Weight training to failure
Is is absolutely necessary to train to muscular failure to build maximum muscle mass? If you would have asked me this 10 years ago and I would have said yes.
However, I've come to realize that using heavy weight and taking your exercise to complete muscular failure is but one of many ways to increase intensity levels. Mind you, it's a very effective way but only one of the ways.
Today, I tend to think that in order to build muscle, you need to improve upon your routine and exercises with each passing workout, day after day, week after week and month after month.
That is, you need to either use more weight that you did last workout for your working set, using the same reps, and rest times. Or, you do more repetitions using the same weight than you did last workout on your work set, using the same rest periods. Or, do the whole workout in less time.
By improving your weight training sessions with each workout, you increase the intensity of each session therefore creating a better environment for muscle growth. However, going to failure on your last set is very effective for muscle growth, but, it is only effective as long as you are improving.
For example, let's say I used 225 pounds last workout for 8 repetitions and I went to failure on the 8th repetition. Now, let's say this workout, I did 8 unassisted repetitions and hit muscular failure on the 9th repetition. I've improved from this workout to the last. You need to keep improving with each workout in order to build muscle mass.