What are the two muscle fiber types?

Ever wonder why some people get bigger and stronger with minimal effort, while others grow like crazy and progress more slowly?

Among the many possible explanations — diet, sleep, the whims of the muscle-building gods — one factor you hear about most often is the variation in muscle fiber types between individuals.

Often oversimplified in fitness, muscle fibers are the stringy, contractile filaments that make up every muscle in your body.

There are several different varieties, but generally speaking, you only need to be concerned with two, fanciful names Type I And Type II

People doing bicep curls

Type I fibers are smaller and more endurance-oriented while type II fibers are larger, stronger and more powerful.

It has long been clear that different people – and different muscles – have different proportions of these fiber types.

This has led many fitness-minded people to conclude that this is why some lifters must build shoulders like half a cantaloupe, while others still look like a Roma tomato.

But is it true? What do we know about these fiber types?

Are we actually stuck with the proportions we were born with, or can we change them?

And how can we apply what we know about fiber types to help us reach our fitness goals faster?

What are the two (-ish) types of muscle fibers?i am

As mentioned above, there are two broad classes of muscle fibers—including subclasses—that differ based on their function, composition, and the type of fuel that primarily powers them.

Type I (“slow-twitch”) muscle fibers

Also known as “slow-oxidative” muscle fibers, these do the most work on a day-to-day basis.

They are more dependent on oxygen (supplied through the bloodstream – hence the fancy “oxidative” moniker) in combination with fat or glycogen – a type of sugar stored in your liver and muscles – for energy production than type II fibers and are better at low-to-moderate intensity steady states. For activities like walking, jogging, light resistance training etc.

They are also your smallest muscle fibers, and when stressed consistently with endurance-focused strength training, they grow less than other fiber types.

However, stress them with aerobic-type activity and they’ll grow more mitochondria—the tiny powerhouses within each fiber where energy production occurs.

All of that makes them more geared toward endurance-based activities.

Type I fibers appear red under the microscope due to the presence of blood-carrying myoglobin, which helps transport oxygen to the cells.

Type II (“fast-twitch”) muscle fibers

Man lifting dumbbells

There are two classifications of type II muscle fibers: type IIa and type IIx.

Type IIa: Also known as “intermediate fibers” or, for the science geeks among us, “fast-oxidative-glycolytic” fibers, type IIa jump into action when a task becomes too intense for type I fibers.

They also run on oxygen and glycogen, but while faster and stronger than type I fibers, they are considerably less energy efficient and fatigue quicker.

Want to do a heavy press set or sprint to catch the departing Uber? Fast-twitchers are fibers for work.

Because they require only minimal oxygen to function, type IIa fibers – and their more explosive cousin, IIx – appear gray or white under a microscope.

Type IIx: A subclass of fast-twitch fibers, also known as “fast-glycolytic” fibers, they are the strongest, most powerful — and least energy-efficient — fibers.

Mitochondria are scarce in IIx, as oxygen is not the primary driver of energy production in these fibers, which run almost entirely on glycogen.

When you’re going for a max-effort squat, broad jump, or 100-meter dash, your IIx fibers are best suited for the job.

Type IIx fibers are a bit mysterious — in active people, less than 2 percent of fibers are pure type IIx.

In addition to these, there are several “hybrid” fiber types that exist on the spectrum among other major types. More on those in a minute.

What are your primary muscle fiber types?

Low shot of woman lifting kettle bell

Given this basic description, most fitness-minded people immediately have two questions:

“Am I more Type I or Type II?


“How can I get more type II and grow bigger muscles?”

The answer to the first question is more involved than you might think.

The only way to get a completely reliable answer is to biopsy your muscles, in which a lab technician takes a core sample of each major muscle in your body with a large needle so he can analyze the relative proportions of different fiber types. microscope

Not a fun — or cost-effective — proposition

Some fitness professionals suggest that you can estimate your fast-twitch/slow-twitch ratio with a performance test.

For example, lift 80 to 85 percent of your one-rep max during an exercise for a given muscle group and see how many repetitions you can do.

“There are a number of problems with this type of testing,” says Trevor Thiem, CSCS, Beachbody’s senior manager of fitness and nutrition content. “Not the least of which is that how quickly you experience fatigue is directly affected by how efficient you are at performing the exercise.”

In other words, your ability to perform a lift is not determined solely by the brute force of the muscles used; How skilled you are in movement also counts.

So that leaves you with a very unscientific approach: taking an educated guess.

