Every single thing you do in the gym should be focused around the law of intensity and progression. Intensity is the amount of effort exerted by the muscles on each given set of each given exercise. Progression is consistently increasing the amount of weight lifted or the number of repetitions performed on each exercise.
This principle is what muscle growth is ultimately based upon, and is absolutely, positively critical to understand and implement if you want to see real results. Gaining muscle is all about following this law, and you will only get the results you are looking for if it is put into practice.
