Training to failure: attempting to lift a certain weight a certain number of targeted repetitions, so that the last rep isn't possible. It is a widely debated topic.
The support for it:
Training to failure has worked for a lot of people. We cannot dismiss proof of concept. Muscle reacts to fatigue and there is no question that pushing a muscle to fail results in fatigue. Without overload, muscle will not grow.
The science behind it:
Recent research compared groups of “train to failure” lifters and “train close to failure” lifters and found the results to be almost identical. Len Kravitz, Ph.D, from the University of New Mexico writes “Studies clearly show that subjects can positively gain strength and power without always going to the strict discomfort and acute physical effort associated with failure contractions.”
Comparisons between groups lifting to failure and those who did not, were still under ideal conditions. Percentages of max lifts were dialed in to the point where as those who did not train to absolute failure were only moments away.
This is why arguments on both sides have valid points. Ending a series of repetitions moments before inevitable failure (pre-failure) can be as beneficial as lifting to failure, providing you know exactly where that point is. Lifting to failure ensures that you hit that point. Failure reps eliminate the guess work during a lift, guaranteeing fatigue. When you lift to failure, you have zero reps left in the tank. When you lift to pre-failure, you have an indeterminate number of reps left in the tank. As you increase strength, your max lift will change. As your max lift changes, so does the point of pre-inevitable failure.
Training to pre-failure, however, is shown to have faster recovery times than training to failure. Longer recovery times can lead to overtraining.
So what do you do?
If you train a muscle to failure on occasion, you can better determine where pre-failure is. Training to failure should not be the goal of every set, and when you do it, you should expect longer recovery times. Fine tune your workout.