Why 100 PAI
The goal of 100 PAI was chosen as a normalized value, but what each person needs to do to achieve 100 PAI is unique to them. As you become more fit, it also becomes inherently more difficult to achieve 100 PAI which makes it appropriately challenging for all levels. PAI is the great equalizer!
It is important to note that 50 PAI is a great goal to start with, as you will still be enjoying 60% of the health benefits with that level of activity.