When you train hard and create microtears in your muscle fibers, you’ve created the primary stimulus for muscle hypertrophy.
Your body will repair the micro tears and ultimately create a bigger and stronger muscle.
How many times do you think you’ve heard this?
Chances are, a lot.
If this is true, the implication is you’d want to consistently train in a way that produces high levels of muscle damage. If you don’t, you’re leaving gains on the table.
However, what if we actually examined the literature on the stimuli behind muscle hypertrophy?
Is damage the primary stimulus for gains, or is it something else?
Well, let’s go ahead and summarize the current literature.
Table of Contents
Part I: Why Do People Believe Microtears Stimulate Hypertrophy?
People have suggested microtears drive muscle hypertrophy based on a few different arguments.
When muscles get damaged, immune cells called neutrophils travel to the location of damage, primarily removing cellular debris.
But neutrophils also produce reactive oxygen species, and reactive oxygen species have been theorized to promote muscle hypertrophy, potentially via increasing activation of the MAPK signaling pathway (a pathway that ultimately increases protein synthesis and thus muscle size).
Moreover, another immune cell called macrophages is also believed to produce something called myokines, and myokines are small proteins that may also promote muscle hypertrophy.
Aside from these immune cells, muscle damage can also cause the build-up of fluid and proteins at the site of damage, resulting in cell swelling.
This swelling puts pressure on various things in the muscle fiber, and it’s thought the cell sees this as a threat to its integrity, subsequently resulting in protein synthesis increases while protein breakdown decreases (ultimately leading to an increase in muscle size).
The final main reason behind why people have speculated damage drives hypertrophy relates to eccentric only training.
During your normal exercises, notice how you’re challenged significantly more during the lifting (termed concentric) phase versus the lowering (termed eccentric) phase.
Eccentric-only training is where a specialized machine or some equipment overloads and challenges you during only the eccentric phase.
This type of training produces substantially more damage than normal or concentric only training, and eccentric training has been proposed to produce more hypertrophy in the long term.
Thus, perhaps the greater damage with eccentric only training causes the greater long-term muscle hypertrophy.
So, the immune cell response, cell swelling, and eccentric only training are all lines of evidence that suggest damage may be important for muscle hypertrophy.
But, does this conclusively mean damage drivers hypertrophy?
Part II: Microtears DO NOT Stimulate Hypertrophy
Regarding the immune cell and cell swelling arguments, though suggesting a possible avenue between damage and hypertrophy, it’s just a mechanistic discussion. It isn’t sufficient to prove more damaging training produces more long-term hypertrophy.
Moreover, although we noted reactive oxygen species may play a role in increasing MAPK signaling, it is also believed to have anti-muscle growth effects, making its overall role in muscle hypertrophy far from crystal clear.
With eccentric only training, some may say this is direct evidence more damage leads to more hypertrophy.
However, this does not seem to be true. The idea eccentric only training produces more hypertrophy comes from older research, conducted before 2010.
Since then, more research has come out, and the whole body of evidence fails to demonstrate eccentric only training is superior (one and two)
Quoting a 2017 review study “We conclude that, when matched for either maximum load or work, similar increase in muscle size is found between ECC and CON RT.”
Interestingly, eccentric-only training, despite similar overall muscle size increases, may be able to produce a different type of hypertrophy versus concentric and normal training, but we’ll save this fascinating finding for another article.
Progressing on, we have some solid data that seriously questions the importance of damage for hypertrophy.
A 2011 study out of the USA by Flann et al. assigned 14 untrained men into a pre-trained or naive group
The naive group performed eccentric exercise on a leg cycle ergometer for 20 minutes at a somewhat hard exertion level (measured using a subjective scale) 3 times a week for 8 weeks.
The pre-trained group did the same thing, but they had a 3-week ramp-up phase before the 8 weeks, where they gradually acclimatized themselves to the training program.
Due to this acclimatization phase, they ended up experiencing little amounts of damage across the training weeks, whereas the naive group experienced much higher damage levels across the training weeks.
Yet, hypertrophy of the quadriceps ended up being comparable between both groups.
Now, the naive group did, on average, see 1% greater gains, but this might just be due to random chance and not the actual training regime as it was not statistically significant.
Another 2016 study out of Brazil by Damas et al. further indicates damage isn’t related to hypertrophy.
They found after untrained individuals performed a single training session, myofibrillar protein synthesis was of course increased afterwards.
However, this increase in myofibrillar protein synthesis did not correlate with muscle hypertrophy, rather it seems it was directed towards repairing the damage induced by that single training session.
As these same subjects continued training for some weeks, which resulted in them experiencing less and less damage (as your body produces adaptations that make you resilient to muscle damage), it was only then the myofibrillar protein synthesis increases after training correlated with muscle hypertrophy.
Re-phrasing all this, this data suggests after an initial workout that produces much muscle damage, the myofibrillar protein synthesis increase afterwards is mainly directed towards repairing the damage, not increasing muscle size. But after a few weeks of consistent training which allows the body the produce adaptations that reduce the damage experienced, then the myofibrillar protein synthesis response is strongly directed towards increasing muscle size.
The indirect implication of this study is excessive damage might actually be counterproductive for hypertrophy, and indeed a 1999 study out of the USA similarly indicates this.
In fact, this study suggests in certain cases, excessive muscle damage causes muscle loss.
They had 6 men perform 5 sets of 10 eccentric biceps curl with 110% of their concentric one-rep max.
Signs of damage and swelling persisted for 7 days after the session, but after this subsided, muscle volume actually decreased by 10% to what it was before the study, and it remained at this smaller size for numerous further weeks.
This might be because excessive damage can cause partial or total destruction of subpopulations of muscle fibers.
A few other lines of evidence similarly imply excessive damage may cause muscle loss.
A final nail in the coffin for the idea that damage is the primary driver of hypertrophy is the existence of indirect evidence finding regimes that produce more damage do not produce more hypertrophy.
Higher rep numbers likely cause more muscle damage, but we now know from numerous studies higher and lower reps can produce similar hypertrophy.
Short rest between sets (1 minute or less) tends to cause more damage than resting longer between sets, yet we know with compound exercises at least, resting 2.5 to 3 minutes between sets produces more hypertrophy than resting for 1 minute or less.
Finally, bro-split routines where you cram all your volume for a muscle group into a single session per week will cause more muscle damage versus spreading your volume for a muscle across more days per week (such as done with full body workouts), yet bro-splits are mostly not superior for building muscle.
Part III: Summary
In summary, muscle damage is clearly not the primary stimulus for hypertrophy.
So what is?
Mechanical tension is likely the primary hypertrophy stimulus.
Muscle fibers contain mechanosensors that can detect tension and then convert this tension into a signaling event that leads to myofibrillar protein synthesis increases.
Thus, to optimize mechanical tension, we’d want to recruit as many muscle fibers as possible and expose each fiber to a decent amount of tension.
Achieving this is not complicated, you’d want to get to or close to failure with your repetitions (likely at least 3 reps away from failure).
This is because getting to or close to failure ensures your effort is high, and the amount of effort you put forth has a close relationship to the number of muscle fibers recruited.
Additionally, as you near failure, many muscle fibers will increase their activation levels.
Notice how this indicates a wide range of things can build muscle.
So long as you get to or close to failure, a wide range of rep ranges and rep tempos are going to be similarly effective.