If anyone shrinks from changing from one system to another (or even taking a 1 to 2 weeks brake from training), than the system they were on had only been giving a false positive that actual muscle mass was developed. Actually the tissue was in a pump up'ed stage, and that is not anywhere near lasting true muscle mass. Just wasting time and effort if positive lasting muscle mass was the goal. Most BB'ers will depend on that pump, which only inflates the tissue with blood....never indicating real muscle gains. If doing 12 to 20+ pump sets a muscle group, may want to re-check your goals for retaining and increasing true muscle mass.
The PHA system (as mentioned before) is an excellent way to insure prime fitness (as most other circuit style training does), though as an extreme muscle mass builder it may be a bit lacking. Me, and others I trained with, used it from time to time during off season football workouts. Based on full body and non stop sessions, lasting from 20 to 40 minutes (some have done much longer bouts with it). A ultimate full body workout, every muscle from wrist, neck, abs, etc besides the larger muscle areas are worked...saying this would be hardcore would be an understatement.
Bob Gadja made the concept popular at the Duncan YMCA, Chicago in the '60's. Sergio also worked on the PHA system, with Gadja for a while. Basic idea was not to ever get anywhere near a pump, but keep the blood flowing from one area to another...non stop. Which might mean hit squats than neck than wrist curls, than chest calves raises,etc, etc, etc... If anyone thinks their in shape, might try PHA and see what happens...may surprise yourself. Excellent for fat burning and muscle sparing.
Good Luck.