So far I've had 8 exercise sessions at Dr. McGuff's personal training studio, and I wanted to report some observations.
In the 6 months I was doing HIT prior to training at the studio, I would do a single set and would select a weight that would have me reach momentary muscle failure at around the 50-70 second mark. However, I often felt that I wasn't tapping into every last bit of my reserves, and so I would frequently do rest-pause reps or other advanced training techniques to get a deeper inroad. But even then, I didn't feel like I was 'hitting the mark'. I would have a deep muscle fatigue for a day, maybe two, and would feel ready to go again a day or two later. Also, just before going to the studio my strength gains had leveled off to where I was only infrequently increasing the weights I used and was only making minor progress in my time under load.
Since training at the studio I've been reaching failure at around the 100-130 second mark and the difference is pretty drastic. The deep muscle fatigue that I felt before now lasts for 4 days after, and I definitely can't exercise more than once a week. My results have been impressive as well. I've gone up 30 lbs on the row machine, 20lbs on the overhead press, and 60 lbs on the leg press and I'm still maxing out my times which means it's time to up my weights
again.
I'm also hyperventilating way more than before, which suggests greatly increased metabolic activity and would mean I'm getting some good conditioning benefits as well.
I know there's more to my results than just a longer time under load. But, having reached momentary muscle failure at 60 seconds and 120 seconds I would recommend people at least consider experimenting with higher times under load. It will mean a slightly longer total exercise time, but I've found it to produce better results.