In the FITT principle, intensity refers to how hard you exercise. It is an important aspect of the FITT principle and can be monitored by heart rate in most patients, although some patients may have pathology or be on drug treatment that affects their HR response to exercise (in which case HR cannot be used to monitor exercise intensity) . The best way to gauge the intensity of your exercise is to monitor your heart rate. For a high-intensity workout, aim to reach 70 to 80 percent of your maximum heart rate. Measures of perceived effort and affective valence (i.e., the pleasantness of exercise) can also be used to modulate or refine the prescribed exercise intensity.