Building Muscle Mass: A Look at How Muscle Grows

Gotta get bigger. Building muscle mass seems like the goal of every red-blooded male that sets foot in a gym.

Problem is, nobody seems to really know what makes that happen. The orthodox gym-rat bodybuilder knowledge is to pump up and break down muscle with lots of sets, lots of exercises (to hit it from all the angles), and usually 8-12 reps each set. The HIT guys will tell you it’s all about intensity, or effort, put into each set. The harder you work, the more you’ll grow.

Gah. If it were that easy. Seriously, if these guys have all the answers, why is it that maybe one guy out of every 100 (if that) actually gets jacked? Well, besides the obvious response, also called “drugs”.

Muscle growth is somewhat more complex, though the actual prescriptions for weight-training aren’t anything to be daunted by.

What Causes Muscle to Grow

Fundamentally muscle hypertrophy is all about protein metabolism – the amount of protein that’s taken into your muscles and converted into contractile tissue, versus the amount that’s broken down and sent elsewhere. This is called protein turnover.

In normal conditions these anabolic and catabolic processes will cancel out so that the net change is more or less zero. This is homeostasis, the condition of stable equilibrium. Your body likes homeostasis, which is why you don’t suddenly start growing or shrinking without some outside cause.

In order to grow, we have to pack more ‘stuff’ inside each muscle fiber. The more ‘stuff’ in a fiber, the greater its cross-sectional area (CSA). A muscle fiber is basically a small tube; if you look at it from one end, it’ll basically be a circle. The size of the circle is the CSA of the fiber. It just happens that the ‘stuff’ that takes up the most space is the proteins which actually make the muscle contract. The more protein in the muscle, the greater the CSA and the bigger that muscle will be.

So that’s our goal – to stimulate maximal protein accretion in the muscle fibers. This involves increasing protein synthesis and doing what we can to decrease protein breakdown.

Muscular Tension and the Tension-Time Integral

So what triggers protein synthesis? Simply put, it’s muscular tension and something called the tension-time integral (Goldspink 1974, Goldspink 1975, Martineau & Gardiner 2001, 2002). Muscular tension is easy to define. Pick a muscle and flex it as hard as you can. Feel how it gets tight? That’s tension in the muscle. When it contracts, that tightness is what makes your skeleton move. Here’s the thing, though – just contracting the muscle isn’t enough. Unless you’re trying to overcome some external resistance, you just won’t be able create enough tension to trigger growth.

This is why we lift weights. Barbells, dumbbells, and even machines are all ways of adding external resistance, which forces the muscle to adapt. The body is always fighting for homeostasis, remember. When you throw it off balance, as you do during a workout when you tax your muscles with an unusually heavy weight, the body will try to return to equilibrium. Protein turnover increases, leaning towards a net positive – meaning that protein accumulates in order to make the muscle resistant to whatever challenged it. The side-effect of this adaptation is a bigger muscle.

Tension is the one absolutely necessary factor, but by itself it is not sufficient to see maximum growth. We also have to consider the tension-time integral (TTI). Without going into fancy-pants integral calculus (which I don’t understand enough to talk about anyway), the TTI can be thought of as the amount of work you do with an external resistance that is heavy enough to stimulate gains. It’s the “area under the curve” on a graph of tension with respect to time, if you’re more mathematically-inclined than I.

This, roughly, translates into the total volume of weight-training: the number of sets you do, the number of reps in each set, and how much total poundage you move around (the tonnage). In fact, this is part of the rationale for doing sets of 4-10 when attempting to build muscle mass. In that range, the tension and the time under tension both reach optimal amounts, assuming that you’re working with near-maximum loads (Atha 1981, Wernbom et al 2007). You don’t have to go to failure on each set, but you do need to be working fairly hard in order for those suggestions to make sense. Training in this way tends to maximize strength adaptations while providing maximum stimulus to the muscle – which will in turn trigger muscle mass gains.

It also suggests an optimal range of total sets and reps for a workout, which we see as well. Several meta-analyses have examined research into the topic and given us some fairly useful guidelines for volume (Petersen et al. 2003, 2004, Wernbom et al. 2007). These papers note that a range of 20-100 reps per muscle group, per session, can trigger muscle growth, with an ideal range of 40-60 total reps. In practice, the actual number will depend on the weights being used, with an inverse relationship between intensity and total reps being the norm.

There’s a trend as an individual advances from fewer sets (four in untrained populations) to more sets (up to eight in the trained athletes studied). Workout frequency shows a slight advantage with three weekly sessions over two in beginners, but this vanishes in more advanced individuals, so roughly two sessions per week is optimal for a muscle group.

While these starting ranges may not be applicable to all groups, they do show a clear trend: intensity must increase over time, total sets done will increase even if the reps-per-set decrease, and frequency of workout sessions for each muscle group will tend to drop. The “optimal” value for total reps done will likely vary for each individual, and may not actually change much over time – anecdotally, more advanced lifters can get away with “more” but there are likely confounds to that (see also: drugs). An advanced natural may well see a drop in total reps per session as s/he gets stronger.

To summarize, you need to work with weights in that are heavy enough to stimulate hypertrophy adaptations, but still light enough that you can rack up enough total volume to maximize that stimulus.

In practice, the actual rep range you use doesn’t seem to matter that much. You can do triples or you can do sets of 10 as long as the weight is ‘heavy enough’ and as long as you’re racking up enough total volume. By ‘heavy enough’, I mean sufficient to stimulate gains. For most people, the cut-off is going to be about 65-70% of your one-rep maximum. For those exercises that you can’t or shouldn’t test 1RM weights, figure that if you can do more than about 12 reps, then it’s too light.

