Of the many possible explanations—diet, sleep, the whims of the bodybuilding gods—one reason you often hear involves the variation in muscle fiber types between individuals.
Often oversimplified as a big part of 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 have to worry about two, imaginatively named type i and type II.
Type I fibers are smaller and more endurance-oriented, while Type II fibers are larger, stronger, and more powerful.
It’s long been clear that different people — and different muscles — have different proportions of these types of fibers.
This has led many fitness-conscious people to conclude that this must be why some lifters develop shoulders like halved cantaloupes, while others still look like Roma tomatoes.
But is it true? What do we know about these types of fibers?
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 types of muscle fibers?
As mentioned above, there are two broad categories of muscle fibers – one with a subcategory – which differ based on their function, structure, and the type of fuel that primarily powers them.
Type I (“slow twitch”) muscle fibers
Also known as “slow oxidation” muscle fibers, these see the most action on a daily basis.
They rely more on oxygen (supplied by the bloodstream – hence that fancy “oxidant” moniker) in combination with fat or glycogen – a form of sugar stored in the liver and muscles – for the production of oxygen. energy than type II fibers, and are good for low to moderate intensity steady state activities such as walking, jogging, light resistance training, etc.
They are also your smallest muscle fibers, and when constantly challenged by endurance-focused strength training, they tend to grow less than other fiber types.
Stress them out with aerobichowever, and they will grow more mitochondria – the little powerhouses in each fiber where energy production occurs.
All of this makes them more geared towards endurance-focused activities.
Type I fibers appear red under the microscope due to the presence of blood-carrying myoglobin, which helps carry oxygen into cells.
Type II (“fast twitch”) muscle fibers
There are two classifications of type II muscle fibers: Type IIa and Type IIx.
Type IIa: Also called “intermediate fibers” or, for the science buffs among us, “rapidly oxidizing glycolytic” fibers, Type IIa fibers kick in when a task gets too intense for Type I fibers.
They also work with a combination of oxygen and glycogen, but although they are faster and stronger than Type I fibers, they are also considerably less energy efficient and faster to fatigue.
Want to beat a heavy set of presses or sprint to catch a departing Uber? Fast-twitch people are the fibers of labor.
Since they requires minimal oxygen to functionType IIa fibers — and their more explosive cousins, IIx — appear gray or white under the microscope.
Type IIx: A subcategory of fast twitch fibers also called “fast glycolytic” fibers, these are the strongest, most potent and least energy-efficient fibers of all.
Mitochondria are rare in IIx because oxygen is not the primary driver of energy production in these fibers, which run almost entirely on glycogen.
When you go for a max-effort squat, long jump, or 100-meter sprint, your IIx fibers are best suited for the task.
Type IIx fibers are a bit of a mystery — in active people, less than 2% of the fibers are pure type IIx.
In addition to these, there are also several “hybrid” fiber types that exist on the spectrum between the other main types. More on those in a minute.
What type of primary muscle fiber are you?
Given these basic descriptions, most fitness-minded people immediately ask themselves two questions:
“Am I more type I or type II?”
“How can I get more Type II and build bigger muscles?”
Answering the first question is more complicated than you think.
The only way to get a fully reliable answer is to take a biopsy of your muscles, which involves a lab technician taking baseline samples from every major muscle in your body with a giant needle so they can analyze the relative proportion of different fiber types under a microscope.
It’s not a fun or profitable proposition.
Some fitness professionals have suggested that you can estimate your fast twitch to slow twitch ratio with performance tests.
For example, lift 80-85% of your one-rep max during an exercise for a given muscle group and see how many reps you can do.
“There are several issues with such tests,” says Trevor Thieme, CSCS, senior director of fitness and nutrition content at Beachbody. “One of the most important isn’t the fact that how quickly you tire is directly influenced by your skill at performing the exercise.”
In other words, your ability to perform a lift is not determined solely by the raw strength of the muscles used; your mastery of the movement itself also counts.
So that leaves you with a very unscientific approach: make an educated guess.
