CALCULATE YOUR NUMBERS: First, figure out your maximum predicted heart rate, or the highest it can safely go, with a simple formula. Start with 220 and subtract your age. Then, multiply the result by both 0.6 and 0.7 to get your target heart rate zone. So, if you're 45, your target heart rate is between 105 and 123 beats per minute. (220-45 = 175 x 0.6 = 105; 175 x 0.7 = 123.)
CHECK YOUR HEART RATE: Find your pulse by placing two fingers on your wrist at the base of your thumb. Count the number of beats in 30 seconds and then multiply that number by 2. This will give your heart rate in beats per minute. If your heart rate is below the target range, push yourself a bit harder (walk faster or do some jumping jacks). If your heart rate is above the target range, take the intensity down a notch.
SWEAT CAN BE AN INDICATOR: Pay attention to the point at which you start sweating because that could mean you've reached your target heart rate zone, says Johnny Lee, MD, cardiologist and spokesperson for the American Heart Association. However, if you haven't worked out in a while or are overweight, you may start to sweat long before you reach this optimal zone. That's why figuring out your target (and keeping that number in mind during exercise) is still beneficial for everyone, no matter what.
BUY A HEART RATE MONITOR: Don't want to worry about figuring out your heart rate while on the run? Consider investing in a heart rate monitor that will instantly display your numbers to help you stay on track. One we like: The strapless Omron HR-210, $32; Amazon.com, which you wear on your wrist, calculates your heart rate in seconds. It also tracks calories burned.