If you have only 45 minutes three days a week to work on fitness, consider spinning. While sitting on a specially engineered bike, you'll be climbing mountains and racing down slopes, simulating an outdoor ride.

"Spinning is the hottest fitness program sweeping the country," says Wanda Perez, of Grass Valley, Calif., a personal trainer who leads spinning classes.

During an indoor group workout, the trainer provides instruction—taking riders through imaginary landscapes or talking them through biking drills—while motivating music encourages increased energy expenditure. The workout can be tailored to a variety of ages and fitness levels. Spinners may also customize the workout intensity by adjusting the resistance control on the bike; hence, spinning can challenge the rider to increasingly reach new intensity levels.

A monitor provides heart-rate feedback, helping the rider assess fitness progress and exercise intensity.

"Spinning works the big muscles—legs and buttocks—that require lots of blood, so the exercise strengthens the cardiovascular system, ultimately resulting in a lower resting heart rate," said Gil Olsen, of Grass Valley, Calif., another personal trainer devoted to the sport.

Combined with a healthy diet, spinning can make a significant contribution to a weight loss program as well. But beware! Considering its popularity, spinning may be addictive!