Muscular Hypertrophy

by Andy Baker, SSC | May 25, 2022

training progression graph from the cover of practical programming for strength training

The process of muscle growth is a confusing topic for many. Clients are confused, coaches are confused, the guys that research it in the lab are confused. Or at least it would seem that way. This is understandable insofar as the underlying processes that occur at the level of the muscle cell are indeed extremely complex and only partially understood.

Depending on what decade we’re in, the explanations for exactly what triggers a muscle to grow might vary. Many mechanisms have been given credit – from the laughable to the highly plausible – but often differing from each other substantially. In recent years, the three most common mechanisms proposed for muscular hypertrophy would be muscle damage, metabolic stress, and mechanical tension. From a purely observational standpoint, it makes some sense to break it down along these lines.

Muscle Damage

For many decades we have witnessed big strong people get even bigger and even stronger by forcefully moving the performance needle in the gym. Often this occurs in spite of their training – simply through sheer grit, determination, and willpower. More weight on the bar. More reps. More sets. Pushing sets deeper towards failure. Intensity-amplifying techniques like forced reps and drop sets, etc. All of these things create muscle damage. Sometimes lots of it. At least some level of underlying and continuous muscle soreness is normal for most hard training bodybuilders or powerlifters. Even a crippling case of lower body DOMS isn’t necessarily abnormal for many trainees. So it might be totally rational to assume that muscle damage was the primary (or one of the primary) mechanisms by which muscle growth is triggered.

We’ve all heard it. We’ve probably all said it. “You’ve got to break the muscle down so the body will repair it and build it back up!” In the “Golden Era of Bodybuilding,” this was pretty much scriptural. Muscles had to be broken down thoroughly for growth to occur. Fibers had to be hit from every angle with as many sets as one could tolerate, as many times per week as one could tolerate.

Muscle Damage was everything.

Metabolic Stress

What about Metabolic Stress or what might be thought of as “traditional” bodybuilding training? Higher reps, lots of sets, and very short rest periods between those sets? Engorging the muscles with blood and metabolic waste products such as lactate (a by-product of the breakdown of energy to fuel lots of sets and reps in a short period of time) in order to create an extremely intense “pump” or “burn” in the muscle?

It feels odd to even refer to this as “traditional” bodybuilding training. Mainly because you rarely see it actually done in practice. For sure on some machine or cable-based exercises for small muscle groups, but rarely if ever for the barbell lifts or even higher stress assistance exercises.

To date, there is no evidence that I am aware of that clearly links the accumulation of metabolic waste products in and around the muscle directly to muscle growth. For a time it was hypothesized that training with very short rest periods and sky-high fatigue would improve motor unit recruitment (i.e. more muscle fibers would be recruited and therefore trained in every rep). This hypothesis is flawed though. It attributes directly to metabolite accumulation what should be attributed more towards volitional effort – the source of motor unit recruitment. As fatigue builds, effort necessarily goes up to complete more reps. But the presence of extreme fatigue probably does more to inhibit motor unit recruitment than enhance it. And it certainly inhibits force production by allowing for nothing but extremely light weights to be used.

To this point, there seems to be only some correlation without a viable explanation for causation. In my own practical experience as a coach and a lifter, the most important role of the “pump” is to help the lifter direct the stress of a given exercise to the target muscle group. This can be most useful on assistance exercises.

The other problem with the “metabolic stress” model is that, by and large, it is impractical – bordering on impossible – to perform with high-stress exercises. This is evidenced by the fact that, observationally, nobody actually trains this way, because you can't. Squatting 5 sets of 8-12 reps with any kind of weight at all on short rest intervals is not an appropriate mechanism of training for anyone I can think of. It’s simply not a progressable framework over any length of time.

For either strength or hypertrophy there is no benefit to training your primary barbell exercises on arbitrarily shortened rest periods in this fashion. The extreme increase in systemic and localized fatigue coupled with massive reductions in force production leaves one scratching one's head as to a target audience for this model. Even if you’re interested in training lower-body local muscular endurance – push the Prowler on repeat. It’s a far better tool for the job.

