How hard your heart is beating during physical activity is a good way to gauge your exercise intensity. According to the Mayo Clinic, calculating your maximum heart rate can then allow you to determine your desired heart rate zone, the level at which your heart is being exercised and conditioned but not overworked. Moderate exercise intensity is between 50% and 70% of your max heart rate, while vigorous exercise intensity is between 70% and 85% of your max heart rate.