“If you struggle to put on muscle but excel at endurance sports, you’re probably Type-I dominant,” Thieme says.

“If you pack on muscle fairly easily and prefer pumping iron or spinning your wheels on a bicycle to pounding the pavement, you’re likely a Type-II dominant,” he adds.

Somewhere in the middle? This will put you somewhere in the middle.

Can you change muscle fiber type?

So now, the burning question: With training, can you change your fiber type profile?

Ask most fitness professionals this question and they will usually paraphrase the school teacher quote: “It comes in fiber type“They will tell you,”Get what you get, and you won’t be upset.”

But emerging research raises some questions about this repeated fitness axiom.

Muscle fiber type appears to change. All the time. And rather quickly.

Research from Cal-State Fullerton shows that the muscle fiber types of identical twins vary significantly, from twin to twin, based on the individual’s preference for physical activity. Genes only tell part of the story.

Remember that hybrid fiber we talked about?

These little buggers can make up up to 40 percent of the muscle fibers of a sedentary human — and are especially sensitive to change.

With training, they can become ‘pure’ fiber types — type Is or type IIs. And the more you train, the more they will change to accommodate the demands you place on them.

Stop training and they become hybrids again.

Why the lingering confusion over fiber-type conversion?

It comes down to advances in measurement: only recently has it been possible to accurately classify large numbers of individual muscle fibers and distinguish hybrid fibers from pure types.

According to Dr. Andy Galpin, PhD, once the guys in the white coats figured out how to accurately count and classify the fibers, the mystery was solved. A leading researcher in the field of fiber types.

Unfortunately for those of us looking for a clear answer, this research opens up the long-standing question of why some people gain muscle and strength relatively easily while others have a much harder time doing it.

Many factors affect muscle growth, some genetic (such as bone structure, tendon length, and many other subtle factors), others behavioral (such as sleep, stress, diet, and training).

But it looks like we can put an end to the long-standing misconception that fiber type is destiny and that everyone is either blessed or cursed with an inherited muscle fiber composition at birth.

“Although we’ve got more questions than answers at this point,” Galpin says, “we can safely say that not only do human skeletal muscle fiber types change, but that it happens often, quickly, and in response to almost everything you do.”

Training by fiber type

Cropped shot of women lifting dumbbells

So, if type I fibers are more persistent and type II are stronger and faster, then common sense would suggest:

To hit your slow-twitch, type fiber: Train with higher reps and lower weights (say, three to four sets of 12 to 15 reps using weights that fail at the upper end of the range).

To hit your fast-twitch, type II fibers: Train with fewer reps and heavier weights (say, two to four sets of three to 12 reps).

You might even guess that the “hypertrophy range”—eight to 12 repetitions—will effectively hit both fiber types.

And as far as research currently indicates, common sense is more or less on target.

Although slow-twitch fibers are still active in heavy lifts and some fast-twitch fibers help at the end of light sets, it is still useful to perform sets that focus on the characteristics of each fiber type (eg, heavy and fast or light and slow), esp. If you are looking to increase muscle mass.

“It’s hard to recruit Type II with light loads unless you’re lifting explosively,” says exercise physiologist Dr. Chad Waterbury. “Doing a set of 25 or 30 repetitions to failure does not recruit the largest type II fibers.”

Conversely, he says, “you need high-rep sets to maximally build type I fibers.”

Ideally, you’re performing those lighter sets to — or near — muscle failure.

So if you’re looking for maximum muscle growth in your type IIa and IIx fibers, lift heavy — using a weight you can probably lift 12 times — most of the time, and throw in some explosive lifts (lifting the weight as fast as possible) when you can.

For your type I fibers, do some high-rep sets (15 or more).

“It’s important that you include both lifting schemes in your training plan to optimize overall muscle growth,” says Thieme.

Worth noting: Different lifters respond differently to different rep ranges.

So you may find that you get better results by skewing your training more toward higher or lower reps.

This may or may not be related to your dominant fiber type: you have naturally large muscles but still respond to higher reps, or are naturally lean and respond better to lower reps.

Sure Thing with Megan Davis is a fitness program that focuses on a science-based approach to fitness called “type training.”

Megan alternates weekly between endurance and strength-based strength training and cardio conditioning to target both slow-twitch and fast-twitch muscle fibers.

TYPE TRAINING is an inclusive training protocol for a healthy, strong, balanced body. Learn more about sure things here.

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