I don't know if he did that during the Colorado experiment but he definitely did later on. I remember reading his training routine in the 80's and he trained with a lot of volume. Why would he ditch HIT if it worked so well?
Yes, he trained for his best ever condition with volume and he used plenty of it. I remember Mentzer commenting about his abandonment of HIT in his writings. Also David Young trained in the same gym as Casey and he said he used around 15 sets a body part. The picture shown below is the result. Was he claiming to be a HIT guy because Arthur Jones was his employer?
On a side note I remember asking how his health was maybe a year before he died because I saw a picture of him looking bloated and fat. He wrote back all his blood and urine tests came back perfect. Needless to say the determination of health is more than lab blood work. Your VO2 max, resting heart rate, blood pressure and how fast your heart lowers after exertion are other important indicators. May he rest in peace. Having said that I could drop dead today too. Don't mistake what I wrote as a dig against him. He was an amazing bodybuilder.
