Related to the second principle of progression is the FITT formula: frequency, intensity, time and type. I'll say it one more time: frequency, intensity, time and type. And this is simply a formula that's used as a guideline for creating an exercise program that's both safe and effective for developing physical fitness. Frequency, we're talking about how often you exercise, once a week, twice a week, five times a week. Intensity, we're talking about how hard that exercise session is, how difficult is that session. Time, how long do you exercise, is it 20 minutes, 30 minutes, 60 minutes. And finally the type, what type of exercise do you choose. And all four of these components, all four of these aspects of the FITT formula, are dependent on a couple things. One, the individual's fitness goals, so, for example, the program for someone who wants to enhance their muscular endurance is going to be different than someone who wants to increase their muscular strength. The second thing that it's dependent on is the current level of fitness of the individual, so someone with very high levels of current fitness should be able to handle a more intense or longer exercise session, maybe more sessions per week, compared to someone who's just getting started who would need lower levels of intensity, maybe less frequent, shorter time sessions first and then progress into those more intense and longer sessions.