The heart is the most extraordinary muscle in the human body. It delivers oxygen throughout the body to sustain life, and it's how blood is delivered to our organs and running muscles. You can actually train the heart to pump more efficiently, and pump more blood—and thus, oxygen—with each beat.
In response to physical activity, heart rate increases in a predictable manner. In fact, the relationship between exercise intensity and heart rate is an extremely linear one: the greater the intensity, the higher the heart rate, with the relationship becoming more curvilinear—heart rate begins to plateau—at very high intensities. Because of its predictability, you can use heart rate to prescribe running intensities, and monitor your progress over time. For example, as your fitness improves, you'll run at a faster pace at the same heart rate, and your heart rate will be lower when running at the same pace.
How to Determine Your Target Heart Rate
There are generally two ways to use heart rate to determine intensity. The first is to simply take a percentage of your maximum heart rate (max HR). Subtract your age from 220 to find your approximate max HR. For example, a 40-year-old's max HR would be approximately 180 beats per minute (220 - 40), and a target range of 70 to 80 percent would correspond to 126 to 144 beats per minute.
The second method of using heart rate to calculate a target range accounts for your resting heart rate. This method is called the Karvonen Method, named after its founder. To calculate your target heart rate, subtract your resting HR from your max HR before multiplying by the desired percentage. Your resting HR is then added back to the product. The difference between the max HR and the resting HR is called the heart rate reserve (HRR). If the 40-year-old in the above example has a resting HR of 60 beats per minute, a target heart rate of 70 to 80 percent HRR would be calculated as follows:
HRR = (220 - 40) - 60 = 120 beats per minute
Lower Limit = (120 x 0.70) + 60 = 144 beats per minute
Upper Limit = (120 x 0.80) + 60 = 156 beats per minute
The Karvonen formula is especially attractive to use since it also estimates the running intensity in relation to your maximum oxygen consumption (VO2 max). For example, 75 percent HRR equals 75 percent VO2 max. There is about a 10 percent difference when comparing either percent HRR or percent VO2 max to percent max HR, however. For example, 75 percent HRR equals about 85 percent max HR.
More: What Is VO2 Max?