What is failure. Not a nice pretty definition of when it's not possible to do a strict form rep. But an explanation of how far is failure???
must there be a burning effect?
obviously(?) no muscle strain or?
must ache for a day or two?
for instance on squats, legs shaking, unable to want to remain standing for 30 seconds or not?
after 3 days bum still aches from squats/leg routine?
{{Help me to ensure my failure is far enough, but not too far- Because I hurt my lower back on squats and was out of the game for 3 weeks..(legs and mind stronger than my low back)}}
Perhaps you legs, mind, and lower back are stronger than your form?
Sometimes breaking form a little can be usefull but you certainly don't want to put yourself in bad position, especially your back.
The way I do things is fairly simple. If I'm close to a max on something my failure will only be my last rep which I need a spot, and maybe one more if I feel I have enough that my spotter isn't going to do all the work. On the sets where I am doing a smaller weight (but still reasonably heavy) I'll burn the muscle until it's really hurtin bigtime or I can't do another rep. As for feeling a leg work out after a few days I think is very normal and why I think legs are the only muscles where there is a serious case for only working once a week.