Mechanical Tension

So the problem with considering either the muscle damage paradigm, or the metabolic stress paradigm is two-fold. One, neither of these models can be divorced or isolated from the third model – which theorizes that Mechanical Tension is the primary mechanism by which muscle growth is stimulated.

Progressive increases in mechanical tension is the only mechanism that can reliably predict a hypertrophic response across broad populations. In other words, it doesn’t matter whether we’re talking about novice, intermediate, or advanced trainees, powerlifters or bodybuilders; a mechanical tension stress at the level of the muscle fiber is needed for growth to occur.

Tension can be achieved and demonstrated in the presence or in the absence (or relative absence) of muscle damage or metabolic stress. So muscle growth can and will occur when increases in mechanical tension are present, regardless of the presence or relative absence of muscle damage or metabolic stress. The inverse is not true, and is also difficult to tease out. In a training environment it’s nearly impossible to create a scenario where we create muscle damage or metabolic stress without mechanical tension.

Muscle fibers are placed under high levels of mechanical tension under two main circumstances: (1) when a heavy weight is lifted – think weights in the 1-5 rep range, or (2) when submaximal weights are lifted with higher reps but taken to (or very close) to failure – let’s say a 12RM. What makes these two events similar (at the level of the muscle fiber) is the involuntarily slowdown of the repetition. The involuntary part is important: simply lifting a light weight with a slow tempo does not substantially increase mechanical tension within the muscle.

So, when a lifter un-racks the barbell for a heavy set of 5 reps (think 80-85% of 1RM) the lifter is under the ideal conditions for optimally placing the most amount of muscle fibers under the optimal amount of mechanical tension. Heavy weights demand that the bigger type-II muscle fibers (those with the most potential for growth and force production) are recruited early in the set – and in this case, from the very first rep. We know this because even for the very first rep of a heavy set of 5 we have to exert maximal or near-maximal effort on the barbell in order to move it.

A similar event happens in a maximum effort set of 12 reps. Perhaps the first 6-7 reps of this set move with a sub-maximal amount of effort (submaximal implies less motor unit recruitment and thus less muscle mass being trained). But then, as fatigue starts to build in the working motor units, the barbell slows down, and the lifter must exert more and more effort to move the barbell for more reps. As the set progresses and voluntary effort increases, motor unit recruitment increases (specifically the bigger type II fibers), the barbell slows and a very high percentage of type II muscle fibers are placed under a high degree of mechanical tension. (Note: I am differentiating between a single 12RM set used here and the earlier example of metabolic stress that involved multiple sets of 8-12 on short rest periods. These are two fundamentally different events).

In essence, the last 5 reps of the 12RM, were similar to the 5RM in terms of the number of muscle fibers trained, even though there are still some qualitative differences between these two events. First, if we are concerned at all with force production, then the amount of weight on the barbell actually matters. Muscle fibers can experience a high level of mechanical tension even with relatively light loads, so long as those loads are pushed right up to the brink of failure. But this is not the same as the force production capacity of the entire muscle, much less the entirety of the musculo-skeletal system.

For force production, there is no end-run around heavy weight. And yes, it is possible to actually achieve some degree of hypertrophic response through the use of submaximal weights taken to failure, and achieve very little improvement in force production capacity at heavier weights (or even a loss of force production capacity).

So if we are primarily interested in hypertrophy, and we have a couple of potential pathways to get there, what do we choose? There are a few factors to consider. First, we already alluded to the fact that training with sets of 5 will automatically select for a load that gives you the dual benefit of placing a large number of muscle fibers under high degrees of mechanical tension and allows for actual strength/force production work to be performed (because the weight is actually heavy, not just perceived as heavy due to fatigue). If we compare to a set of 10 or 12 taken towards failure, we may potentially get the hypertrophy response, but we utterly lack the component that trains force production.