Now there are some nuances here as far as how and when to add weight, and there are considerations to be made as far as neuromuscular or strength adaptations. Unfortunately it’s not as simple as just piling more weight on the bar for the vast majority of people. That discussion is well beyond the scope of this article, though. Maybe another time.

Building Muscle Mass as a Long-term Adaptation

Ultimately progress over the long-term is going to come down to getting fundamentally stronger. This can link to getting stronger in the basic exercises, and for most of one’s lifting career I’d submit that this is sufficient. However I do need to stress the point that your goal is to make the muscle groups stronger, not necessarily ‘just’ getting your lifts up. The thing is, most of the time those goals overlap. All the arguments about training for bodybuilding vs. training for powerlifting mostly boil down to semantics and a misunderstanding about specificity.

That’s why I’m so adamant about beginners just sticking to basic routines. You don’t need anything else, since getting stronger just happens to correlate with exposing your muscles to high tensions and sufficient volume to grow. It’s hard to improve on things like the 5×5 routine for that purpose when you’re just starting out.

I do see a potential case to be made for bodybuilders gravitating away from a pure strength emphasis in some scenarios, however. Mainly this happens when you get beyond a certain threshold of strength. You’ll eventually get to a point where just going to the gym and knocking out some sets isn’t enough to keep getting stronger – progressive overload will stop working. Once this happens, you can still get stronger, sure – but this won’t necessarily correlated with maximal hypertrophy training anymore.

At this stage, getting stronger has split away from the goal of getting bigger muscles. Once these goals separate, you have to pick one to specialize. If you want to get stronger, do that. If you want bigger muscles, you have to go after that too. The good news is that these goals never stop being complementary.

Whereas just bench pressing and overhead pressing might have been sufficient to get big shoulders and triceps early on, you might have to start doing more specific exercises and specific set/rep schemes once you’re strong enough. The big lifts are a foundation of a house; the specific muscle-developing work is the paint on the walls. You still need both to have a nice-looking house, but the relative importance is obvious. So there is a rationale for using “isolation” exercises and other more specific training than you’d find in a “strength” program, even though the “strength” program should always be a foundation of your training.

Besides the actual exercise choices, there are also some physiological considerations. It’s been a running hypothesis of mine for awhile now that over the long term, satellite cells and all the chemistry associated with them will play a progressively larger role in the hypertrophy process. This is due to some quirks of human physiology with regards to muscle adaptation. Simply put, we can’t grow new fibers, only enlarge the existing ones – yet we still have immature muscle fibers. Turns out that these fibers actually fuse with the mature muscle fibers after a workout, which makes them larger and allows them to support more protein synthesis.

Here’s my thinking: as a beginner, it’s easy to trigger adaptations because the muscles are ‘soft’ for lack of a better word. Anything is a stimulus and can trigger all the adaptations. As you get more adjusted to a given level of stress, it takes more ‘oomph’ to trigger equal gains. Earlier I said that there was a trend towards heavier weights, less-frequent workouts, and more sets of lower reps, and I’m thinking that this may be one avenue involved. The muscles become so adapted over time that providing hard “hits” to the muscle becomes necessary to sustain growth. In effect, it becomes more important to actually stimulate the satellite cells, as opposed to “just” stimulating protein synthesis with a workout.

We’ve got some indications that satellite cell behavior, from division, maturation, and fusion, is linked to hormonal levels and to inflammation responses, so it could well be that experienced types could well benefit from doing less-frequent but harder and more stressful workouts. Bear in mind that this is just speculation, though – while this does follow from the data, it’s likely that there are other explanations (neurological adaptation comes to mind), and that if this is involved then it’s only one factor. There’s also the teeny tiny fact that satellite cells are strongly signaled by anabolic steroid use, so, uh. Yeah.

How to Train for Building Muscle Mass

We’ve got a pretty effective blueprint at this point; in fact, the suggestions given already are pretty much sufficient to create a workout.

1) Weights that are from 70% to 90% of your 1RM
2) A range of 20 to 60 total reps per muscle group, each workout session
3) Roughly two sessions per muscle group each week

I know a lot of you readers are going to sweat the details, though. The good news is that this already correlates with the workouts I’ve suggested on this site. That’s basically what the upper body/lower body split is, and that’s the rationale behind sets of 4-10 reps. Doing that puts you right in the sweet-spot for mass gains.

As far as picking out exercises, remember the house analogy. You need a basic routine focusing on the big barbell exercises: your squat, deadlift, bench press, overhead press, barbell row, weighted chin-up, and all the numerous variations on those exercises. That’s your foundation. If you want to toss in “pumping” crap for individual muscles after that, you’re welcome to do so. You might want to keep your big lifts somewhat heavy, and then use a higher rep range for the bodybuilding stuff.

Further, you need some sort of on-going progressive overload in order to keep up with the adaptation of the muscles and indeed the entire body. The more muscle mass you add, the harder it becomes to add more. You have to constantly try and out-race that process of accommodation. That’s a job for the periodization process, and it’s really not all that hard either.

Finally, we can’t forget rest. Although this kinda falls under the periodization heading, for every interval of “hard” training you do, it needs to be balanced with “light” training. This can be heavy and light workouts each week, or it can be entire blocks of heavy training and light training, but you have to do it if you really want to grow.