started."