Circuit training is short bursts of resistance exercise using moderate resistance and frequent repetitions, followed quickly by another burst of exercise targeting a different muscle group.

Because the exerciser switches between muscle groups, no rest is needed between exercises. This gets the heart rate up, which usually doesn't happen during resistance exercise. To up heart rate further, aerobics stations are sprinkled between the resistance exercises.

An ongoing CD with a recorded voice informs you when it’s time to switch to another piece of equipment or to a cardio station.

Westcott cites the "classic" Cooper Clinic study done in 1982, which studied the effects of doing a circuit workout three times a week. The study had 77 participants, who were divided into three groups.

Not surprisingly, the group that didn't train saw no improvement in its cardiovascular fitness. The weights group improved cardio fitness by 12%. And the weights-and-jogging group improved 17%. (The weights group also improved strength by 17% and the weights-and-jogging group improved strength by 22%.)

According to Westcott, a circuit workout improves both strength and endurance, and jump-starts metabolism.

Although weight training has traditionally been a male pastime (think Ah-nold), it's important for women, who tend to lose muscle mass at the rate of 1% per year in their late 30’s and 40’s. This muscle often gets replaced by fat. But you need muscles to cushion joints and help protect against osteoporosis, among other benefits. That's not to mention the trimmer, tighter appearance you'll gain by toning up.

Circuit training works because it's short and sweet and people actually do it. "It's brief, it's basic, it's consistent; no frills, over quickly" is how Westcott puts it.