As John's fitness level improves and his workouts become easier, he will need to make more adjustments to his leg routine. The principle of progression says that he must progressively or gradually increase the workload for improvement to continue. Now that John's legs have adapted to his new workout, he must use the overload principle once again to progress to a new level of fitness. He might add more weight, increase the number of reps, lengthen the amount of time he does the workout, or try more difficult exercises.
Both progression and overload can be achieved by using the FITT approach to guide your changes.
Frequency - how often you train (once or twice per week)
Intensity - how hard you train (workload, high or low intensity)
Time - how long you train for (20 min or an hour)