Fast And Slow Twitch Muscle Fibers

What are those “fast twitch” and “slow twitch” muscle fibers you hear about? Here’s a simple explanation!

Fast Twitch, Slow Twitch

When you are into bodybuilding, running or fitness in general, you have heard the terms: “fast twitch” and “slow twitch” muscle fibers. But what are they actually?

Here’s what they do, if slow or fast twitch fibers make you bigger and what type bodybuilders, marathon runners and sprinters benefit from. We’ll also check if you can influence how much of each type you have and if it’s really necessary:

Fast Vs. Slow-Twitch: The Basic Distinction

It’s a bit complicated, so I also wrote it down and added some more detail.

As I said in the video, skeletal muscle is made up of individual fibers, and they fall into two broad categories based on how they contract and where they get their fuel from:

Type I: Slow-twitch (“oxidative”)

• Contract slowly, produce lower force
• Take a long time to fatigue
• Rich in mitochondria (energy generators) and myoglobin (a protein that helps them get oxygen)
• Many capillaries (blood vessels)
• Primarily used for aerobic activities (fatty acids + glucose with oxygen (the “oxidative”)
• Think: holding a specific posture, your endurance, long steady efforts

Type II: Fast-twitch (“glycolytic” / “oxidative-glycolytic”)

• Contract quickly, produce high force
• Fatigue much faster
• Less myoglobin, fewer mitochondria and capillaries
• Rely more on anaerobic glycolysis (turning blood sugar into energy without oxygen)
• They are subdivided into:
  • Type IIa — fast but can generate some energy with the help of oxygen (a hybrid)
  • Type IIx (also called “IIb”) – the fastest, most powerful, but fatigues very quickly and is least efficient aerobically

A (Hopefully Useful) Car Analogy

Here is another of my car analogies. Think of these muscle fibers like engine types:

• Type I is a diesel engine: low RPM, high torque over long durations, very fuel-efficient, but won’t get you to 100 mph quickly
• Type IIx is a V8: massive power in a burst, drinks fuel, overheats fast
• Type IIa sits in the middle: it’s quick, but can sustain moderate output longer

Fast-Twitch Vs. Slow Twitch In Practice

• Sprinting, jumping, heavy lifting rely on Type II (especially IIx) heavily
• Distance running, cycling, posture rely on Type I
• Training shifts the balance between IIa and IIx somewhat (IIx can shift toward IIa with endurance training; IIa can shift to more power generation via blood sugar with strength training), but the proportion of I vs. II you’re born with is largely fixed by genetics
• The fiber type distribution not only varies between people, but also between muscles: your vastus lateralis (one large muscle that is part of your thigh) has more II than your soleus (part of your calf), for example.

The Evolutionary Angle (If You Really Want To Know)

The I/II ratio is under strong genetic control and twin studies show high heritability. Which makes sense from a selection standpoint, let’s call it the “economy of movement.” A population’s typical fiber type distribution tracks loosely with the kind of movement demands that shaped it (persistent and patient hunting vs. ambushes and sprints). 

Last But Not Least: You Can’t Really Convert!

You can’t really convert I to II or II to I in any meaningful sense. What changes with training is mostly within the II group (IIa and IIx), the hypertrophy of existing fibers, and the recruitment efficiency (neural adaptation – the “mind-muscle connection”). The “I became II” stuff you see in popular articles is overblown, sensationalist hyperbole to generate clicks.

An elite long-distance runner will never have the bulges of a bodybuilder and a bodybuilder will never have the amount of slow-twitch muscles fibers to be a champion long-distance runner.