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Muscle failure doesn’t provide a significant benefit over stopping on the last quality rep you can do.
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Going to muscle failure SIGNIFICANTLY increases central fatigue, which reduces the strength of the excitatory drive to the muscles, making it harder to recruit the high threshold motor units (which require a stronger drive to get activated, hence the HIGH THRESHOLD name) which are connected to the fast-twitch fibers; the ones with the significant growth potential.
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The more sets you go to failure on, the more of a decrease in effectiveness you will get from the later sets in the workout. Basically going to failure reduces the number of effective sets you can do in a workout. So even if going to failure give you one more “growth promoting/effective rep” per set, it significantly decreases the number of effective reps you can accumulate in your workout.
So in THEORY going to failure will stimulate more hypertrophy than stopping 1, 2 or 3 reps short (albeit not by as much as proponents of failure training would believe). HANG ON, THIS IS NOT A REASON TO GO TO FAILURE.
That’s why some studies comparing going to failure or not show that going to failure might give more results. But they normally do a super small number of sets for both protocols.
If you only do one set of a single exercise in your workout, YES going to failure would be more effective than not.
BUT nobody does that. People tend to do multiple sets (2-5) of multiple exercises (4-8) per workout. And that’s where failure becomes a problem and is counterproductive.
Going to failure has a MUCH stronger impact on central fatigue. Central fatigue refers to a weakening of the excitatory drive toward the muscles. The more of it you have, the weaker the signal to turn on muscle fibers get, the less likely you are to recruit the FT fibers (even if going to failure again).
Central fatigue is impacted by several factors, including duration of effort, complexity of the exercise, load and, most of all, hitting failure. Weirdly enough, and this is important, the feeling of pain, discomfort and struggle actually is one of the main causes of central fatigue! That’s because these things all send powerful signals to the nervous system and it leads to a downward modulation of the excitatory drive as a protective measure.
That last point is the main reason why going to failure causes so much more central fatigue than stopping even just 2 reps short.
Why is all of that important? Because if in theory, going to failure is superior, that’s only true if you look at a set in isolation.
What happens in real life is that going to failure RAPIDLY causes an accumulation of central fatigue. Making each set less and less effective even if they “feel” effective and hard. You simply do not recruit the FT fibers anymore, even if you go to failure.
Everytime, you go to failure, you make all the subsequent sets less effective (unless if you rest for like 6-7 minutes between sets, which is also not realistic or practical).
So while “in theory” you get more effective reps for a set to failure vs not to failure (e.g. 5-6 rather than 3-4) in the context of a whole workout you get DRAMATICALLY fewer effective reps when you go to failure on most sets.
Sorry for being long-winded. It is actually a much more complex topic than what most believe.
The benefits of going to failure are:
a) It’s the easiest way to be sure of the effort level provided in a set. Not everybody accurately stop 1 or 2 reps short of failure
b) It allows you to get a proper stimulus in less sets. But the downside is that once you are adapted to that stimulus you can’t crank it up… you are already going to failure and going to failure create more central fatigue which makes adding volume ineffective