Bill Roberts touched upon this in a more eloquent fashion than I’m capable of, however I’ll add my personal experience to his scientific explanation.
The only time that I experienced changes in my body (positive changes) from HIT style training was after longer periods of very high volume AND high intensity (i.e. a lot of overall volume with regards to muscular work and a lot of CNS stress from utilizing heavy weight as well). Essentially I overreached for a significant period of time, proceeded to take roughly 10 days off from ALL training and then did about 4 weeks of HIT style training. My training consisted of, no joke, two 30 minute, full body sessions per week. I did about 1-2 exercises per body part…one true working set that was taken WAY passed the point of failure, utilizing numerous methods: extremely long tempo reps which accumulated quite a lot of metabolic waste byproduct, forced reps and long negatives, drop sets, isometric holds, extended sets etc. I can promise the level of intensity because it was done under the eye of a trainer specializing in HIT.
Summation of my experience:
-
My body responded relatively favorably, however, nothing out of this world. I was only on for a brief four weeks (not really fair to judge based on the duration). This was attributed, IMO, simply due to a severe reduction in overall volume, exercise duration etc.
-
I hated absolutely EVERY. SINGLE. SECOND. This type of training (when done TRULY to and passed failure) is simply not for everyone. It certainly is not for me…one of the primary reasons is the amount of discomfort and the amount of mental fortitude it takes to truly go that far passed failure was exhausting in every sense of the word. I dreaded training…even though it was only two 30 minute sessions per week!..I literally could not sleep the night before because of the sheer dread I had. Perhaps, I’m a pussy…whatever. For me, I can tolerate hours of volume…zero rest days…hell, I’ve even squatted every day, some times twice a day for weeks on end! Nothing I have ever done has equaled the amount of mental and physical pain of HIT style training. The second session I did, I literally could barely make it to the locker room…I was SO nauseated and so exhausted that I could hardly speak to anyone let alone get my ass off the couch in the locker room. In fact, I had to shit my brains out, yet I couldn’t muster the strength to get my ass up and go to the bathroom for almost 30 minutes until I thought I might shit my pants if I didn’t/
I only stayed on this for four weeks because of the above reasons. It truly made me HATE training and training is one of the few things I hold dear to my heart and enjoy…I look forward to it. I didn’t like not liking training. The other reason was the amount of time being out of the gym. To be in the gym for only 1 combined hour per WEEK was just too difficult for me. I felt like a lazy piece of crap and I couldn’t really will myself to do cardio much due to the amount each session took from my physically and mentally; surprising how much those very brief, 30 minute sessions took out of me. Needless to say, this type of training was not for me, and I found that I got the same favorable effects from simply taking deload/taper weeks or a full week off after long periods of heavy volume.