For more than ten years, Mentzer's Heavy Duty program involved 7–9 sets per workout on a three-day-per-week schedule. With the advent of "modern bodybuilding" (where bodybuilders became more massive than ever before) by the early 1990s, he ultimately modified that routine until there were fewer working sets and more days of rest. His first breakthrough became known as the 'Ideal (Principled) Routine', which was a fantastic step in minimal training. Outlined in "High-Intensity Training the Mike Mentzer Way", fewer than five working sets were performed each session, and rest was emphasized, calling for 4–7 days of recovery before the next workout. According to Mentzer, biologists and physiologists since the nineteenth century have known that hypertrophy is directly related to intensity, not duration, of effort (Mentzer 2003;39). Most bodybuilding and weightlifting authorities do not take into account the severe nature of the stress imposed by heavy, strenuous resistance exercise carried to the point of positive muscular failure.