Muscle exhaustion or Muscle Strain?

[genius1265]

I was doing doing preacher curls when the I hit failure around 7 reps, (now i believe in going to failure dont try and change my mind on this because your waisting your breath) I was at that point though where you are about halfway up and sometimes you just say "its not going to happen". However last night I said F it, and no one was in the gym so I just put the scary face on and dropped every last ounce of whatever it is that gets that bar up, into it. So i totally pushed through on this one unlike i think i ever have before. Normally i put a really high effort in, the rep before that was extremely difficult. Now dont get me wrong I have made progress with the way I have been but I was wondering if my whole philosophy is just a bit off. I normally do a few sets with what i consider very high intensity, untill i just cant do anymore. But now im wondering does "Microscopic tears in muscle fibers that lead to muscle growth come from muscle exhaustion or from more intense strain that is placed on them?" Thanks all in advance.