How Do Muscles Grow: Size and Strength Not Always Related

Muscle Size StrengthIf you’re wondering how do muscles grow, there’s a good chance you’ve observed the following scenario a number of times:

A guy with a massive chest and shoulders bench presses 225 pounds for reps; pretty impressive. He works his way up to 275 for lower reps; even more impressive.

A few days later you spot a guy who looks like the average Joe, albeit stocky but with some muscle going on, chatting with some muscled-up guys at a bench press station.

Looks like he’s been doing some lifting, maybe to shed some of the belly.

You pay no mind until you hear some rather loud grunting 30 minutes later. The average looking man is cleanly bench pressing a bar with four plates on either side—405 pounds! Where is this strength coming from?

The bigger guy you saw the other day looks like he could easily body-slam this smaller man, but then…the smaller man is way stronger! How could this be? After all, doesn’t the amount of muscle mass correlate to the amount of weight lifted?

This is not the first time you’ve observed something like this, in which a person can lift considerably more than what he or she looks capable of, and usually, the type of lift involved is a bench press, deadlift or squat.

How Do Muscles Grow?

Muscle ProgressTo understand what makes muscles grow, it helps to have some basic understanding of muscle cell anatomy.

If you slice cleanly through a muscle cell to view a cross-section of it, it looks like a tube with a bunch of skinnier tubes or tubules packed inside of it.

There’s “room” or space” between the “tubules.” This space is called sarcoplasm. Each “tubule” is called a myofribril. The myofibrils are made up of many filaments that contain actin and myosin, which are proteins responsible for muscle contraction.

There are two basic types of muscle fiber (muscle fiber is composed of muscle cells): slow twitch and fast twitch. It is the fast twitch type that muscle building exercises recruit.

There are then two classes of fast twitch fiber: type a and type b (IIa and IIb), or sometimes they are referred to as type II and type IIx. The “b” or “IIx” are the bigger of the two, and the most powerful.

In heavy weight lifting, they are recruited last. In a short weight lifting set, the type IIb fibers won’t get fatigued much. The longer a set, the more fatigued the IIb fibers will become. Failure sets guarantee that the IIb fibers will get hit hard.

When a fiber fatigues from going to failure in a set, it’s forced to adapt so that next time around, it could better handle the training stimulus.

Adaption to failure training partially comes in the form of an increased number of the contractile proteins, but not mostly in this form.

Adaption mostly occurs in the form of more “metabolic gear.” The increase in metabolic gear is largely responsible for what is known as muscle growth, i.e., hypertrophy or mass gain.

The ratio of added metabolic gear to added contractile proteins changes when the training protocol changes.

When a person lifts in the 8-12 rep range to failure, meaning, a ninth to thirteenth repetition is not possible, this triggers a big increase in the metabolic gear.

What Does This Metabolic Equipment Consist Of?

Muscle Structure

  • Glycosomes, which are particles that contain glycogen (stored sugar), a muscle cell’s chief form of fuel
  • Mitochondria, little “machines” inside every cell of the body, including muscle cells, that are headquarters for a cell’s energy
  • Capillaries, microscopic blood vessels that training causes to increase in density, a process called capillarization

This metabolic apparatus is located within the sarcoplasm—the “space” between the “tubules” (myofribrils) inside the muscle cell. The increase in metabolic equipment attracts water to the cell.

The metabolic gear enables the muscle fibers to sustain an intense, relatively long set (8-12 reps).

This is easy to understand once you know that more glycosomes mean more sugar-fuel; more mitochondria mean more energy; and more capillaries mean more oxygen-carrying blood.

Sets to failure (or even near failure) involve a lot of time (relatively speaking) under tension. Fatigue to the muscle cells starts setting in after 10 seconds with this kind of training.

The result? Increased volume in the sarcoplasm within which the myofibrils reside. The muscle cell thus gets bigger (hypertrophy). In this case, the increase in size is called sarcoplasmic hypertrophy.

The cell is bigger due to an expansion of the gel-like sarcoplasm, because this substance now contains more metabolic equipment.

From the outside when one views a bodybuilder, this appears to be a bigger biceps, bigger lats or bigger quads. However, though the muscle cell is bigger, it has not become denser.

There are more capillaries, more glycosomes, more mitochondria, more water and more non-contractile proteins, causing the sarcoplasmic area to enlarge, thereby enlarging the entire muscle fiber unit, therefore enlarging the muscle belly (which we see as the biceps or the hamstrings or the deltoids).

What about the contractile proteins inside the myofribrils? As mentioned already, those, too, increase in numbers from 8-12 RM workouts—but not enough to produce maximal strength in the trainee (e.g., benching 405 for reps).

This is why oiled-down bodybuilders can appear as though they can bench press 500 pounds, but often, work out with only half that much.

This isn’t to say that bodybuilders aren’t strong; heck, they’re mighty strong alright, but…there’s a reason they are called bodybuilders and not powerlifters.

All that sarcoplasmic hypertrophy has ballooned up and sculpted their muscles, and in combination with very low body fat, they look spectacular.

However, in the corner of the gym, a man minus bulging muscles and humongous “guns” is bench pressing 425 pounds for three reps. His sarcoplasm is not loaded up with metabolic gear because his training sets are relatively short.

To get as big and rippling as possible and as symmetrical as possible, one needs to execute sets that exceed the immediate energy that cells have available.

