The principle of progress can also be called load progression and is one of the most important triggers for generating hypertrophy in a weight training workout.
The principle of progress is nothing more than to apply an increasing load on the muscles to stimulate the adaptation, forcing the body to get stronger and stronger.
Imagine that you lift 20kg for 8 repetitions with great difficulty. As time goes by and with you insisting, your body will adapt so that you are able to use this load for the number of repetitions, getting bigger and stronger.
