 He began training clients in a near-experimental manner, evaluating the perfect number of repetitions, exercises, and days of rest to achieve maximum benefits.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.