This immediate energy comes from the mitochondria, in the form of adenosine triphosphate (ATP), and creatine phosphate.

This short-term energy is gone after 10 seconds, after which, the stored glycogen will be used for energy.

The breakdown of this stored sugar produces lactic acid, which is the “burn” that one feels as the set nears failure; this burn is often what forces the muscle builder to quit the set. (In the case of a powerlifter, he’s forced to quit the set due to mechanical failure—the weight simply becomes too heavy to lift).

The set must exceed 10 seconds to cause sarcoplasm to swell. Failure training is also known as fatigue training.

However, this doesn’t mean that the fatigue that’s experienced while doing 40 repetitions of 30-pound barbell shoulder presses will produce the same result!

This kind of fatigue-inducing workout will tap mostly the slow twitch, type I fibers, which are designed for endurance and do not grow in size.

Let’s back-step to that lactic acid. The production of lactic acid causes more human growth hormone to be released, and more HGH means more muscle growth, in that HGH increases the size of connective tissue (to adapt to the training stimulus).


HGH does not increase contractile proteins, but does elevate fat burning, making muscle development more visible.

Thus, we have the entire process of maximizing muscle hypertrophy: the bodybuilder or “jacked” look, complete with big-peak biceps, towering traps, flexing pecs, and bulging quads, gigantic hamstrings and calves like cantaloupes.

You won’t see this kind of exquisite leg muscular development in men who compete in squatting competitions.

Time under tension can also be achieved by slowing the tempo of repetitions or by including supersets with the same muscle group.

Deceivingly Strong

Woman Weightlifter

The powerlifter or Olympic weight lifter uses a heavier weight load than does the bodybuilder, and performs fewer reps, ranging from one to seven reps.

These athletes are mighty strong, yet are less muscular, or, to put it another way, not nearly as jacked as Mr. Physique.

Now, the muscles do indeed experience hypertrophy, but not to the extent that bodybuilders develop. The smaller size is due to the sarcoplasm not being as loaded up with as much metabolic machinery that occurs in bodybuilders.

There are some sarcoplasmic changes, but not as much as in the bodybuilder because the time under tension in powerlifting or Olympic lifting ends in less than 7-10 seconds.

Not enough fatigue sets in to cause all the capillarization, increased glycosomes, increased water, etc., that occurs with the bodybuilding protocol.

However, something else happens that explains the whopping strength and power. The shorter sets, due to fewer reps, mean that the type IIb fibers don’t get hit with fatigue.

Furthermore, powerlifters and Olympic lifters usually do not train to failure.

So, the type IIb fibers do get used, but in a different way: to lift very heavy loads without having to sustain this long enough to use up all the ATP.

The result is improved firing rate of muscle cells, better synchronization of the motor units and motor recruitment, and improved galvanization of the contractile machinery.

In this case, strength can be viewed as more neurological; a complex skill that has been developed. This doesn’t mean that the type IIb fibers don’t experience any fatigue at all.

They do, which is why the powerlifter or Olympic lifter has visible muscle development, but certainly doesn’t rock the musculature that Mr. Universe contestants do, or even that of male fitness models.

The hypertrophy in power- and Olympic lifters comes mostly from an increase in contractile proteins, which are contained inside the myofibrils. This is called myofibrillar hypertrophy.

The myofibrils become denser, due to an increase in the number of filaments. These filaments are made up of the contractile proteins myosin and actin.

These filaments are called sarcomeres. With more sarcomeres and therefore denser myofibrils, a lot more strength and power results than that which occurs in “muscle building.”

The contractile unit of the muscle cell is more compact, meaning that more force and power can be generated.

What Is The Best Training Protocol?

TickThis depends on one’s goals.

If one wants to look like a bodybuilder or have a physique like the men who star in today’s action films, then the regimen should be based upon fatigue training or failure training with 8-12 rep maxes.

Some fitness professionals insist that reaching failure is not necessary to promote a lot of hypertrophy (sarcoplasmic) or to develop muscles to maximal size, while others insist that going to failure makes a difference.

Another school of thought is to do failure training for 25 percent of the program. In general, here is how it all pans out:

  • Greater than 15 reps: promotes endurance and toning with minimal hypertrophy (sarcoplasmic), minimal strength gains and no recruitment of type IIb fibers.
  • 8-12 reps: maximal hypertrophy of the sarcoplasm, excellent strength grains, but not maximal; a candidate for physique and bodybuilding contests, or fitness modeling
  • 6-7 reps: a happy medium between sarcoplasmic hypertrophy and myofibrillar hypertrophy—a muscular and very strong individual, the type seen in strongman events in which multiple reps within a limited time frame are judged (e.g., how many times heavy bucket-shaped weights can be tossed over a high bar within a set time) – these events do not consist of one-rep maxes.
    Some of these athletes are huge (and some of that size is from body fat), but they still lack the “muscularity” of the competitive bodybuilder.
  • 1-5 reps: greatest increase in the density of myofibrils (maximal increase in the contractile protein myosin-actin filaments), with less addition to the metabolic gear in the sarcoplasm; highest degree of strength/power development; these athletes can be seen in competitions deadlifting, squatting or clean-and-jerking enormous amounts of weight in which the event is judged on a one-rep max.

It should now be clear just how do muscles grow, in terms of what is visible from the outside looking in, and what happens from the inside looking out.

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