"The Last Two Reps Are the Most Important!"
You've heard that before in bodybuilding circles. Is it true or just a macho catchphrase? Let's explore it.
This idea refers to pushing a set to the point where it's almost impossible to complete another rep – hitting failure. As a general concept, yes, pushing yourself hard can be useful. Most people don't push their moderate or high-rep sets far enough to stimulate hypertrophy. And the quote does emphasize the importance of reaching a certain threshold of effort to trigger growth.
But if you take it literally, this quote would mean that all the reps done prior to those last two have no value in helping you get stronger or bigger. What's worse, it also implies that if you don't reach failure, you're wasting your time.
Does it hold water? Well, there's no doubt that hitting failure is a strong trigger for muscle growth. You'll create a lot of muscle fiber fatigue and accumulate metabolites (lactic acid, hydrogen ions) that are connected to a release in local growth factors.
So the quote is partially right. Those last two reps can be the most important in that hitting failure ensures a more thorough fiber fatigue and growth factor accumulation. It might be reasonable when doing bodybuilding-style work on exercises that have a lower neurological demand. But...
Here's Where It Falls Short
First, the reps preceding the last few painful ones can be just as important. They increase neural activation and are fatiguing to the muscle fibers. Both of these lead to greater fiber recruitment in the last part of the set. Without the earlier reps you can't reach the fatigued state. It's the bulk of the set that puts you in the physical state to make those last two reps so hard.
Second, the earlier reps are better for motor learning. It'll be easier to maintain perfect technique or focus on maximally contracting the target muscle. Once lactic acid has set in and it becomes painful to do a rep, you'll be more focused on surviving and tolerating the pain than on the intensity of the muscle contraction or technical efficiency.
The easier reps are a great tool to practice contracting and recruiting your target muscle. The better you become at doing that, the more growth you'll be able to stimulate.
And finally, on big compound lifts, going to failure often isn't a good investment. When you hit failure on a squat for example, not all of the muscles involved have hit failure. You just can't produce enough force from the combined muscles to lift the weight.
Plus, going to failure on those lifts can create an amazingly high neural stress which can negatively impact the rest of your workout or even the subsequent workouts. I've known lifters who've experienced a decrease in performance for a week after going to failure on deadlifts.
The Technical Element
On those big lifts, especially those with a postural component like squats and deadlifts, your form will likely start to break down before you hit failure. This both increases the risk of injury and can lead to bad motor learning.
And remember, hitting failure isn't the only stimulus for growth. People have built awesome physiques without going to failure regularly. You can trigger muscle growth via mTor activation without hitting failure by emphasizing the eccentric (negative) or the stretch position. You can create muscle microtrauma without going to failure. You can even stimulate local growth factors by building up lactic acid without hitting the point where you can't complete a rep.
You can also create enough fiber fatigue to stimulate growth without having to reach that last impossible rep. Going to failure ensures that you did all you could to fatigue a muscle, but it's not a requirement.
If strength is your main goal, hitting failure is likely going to do more harm than good (on the big strength lifts). It can lead to faulty technique and decrease the amount of work you can do in your workout and training week.
Strength is a motor skill, not just a physical capacity. The more you practice it, the better you become at it. If training for strength, the neurological cost that comes from going to failure will limit how much you can train and will diminish your strength gains.