Mike Mentzer was one of the most popular bodybuilders during the Golden Era of the sport. He was famous for his thick physique and constant fights with Arnold. Mike is also the person who made High Intensity Training (H.I.T.) protocols famous through his Heavy Duty books. Please keep in mind that the main principles were originally presented by the creator of the training equipment Nautilus - Arthur Jones. While there are some good ideas behind H.I.T. it usually fails miserably when followed as prescribed by Mentzer unless you are using steroids on regular basis.
The main principle behind H.I.T. is that you should perform just one working set to complete muscle failure on a few exercises and call it a day. This blows if you are a natural bodybuilder because one set even taken beyond failure is simply not hard enough on the body to cause growth while being extremely stressful on the mind and Central Nervous System (CNS). Everybody who has ever trained the H.I.T. way knows the feeling before starting a set - your mind is troubled and already crushed because you cannot stop a single rep before failure. This gets brutally hard as time goes by.
While science had advanced tremendously people still argue to this day what causes muscular hypertrophy. I don't have a definitive answer to that either but less work sure as hell does not cause more hypertrophy unless you are on hormones or dreaming. The body always responds to stimulus and the more the better as long as you can recover for your next training session. This brings me to the next idea presented by the H.I.T. bro scientists - you should train once every 7-10 days. This is quite wrong and I advise you to try the following example - train one day and for the next 10 days do not train at all. You will be sore. On the 11th day repeat your workout. You will be sore again. After 10 more days repeat again - you will be sore. Why? Because your adaptation to training is slowly fading away during those 10 days of rest.
The fact that you are sore after every workout means one thing - your body has more than enough time to recover and the adaptation process is interrupted by the extra rest days. In other words your body is slowly going out of "beast mode" and slowly switching to "weak sauce mode."
Try playing a piano song every day for 10 weeks - you will get better at playing it. Now play it once every 10 days - you will barely maintain your skill level. Even professional musicians need to practice when working on a song which is not part of their usual repertoire.
Try training each body part twice a week or every 5-6 days. Most likely you will not be sore after your workouts except for the first week. Why? Because of adaptation. When training more often and with more volume the body adapts and recovers faster.
High intensity training to failureis not practical on exercises like bench press, squat, deadlift, good mornings...etc. If you go to complete failure when doing deadlifts for example you are only risking injury while accomplishing nothing. It's better to do a simple "tight" set of 6-7 reps with good form. That's the reason why you see so many wannabes doing H.I.T. workouts using machines - there is less risk and you can go really slow and feel the burn. How many gyms have all that unnecessary equipment? Who likes to train like a 90 y.o. grandpa using mainly machines?
One set Squats
One set Close-grip, Palms-Up Pulldowns
rest 5-7 days
One set Regular Deadlifts
One set Dips
rest 5-7 days
I am sorry to bring it to you but this routine is only good for maintaining some muscle mass and will not build strength nor size due to the extremely small amount of weight (overall tonnage) lifted despite the "high intensity". Mentzer himself never used similar routines and many people have said that he never actually trained the way he preached to others. But even if he did he was also abusing high doses of steroids - Casey Viator said in a interview that Mentzer was taking up to 2.5 grams of Deca on weekly basis.
I still believe that some of the ideas behind H.I.T such as pre-exhaustion, and more rest for recovery are really good but natural bodybuilders simply need to do more work and cannot afford to rely on synthetic hormones.
P.S. Arnold's way of training was rightfully criticized by Mentzer but his alternative was extreme.