The FITT Principle describes how to safely apply the principles of overload and progression:
Frequency. Frequency is how often a person performs the
targeted health-related physical activity. For
each component of health-related fitness, a safe
frequency is three to five times a week.
Intensity is how hard a person exercises during
a physical activity period. Intensity can be
measured in different ways, depending on the
related health-related component. For example,
monitoring heart rate is one way to gauge
intensity during aerobic endurance activities,
but gives no indication of intensity during
Time. Time is the length of the physical activity. As
with the other aspects of the FITT principle,
time varies depending on the health-related
fitness component targeted.