Training to failure is how I've always trained, ever since I could remember. Lift the weight, till you can't do another full rep. But, ive been reading more and more articles about how not training to failure, to stop 2 reps short. I read one article that says, the moment you explosiveness startes to fade, and you start grinding out the reps, you should stop the set. I now beleive its not the intensity that builds muscle, but just the over all amount of work done. And as long as you keep progressing in your workouts(adding weight) you will get bigger. I have for years would look and feel great for a few weeks, then all of sudden, id look alot smaller and feel like shit. only when I took a few days off, would i feel better again. Since i train each bodypart 2x per week on average, something had to give. I was already doing a fairly low volume workout. So now, I will be reducing the intensity to 1 or 2 reps shy of failure.
I actually think that training to strict form failure is one of the least stressful training methods ON THE MUSCLE. Even training to 8+ rep failure with forced reps is not that stressful on the muscle because the first 2-3 reps of a set will produce the greatest force on the muscle either because you can do 2-3 reps heavier than you can do 8-12, or because all of you fibers will be unfatigued.
I do know this, there are at least 2 real different types of failure. In method one, you train to momentary failure at around 8-12 or sometimes up to around 15 reps. You run out of energy and outrun your oxygen and build up lactic acid. Ideally your whole system has enough fitness to take the individual muscles to the point of outrunning the energy supply. It basically means you used up all of your ATP and CP, and got far enough ahead of their replenishment that you couldn't keep going and you needed to rest for some time after the set. I think it is important that you can outrace the energetic depletion and put yourself 4-5 reps in the "hole" before you can't move the weight for lack of energy.
In method 2, you expose your muscle fibers to a heavy load. An example protocol would be that I will do 1 heavy hack squat every 15 seconds, ie one at 0, one at 0:15, one at 0:30, one at 0:45, racking fast between reps for about 10-12 seconds. Maybe you do a double every 30 seconds or a triple every 45 seconds. At some point you will get a rep where you can not contract hard enough to move the weight, but your breathing is fine, your muscles are not burning (though they might burn a little between segments).
It can be a fast/explosive rep, or an exercise with a peak contraction, as long as you have to flex real hard at some point on each rep.
What seems to work for me now is kind of the opposite of what I've seen suggested in some popular programs like DC ie doing 3 sets to failure with short rest period.
What I do now, and have gotten real new muscle from is to do the second method, say a heavy but powerful 2-3 reps every 30-40 second (less on smaller exercises) for 4-6 segments, or maybe 2-3 minutes total, THEN take a final segment to energetic failure.
So on the hack squat for example, you might take an 8 rep max weight, but do 2-3 reps every 30 seconds, at time 0, 30, 1:00, 1:30, 2:00, 2:30, then at the 3 minute mark you try to get 5-8 reps. That way you get 12-18 heavier reps and can focus on feeling the target muscles, and still get to challenge your energy supply.
As for volume versus intensity, I alternate. I'll go through 4 workouts with higher volume, like say I'll try to get 25-30 good reps in in 5 minutes, but rest a minute when I stop feeling the working muscle 100%. So I might get 8, 6, 5, 4 and 4 reps in in 5 minutes stopping each time when I lose full focus on the muscle.
Then I'll go through a second week with higher intensity, like 15 reps rest-paused on the squat, or 7-10 singles with 10 seconds rest in between. Or I'll do 5 reps strict, 4-5 forced reps, and maybe a real slow negative at the end.
If you only do volume, you will lose your balls, but if you do intensity workouts one week, then you will still push hard when you go back to the higher volume workouts, but if you only do intense sets you may tend to not focus on feeling the muscle in exchange for forcing up heavy weight.
Basically for a given exercise, week 1, warm and ramp up, then do 20-30 strict reps within a 5 minute time frame, feeling the working muscle do the work. Then week 2, warm and ramp up, and push to the limit over 1-2 minutes using rest pause, forced reps, holds, negatives.