“If you’re struggling to build muscle but excel at endurance sports, you’re probably a type I dominant,” says Thieme.
“If you build muscle fairly easily and prefer pumping iron to pounding the pavement or spinning your wheels on a bike, you’re probably a Type II Dominant,” he adds.
Somewhere in between? That would put you… somewhere in between.
Can you change muscle fiber types?
So now the burning question: with training, can you change your fiber type profile?
Ask most fitness professionals this question and they’ll usually paraphrase the teacher’s adage: “When it comes to fiber typethey will tell you,you get what you get, and you don’t get angry.
But emerging research raises questions about this oft-repeated fitness axiom.
It turns out that muscle fibers change type. All the time. And rather quickly.
To research from Cal-State Fullerton demonstrates that the muscle fiber types of identical twins vary considerably, from twin to twin, depending on the individual’s physical activity choices. Genes only tell part of the story.
Remember those hybrid fibers we talked about?
These little buggers can represent up to 40% of the muscle fibers of sedentary people and are particularly susceptible to change.
With training, they can become “pure” fiber types – type Is or type IIs. And the more you train, the more they change to meet the demands you place on them.
Stop training and they become hybrids again.
Why the longstanding confusion over fiber type conversion?
It comes down to advances in measurement: only recently has it been possible to accurately categorize large numbers of individual muscle fibers and distinguish hybrid fibers from pure types.
Once the guys in white coats figured out how to count and categorize fibers accurately, the mystery was solved, according to Dr Andy Galpin, doctorate. a leading researcher in the field of fiber types.
Unfortunately for those of us looking for a straight answer, this research leaves open the long-standing question of why some people gain muscle and strength relatively easily while others have a much harder time.
Many factors influence muscle growth, some genetic (eg, bone structure, tendon length, and many others esoteric factors), other behavioral (e.g. sleep, stress, diet and training).
But it looks like we can put an end to the long-held misconception that fiber type is fate and that everyone is blessed or cursed with the muscle fiber composition they inherit at birth.
“While we have countless more questions than answers at this point,” says Galpin, “we can safely say that not only do human skeletal muscle fiber types change, but it happens often, quickly, and in response to almost everything you do.”
Training by type of fiber
So while type I fibers tend to be tougher and type II fibers stronger and faster, common sense would suggest:
To hit your Type I slow twitch fibers: Train with higher reps and less weight (eg, three to four sets of 12 to 15 reps using a weight that results in failure at the higher end of the range).
To hit your type II fast twitch fibers: Train with fewer reps and more weight (eg, two to four sets of three to 12 reps).
You might even deduce that the “hypertrophy range” – eight to 12 reps – 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 always active on heavy loads and some fast twitch fibers help towards the end of lighter sets, it is still effective to perform sets that focus on the properties of each fiber type. (for example, heavy and fast or light). and slow), especially if you are looking for muscle growth.
“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 reps to failure doesn’t recruit the largest type II fibers.”
Conversely, he says, “to build the most Type I fibers, you need higher rep sets.”
Ideally, you perform these lighter sets until – or close to – muscle failure.
So if you’re looking for maximum muscle growth in your type IIa and IIx fibers, lift heavy weights – using a weight that you can lift maybe 12 times at most – most of the time, and throw explosive moves ( lift a weight as fast as possible) when you can.
For your Type I fibers, do higher rep sets (15 or more).
“It’s important that you include both lifting regimens in your training plan to optimize overall muscle growth,” says Thieme.
Note: different risers respond differently to different rep ranges.
So, you may find that you get better results by gearing your training more toward higher or lower reps.
It may or may not have to do with your dominant fiber type: you naturally have big muscles but still respond to higher reps, or are naturally lean and respond well to lower reps.
Sure Thing with Megan Davies is a fitness program that focuses on a scientific approach to fitness called “TYPE TRAINING”.
Megan alternates weekly between endurance and power-based strength training, as well as cardio conditioning to target slow-twitch and fast-twitch muscle fibers.
TYPE TRAINING is an inclusive training protocol for a healthy, strong and balanced body. Learn more about Sure Thing here.