The most import word here is failure.
You have to work till failure, cuz it's then that your muscles will say : We have a problem with keeping up with this guy; and they'll make some sort of protection ( muscle ) to keep up with you better next time. Next time you higher your breakpoint again.
My personal trainer said that this is the most important part of training and that 90% of the people don't realise this and thus, don't gain such an amount of muscles.
He also advices to rest 3 days between each workout , in order to fully recover your muscles.
I used to train 4 times a week, where I did just 3 sets of 15 reps for each exercise.
Now I go 2 times a week with 1 ;;yes 1 ; set of 8 reps-12 reps until failure ( which means I always stop at the 10th or so rep because of failure ) and I have great improvement.
In opposite to high interval training, I don't work on an interval but on a target of 8-12 reps ; and I do them very slow. ( 4 secs up, 2 hold, 4 down, 2 hold )
I believe that lifting weights fast is not good.
You make use of momentum instead of your muscles.
If interested on high intensity training, check out Mike Spencer un derground seminar on youtube.
High intensity is the best way to train. Glad you brought it up.
V