This has consequences in terms of long-term progressibility of a system that relies solely or mostly on high-repetition work. And progressibility must be accounted for. If there is no mechanism in the program that trains force production, then there is no mechanism that creates a catalyst for long-term progress. As odd as it may sound, your ability to squat 315 x 20, will by and large be predicated on your ability to squat 495 x 5 first. The best way to increase your capacity with submaximal loads is to increase your capacity with near-maximal loads.

So How Do We Train?

It might be safe to say that both muscle damage and metabolic stress are simply inevitable by-products of a training system that is focused on progressive increases in Mechanical Tension. Since we can’t separate them out entirely, just focus on what is causative. Damaging a muscle beyond its ability to recover is both unnecessary and counterproductive. But some muscle damage is unavoidable. Both heavy loads (sets of 5) and submaximal loads taken to failure create both mechanical tension and muscle damage, and so we can’t really get all of the good without at least some of the bad no matter how we train.

But chasing muscle damage as a primary objective can actually be self-defeating insofar as it creates a competition for resources by two competing processes. Both damage repair and growth require an uptick in the rate of muscular protein synthesis (MPS) in order for the individual processes (growth and repair) to occur – but these are not the same processes. To simplify, repairing a damaged muscle fiber is not the same as growing one. So it may be that by forcing your body to focus on repair, you are actually inhibiting growth.

So again – getting back to the main point – if we aren’t chasing muscle damage and we aren’t chasing “the pump,” what are we exactly chasing in each individual workout?

It’s Still All About the PR

So even if performance isn’t the goal, performance is still the goal.

If Mechanical Tension is the mechanism by which growth is signaled, how do we quantifiably increase it over time? The only real metric we have is weight on the bar. So the weight on the barbell must go up over time if increases in tension – and therefore increases in muscle mass – are to occur. As it so happens, PR sets of 5 are an excellent measuring stick for the bodybuilder or physique competitor. They encapsulate the right balance of mechanical tension, force production, low fatigue, and predictability.

By predictability I am referring to the ability of the trainee and coach to rely upon the transference of radical improvement in a trainee's 5RM to the broader goal of increased muscle mass. To simplify, taking your squat from 315 x 5 to 405 x 5 will result in a bigger pair of legs. It will also increase your 1-rep max and your 20-rep max, without ever training either. Sets of 5 are fantastic in that they don’t necessarily rely on developing other physical capacities to get good at them, and thus they make an excellent training as well as diagnostic tool.

One-rep maxes are irrelevant for physique-oriented trainees. There is a skill element inherent in performing one-rep maxes that is not a productive use of time for the non-powerlifter. One-rep maxes are highly fatiguing and take away energy and resources for training that does a better job of actually building the muscle. Sets of 15-20 on a barbell exercise are as much about local muscular and cardiovascular endurance as they are about anything else. This is an unnecessary obstacle to overcome when sets of 5 will take you directly towards your goal.

The Importance of Progressive Overload

There are many many variables when it comes to training. For example:

  • The type of training split you use.
  • The exact exercises you perform.
  • The rep range you train within.
  • The total volume you perform.
  • What, if any, assistance movements you perform.

All of these things matter some. None of them matter at all in the absence of progressive overload. Bottom line: if you are squatting 315 x 5 today, and this time next year you are squatting 315 x 5, you will not have grown. Period. End of story.

It is irrelevant how much per-session volume you performed, whether you squatted high bar or low bar, how much rest you took between sets, how many times per week you squatted, or what assistance movements you did or did not do. None of these things matter in the absence of progressive overload.

An alternate case is also true. If you squat 315 x 5 today, and this time next year you squat 365 x 5 or 405 x 5, you will have grown.

How you get there is also irrelevant. Whether you get there squatting once per week or 3 times per week does not matter. Whether you get there with one big work set or multiple work sets does not matter. Whether you use assistance work or none, does not matter. These are all programming considerations that play out on an individual level. Some will need less, some will need more. But we never do more for the sake of more, or less for the sake of less. Do what is needed to allow you to progress.

In your own analysis of your programming, do your best to distill things down to what really matters. What you will find is that in the absence of progressive overload, none of the things you obsess over actually matter at all.

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