Do you know what your anaerobic threshold is? Do you care? You should—the higher your anaerobic threshold, the longer, harder and faster you can exercise aerobically before your muscles cry "uncle."

When you exercise, your body uses two primary energy sources: fat and carbohydrate. When you exercise at an easy level (i.e., a long, slow run), there's no rush to supply energy, so your body can take the time to metabolize fats. However, during vigorous exercise (say, one to three miles of high-speed running), your body uses more carbohydrates because they are easier to "grab" and put to use for energy than fat.

The anaerobic threshold (AT) is that level of intensity where carbohydrates take over as the major supplier of energy. It's also the point at which the oxygen supply to your muscles starts to dwindle. The result is a buildup of lactic acid—that's why it's also called the lactate threshold.

As you know from your own experience, excess levels of lactic acid inhibit muscle contraction and cause a feeling of heaviness, fatigue, and possibly even cramping. This is what you might call "hitting the wall."

To improve your performance, you want your AT to be as high as possible. If you gauge it by your heart rate, which is an excellent measure of intensity, the AT typically occurs at around 75 to 80 percent of your maximum heart rate. (To determine your maximum heart rate in beats per minute, subtract your age from 220.)

If you're 30 years old, for example, your maximum heart rate may be 190 and your AT is likely around 143 to 152 beats per minute. (If you really want to keep track of your workout intensity and your AT, use a heart rate monitor or go to a sports medicine clinic where they can scientifically pinpoint both your max heart rate and AT.)

To improve your AT, you should exercise at or slightly above your threshold level for a minimum of 20 minutes about three times a week in the beginning. After three to four weeks, you can go to twice a week. With the proper training, you can push your AT to 85 percent or higher. A well-trained athlete can approach 94 to 95 percent of maximum.