This principle suggests that when designing a personal training program, the frequency, intensity, time, and type of exercise must be considered. Specifically, frequency is how often clients should be performing a particular component of their program. Intensity is the difficulty level of the program component (as measured by heart rate, load, reps, watts, and so on). Time is how long each component should last and how much rest clients should have (usually measured in minutes or seconds). Type refers to the choice of exercise used for a given component of their program (usually defined an exercise, piece of equipment, or activity).