It isn’t necessary to exercise at maximum heart rate to improve fitness and gain the health benefits of exercise. Fitness adaptations occur at lower heart rates than maximum, with a much lower risk of injury. Your target heart rate zone— the rate at which you should exercise to experience cardiorespiratory benefits— is between 65 percent and 90 percent of maximum heart rate. Estimate your maximum heart rate (MHR) by subtracting your age from 220. Multiply your MHR by 65 percent and 90 percent to calculate your target heart rate zone. Very unfit people should use 55 percent of their MHR for their training threshold. Scientists have suggested other methods of choosing exercise intensity, but these differences really don’t matter. The exact intensity of your program is not that important. More intense exercise builds fitness faster, but is difficult to sustain over time. Less intense workouts build fitness slowly but may have better long-term health benefits.
Source: University of California, Berkeley Wellness Letter, December 2014