Find the Right Dose
Taking your reps to failure is useful under the right circumstances and in the right doses. And it's not the only stimulus for growth, maybe not even the best one.
Granted, there's been some research showing that the weight used isn't as important as going to failure when it comes to hypertrophy. One experiment even showed that a load of 30% of a lifter's max (light weight) led to similar hypertrophy as a load of 80% (much heavier).
This would indicate that failure is required in stimulating growth. But nothing is ever that simple. You can't ignore all the bodybuilders and athletes who've built a lot of muscle without going to failure.
Are You Really Hitting Failure?
So is hitting failure really the stimulus for growth? Or is it only the outcome of pushing what's needed for growth to the extreme?
Here's why it seems like training to failure will work: When you reach failure you've exhausted the muscle fibers so much that they can no longer produce enough force to lift the weight. But failure isn't always due to fatigue of the muscle fibers.
Muscle failure can be due to energy depletion (phosphagens or glycogen in the muscle), and mitochondrial failure can also occur with very high reps. And a common cause of muscle failure is the acidification in the muscle. Lactic acid and hydrogen ions accumulate as the result of mechanical work. These interfere with the contraction of the muscle fibers. If it interferes enough, you hit failure.
When doing sets for 40-60 seconds (12-20 reps) that's likely what causes failure. On top of that, fast twitch dominant individuals hit that point much sooner because they produce more lactic acid. So a person with more fast twitch muscle might hit failure due to acidity or contractile interference during sets as short as 25-30 seconds.
So hitting failure doesn't necessarily mean that you fatigued the fibers as much as you could. Furthermore, in many exercises hitting failure doesn't mean that the target muscle even failed. It's more likely due to other factors: the muscles involved can't generate enough force; proper lifting posture and mechanics can't be maintained decreasing efficiency; or even CNS fatigue.
Bottom Line: Going to failure can be useful with certain types of training and in some individuals, but it's certainly not to be used universally on all exercises.
Sure, you can go to failure to recruit more muscle fibers without having to use heavy weights. The nervous system pretty much always recruits the slow twitch fibers first. If the weight is too heavy for the strength of these fibers alone, it will bring the intermediate fibers into play. And if it's even heavier, the fast twitch fibers will be added to the mix.
Failure or Not, Get Maximal Muscle Fiber Recruitment
The average person needs to lift around 80-85% of his or her maximum. People who are very fast twitch dominant, or neurally efficient, might be able to do so with as little as 65-70%. Of course, this is for normal reps. If you're lifting explosively, you can recruit the fast twitch fibers even with a light load.
The other way to recruit fast twitch fibers is to use muscle fatigue to make the weight heavy. In a normal set you lose 1 to 4% of your strength per rep. The longer the set lasts, the more decrement there is from rep to rep.
For example, in a set of 20 reps you have a much larger strength drop from rep 19 to 20 than between reps 1 and 2. That's because in the latter reps more fast twitch fibers are recruited and fatigue faster. There's also a greater accumulation of lactic acid.
And after several reps with 60% of your max you can recruit as many fibers as you would if you were using a weight of 80% because as you fatigue the weight becomes heavier relative to your current strength potential. During a set of 15 reps at 60% here's what each rep "feels" like:
Parameters set at 60% of 1RM
- Rep 1: Feels like 60%
- Rep 2: Feels like 62%
- Rep 3: Feels like 63%
- Rep 4: Feels like 64%
- Rep 5: Feels like 65%
- Rep 6: Feels like 66%
- Rep 7: Feels like 67%
- Rep 8: Feels like 68%
- Rep 9: Feels like 70%
- Rep 10: Feels like 72%
- Rep 11: Feels like 74%
- Rep 12: Feels like 76%
- Rep 13: Feels like 78%
- Rep 14: Feels like 80%
- Rep 15: Feels like 83%
Remember, maximum voluntary fiber recruitment occurs at 80-85%. So by rep 14 the accumulated fatigue makes 60% feel like 80%. By rep 13 or 14, because of the accumulation of fatigue, you're recruiting all the fibers you can recruit. What would happen if we continued the 15 reps set with 60% until we hit failure? Here's what extending the set would feel like:
- Rep 15: Feels like 83%
- Rep 16: Feels like 85%
- Rep 17: Feels like 88%
- Rep 18: Feels like 91%
- Rep 19: Feels like 95%
- Rep 20: Feels like 100%
Failure would likely happen at rep 20. The first 13 reps, up to what feels like 78%, is where you prepare the body for the "money" reps and build up the fatigue that'll force your body to recruit higher threshold muscle fibers. Reps 14 to 18 are the money reps where you have full recruitment and can fatigue/stimulate the muscle fibers most prone to growth.
The final few reps are the danger zone, where you're likely to hit failure. They place a bigger burden on the nervous system and impair your capacity to do more work. Think of each set where you reach that point the same way as you'd think of a set where you lift your 1RM.
This is important to understand: Going to failure has the same neurological impact as going for a 1RM because you hit failure when the relative weight of the resistance is slightly higher than your strength potential at the moment. In other words, the rep you hit failure on is the rep where you couldn't lift the weight despite a maximal effort.
Training to failure when building muscle is like a powerlifter always doing max lifts in training. Neurologically it has the same impact because that last rep is a maximum effort: it can be very effective, but you'll burn out fast. Going to failure is only an insurance policy. It helps you make sure that you recruited as many fibers as possible. The thing with an insurance policy is that it costs you. The cost is neural fatigue and eventually hormonal fatigue.
If you're going to do it, know what exercises are okay for going to failure. Only do it with ones that have a low neurological demand – isolation exercises and machines mostly. Avoid hitting failure on compound exercises.