Stronger, Faster, Better: What Training does to your Muscle Fibers

Today we’re doing an experiment. Obviously we’re in a lab, as evidenced by our lab coats and presence of microscopes and Bunsen burners. We both like to lift and get jacked and tan, but we wanna know: what’s actually going on inside our muscles when we lift weights or go for a jog?

We know muscles are comprised of fibers, and we want to take a closer look under a microscope. In order to do that we need a muscle biopsy.  And in order to get a muscle biopsy, we need to take a very large needle – about the size of a #2 pencil – and poke someone in the leg with it.

Our participant taking an enormous needle to the thigh happens to be my younger sister and favorite target of torment for her entire life, Holly. Why? Because she deserves it. Because she’s a former high school and collegiate track athlete, which means she’s used to running very short distances very quickly.

So we strap down our participant and probe our subject in the quadriceps to get our biopsy sample. Sample harvested, we give Holly a lollipop, dye our tissue, and slide it under a microscope for examination.

There are three predominant muscle fiber types in every muscle in every person: Type I, Type IIa, and Type IIx. They all stain a bit different on the slide, so we can tell which is which and how much Holly has of each one in her quadriceps.

via Wikimedia Commons

The reason that we have all three, and the reason we can largely tell them apart, is because they’re all slightly different in terms of size and mitochondrial density. From a practical standpoint, each muscle fiber type uses different energy sources, produce different levels of force, and are more or less fatigue-resistant. But let’s take a closer look:

Slow Twitch/Type I

  • Under the microscope, slow twitch fibers tend to be smaller and darker in color. 
  • They contain myoglobin, an iron-containing protein responsible for delivering oxygen to the mitochondria present in the muscle cell. 
  • Have the greatest density of mitochondria, which produce an abundance of ATP or “energy currency” that the fibers use to contract
  • Are responsible for most low-intensity movements (think walking, jogging, moving light objects around the office, petting your dog, etc.)
  • Produce the lowest amount of force (not useful for lifting weights or sprinting)
  • Fatigue resistant (very useful for long-duration activities)

It’s these qualities that make slow-twitch fibers ideal for long-duration, low-intensity activities like walking, jogging, or cycling long distances. As your lungs bring in oxygen, your heart pumps it to these working muscles, where mitochondria turn out lots of ATP to keep you going for a long time.

Fast Twitch/Type IIa

Adjacent to those dark slow twitch fibers are slightly larger, and less dark, Type IIa muscle fibers. These muscle fibers

  • Produce a lot more force
  • Contract much faster
  • Are only recruited when slow twich fibers are unable to produce adequate force (think heavy lifting, or turning up the pace from a light jog or cycle to a sprint)
  • Have far fewer mitochondria (so they appear white-ish under our microscope)
  • Are more susceptible to fatigue (able to maximally contract for only a few seconds)

It’s the presence of these fibers that allow us to sprint, jump, or lift heavy weights around the gym, but find ourselves fatiguing much faster or unable to perform lots of reps with good form – once these fibers start to fatigue, they’re unable to produce force, and we stop lifting or sprinting or jumping.

Super Fast Twitch/Type IIx

Lastly, we have our biggest, strongest, but most easily fatigued muscle fibers, Type IIx. The largest difference between IIx and IIa has much to do with the energy and force demands. But they’re very similar otherwise. These fibers allow us to move very fast or lift super heavy weight very quickly. Think short distance sprinting, Olympic lifting (clean, jerk, snatch), or super heavy bench press or squats.

Large, forceful, but unable to keep us going for very long.

The activities you do, whether that’s running, cycling, or heavy weight training, the exercise will hypertrophy the muscle fibers most necessary for that activity. In other words, the muscles most needed for the activity will become larger, contract faster, and become more efficient with time, making you a better athlete in the process. This is largely because these activities stress the muscle fibers, damages them, and our bodies use protein and adequate rest and recovery to build and repair those fibers for future use.

So if you like to run, your body will make your slow twitch muscle fibers larger, denser in terms of mitochondrial density, and more vascular – meaning that you’ll be a much better long-distance athlete over time.

Conversely, if you like to lift weights and get jacked in the gym like me, your body will make the Type II fibers much larger and more powerful, making you bigger, stronger, and faster in the gym or faster on the short-distance track. And of course if you like to do a little bit of both, you’ll likely reap the benefits of both.

progress pic from fat loss program “Jacked and Tan” summer 2019

Two interesting facts:

  1. Type IIa and IIx fibers can convert back and forth, depending on your fast-twitch activities. So if you like to strength train, olympic lift, sprint, or if you’re a Crossfit athlete, your fast twitch fibers will adapt in whatever direction based on need and stimulus. It is currently unclear, however, if the human body can convert slow twitch to fast twitch, or vice versa.
  2. Most of us mortals have roughly a 50/50 split with Type I and Type II fibers. That means that we can become much more efficient and much stronger with whatever activity we choose. So whatever it is you like to do, do that! Elite athletes, however, might have a much different profile, depending on their sport. Is that what makes them elite? Maybe. Or maybe they train hard regardless of their muscle fiber distribution.

Slow Twitch Activities
Office work
House chores
Light jogging, cycling
Marathon running (constant pace)
Distance swimming
High Intensity Interval Training (lows)
Fast Twitch Activities
Jumping (hurdles, boxes)
Medball slams/throws
Olympic lifting
Weight training
Fighting (punching, kicking)
High Intensity Interval Training (highs)
Kettlebell swings use fast-twitch muscle fibers because the ‘bell is heavy and the movement is explosive. Fast-twitch muscle fibers produce the most force in the shortest amount of time, but fatigue relatively quickly.

Bottom line: because your body doesn’t like to be bad at exercise, it will adapt to whatever stimulus you throw at it, and in time will adapt to each new stressor to become faster, stronger, and more capable of enduring and thriving during exercise.

A question on ethics.

I’d like to apologize to Holly for stabbing her quad with a giant needle in this hypothetical lab experiment. But further, in a world of genetic testing for literally anything you can think of, I can’t help but worry about the ethical ramifications of muscle fiber testing. Right now the only way you’d get a muscle biopsy in the U.S. is for medical reasons and muscle-related disease testing.

But that doesn’t mean that companies aren’t looking for ways to cotton-swab your genetic profile otherwise. I can only imagine an obsessed helicopter parent looking to give their young athlete a competitive edge and pushing their kids towards a specific sport (or, being rather disappointed to find their offspring to be like most of us, 50/50).

While I would find this information incredibly fascinating, hopefully it won’t be abused and would rather give us a better look at the human body, and all it’s capabilities, in a more productive way, like getting more of the general population moving and exercising regularly. 

In the meantime, continue with the exercises and activities you enjoy most, and on occasion think about how each muscle fiber in your body contributes to your everyday living and performance. It’s pretty neat.

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