By Sarah Unke

To get the most out of a cardio workout, you need to get your heart rate into your target heart rate zone. However you keep your heart rate elevated—by running, doing intervals on an elliptical machine, on taking a spinning class—the American Heart Association recommends that you do “moderately intense aerobic exercise” for at least 30 minutes most days of the week.

Rebecca Naden-pa/AP Photo Venus Williams gets her share of target heart-rate training on court.

Here’s how you do it: According to the American Heart Association, a person’s target heart rate is 60–80 percent of his or her maximum heart rate. To find your maximum heart rate, subtract your age from 220. So, take a 35-year-old, for example. His or her maximum heart rate is 220 minus 35, so 185 beats per minute (bpm). That’s the maximum rate he should reach while exerting himself. The target heart rate zone, or where he should try to keep his heart rate for the most safe and effective cardio workout, is 60–85 percent of the maximum, so 111–157 bpm.

Or, if you don't feel like making arithmetic part of your workout, just check out this chart from the American Heart Association.

A heart rate monitor is the most convenient way to keep track of where your heart rate is during a workout. But you can also do it the old-fashioned way by checking your pulse. Just count how many times your heart beats in 10 seconds at your wrist or neck and multiply that number by 6 to get your beats per minute. That way you can tell whether you need to pick up your intensity, dial it back, or stay right where you are.

Sarah Unke is the managing editor of TENNIS and SMASH magazines. She writes a blog, The Healthy Player, for TENNIS.com.