If you're ready to take the next step up in intensity, burn some more calories and inject some novelty into a stale routine, interval training might be just what you've been looking for.

The idea behind it is relatively simple: Take a ho-hum run-of-the-mill aerobic workout and spice it up with a tough but short period -- called, appropriately, an interval -- of challenging intensity. Then bring it back down for a "recovery period" and do it all again. This basic template can be modified for levels of fitness ranging from virtual beginner to professional athlete.

Although the basics may be simple, the execution can be as easy or as difficult as you want to make it. To illustrate, though, we need some tangible measure of intensity. Exercise physiologists like to talk about "percentage of V02 max," which is a basic measure of how much oxygen you are using and can be estimated from your heart rate. Too complicated. For our purposes, let's use a simple scale of 1-10, with 1 being so easy it's practically like staying in bed and 10 being the hardest effort you're capable of putting out, one you couldn't possibly sustain for long.

Now let's further assume you've just begun a fitness program and have worked up to a simple, brisk 20-minute walk. Let's say that you would rate that brisk walk as a 3 on the scale of 1-10. Great. A basic interval program for you might look like this: Walk for a while at 3; now break out at 4 1/2 or 5 for a short spurt; return to 3 while you catch your breath, and, when you're ready, do it all over again.