It takes Metallica to get some folks' heart racing; for others, Tom Jones does the trick. But when you're working out, you want to keep your heartbeat pumping at about 50 percent to 75 percent of your MHR (maximum heart rate).

That won't overburden your cardiovascular system. But what is your MHR, exactly?

For years the rule has been "MHR equals 220 minus your age." And working out in your target heart rate range of 50 percent to 75 percent of MHR provides the intensity and safety you need to build endurance, strength and aerobic capacity. (We like 80 percent or more for 20 minutes, three times a week if your doc agrees.)

One recent study discovered that the old formula can underestimate your MHR by up to 40 beats per minute if you're 65 or older.

Another study presented at this spring's American College of Cardiology annual scientific sessions looked at 25,000 stress tests and discovered that older men and women's MHR are distinctly different. Women 40-89 can find their MHR by 200 minus 67 percent of their age; in men 40-89, it's 216 minus 93 percent of their age.

These new formulas will help you determine your target heart rate. But if you're less number-driven, stick with this formula: low-intensity workouts feel, well, easy; moderate intensity (what most of you are aiming for) breaks a sweat, but you can talk comfortably and sustain your effort; high intensity is when you're really sweating (that's 80 percent or more of your MHR for 20 minutes) and not talking much.

Any way you get there is maximum great!

© 2014 Michael Roizen, M.D. and Mehmet Oz, M.D.
Distributed by King Features Syndicate, Inc.