Heart rate is generally accepted as a good way to measure intensity during running, swimming, cycling, and other aerobic activities. Exercise that doesn't raise your heart rate to a certain level and keep it there for 20 minutes won't contribute significantly to your cardiovascular fitness.

But how do you know what your Target Heart Rate is? One of the simplest ways to calculate it is:

Maximum heart rate (220 minus your age) times 70%

Thus, for a 40-year-old, the target heart rate would be 126: 220 minus 40 = 180 x 70%.

You can check your heart rate during a workout by taking your pulse within 5 seconds after interrupting your exercise. Count your pulse for 10 seconds and multiply it by 6 to get the per-minute rate.