“A few years ago I started training champion bodybuilder Sharon Bruneau. After her first week of three workouts, she told me she wanted to stop training for a while because she’d already seen an increase in size. She was worried she’d be too big for the Ms. Olympia. After three weeks of no training, I put Sharon through another week of three workouts. Curiously enough, she was stronger on every set of every exercise in that second series of three workouts. That meant she’d succeeded in stimulating an increase during the first series of workouts and that her body had not only produced but also maintained the increase—for three weeks. If the body does in fact decompensate after 96 hours, she would have been weaker, but she was considerably stronger after three weeks of absolutely no weight training."
Just like everyone else, Mentzer was selling something, Heavy Duty is not what made the Mentzer brothers pros, but it was a nice new unused gimmick to push, a variation on Arthur Jones' system to sell Nautilus machines.
Weider & Gironda had theirs, Bob Kennedy had pre-exhaustion, etc. Mike himself didnt do 1 or 2 sets as he prescribed, when called on it, he would say the other 3 or 4 sets were warm-ups.
This is like when Darden mentions in articles and books how by following Jones' superior methods, Arnold, Sergio, Viator, Coe, Wilson, etc all surpassed plateaus, made unbelievable gains, smashed records.... So why did all these names go back to traditional training after finding the holy grail? Were they scared they'd get too big?