The pedometer tops the list of simple tools that track fitness, and most fitness experts will tell you to rack up 10,000 steps a day on yours. But while a pedometer can tell you how many, what it can't do is tell you if you're stepping fast enough to really do your body any good.

Moderately intensive physical activity -- we know we need to get at least 30 minutes of it a day, but what exactly is it? Researchers at San Diego State University think they know. They put volunteers on treadmills, then used pedometers to count their steps and monitors to track the volunteers' heart rates. The results? 100 steps a minute is the perfect pace for fitness.

"Because health benefits can be achieved with bouts of exercise lasting at least 10 minutes, a useful starting point is to try and accumulate 1,000 steps in 10 minutes, before building up to 3,000 steps in 30 minutes," says lead researcher Dr. Simon J. Marshall in a news release.

I'm not convinced this rule accounts for varying fitness levels. Put a gym rat and an exercise newbie on side-by-side treadmills and ask them to walk 100 steps a minute. I'd bet you'll see a big difference in exertion, both perceived and in the heart rate numbers. Still, it's smart to define "moderate exertion" so that people aren't out there wasting their time on inefficient workouts.

If you forget your pedometer (or don't own one), the talk test is another easy way to gauge whether or not you're working hard enough. Fitz shared her "keep it simple" version last year, and one of my favorites is this interpretation of the Borg Scale of Perceived Exertion from About.com. The basic idea is that if you feel breathless but can still talk to your workout partner, you're probably in the moderate zone.