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Slow Twitch? Fast Twitch? Understanding Muscle Fiber Types & Recruitment for Endurance Running

Muscles are just muscles, right? They contract and relax as one, and that's how we can do things like run! ...right? Au contraire, mon ami! Your skeletal muscles (the muscles that are attached to your bones and produce force) are actually a complex network of tiny units working together to help you move, lift, and do pretty much anything physical. These units are the muscle fibers, and they're like the building blocks of your muscles.


Picture your quads: just a big ol' muscle, right? Actually, each muscle is made up of muscle fascicles (a bundle of skeletal muscle fibers surrounded by connective tissue). Each fascicle is controlled by one nerve. This is also known as a motor unit. What type of muscle fibers each fascicle contains and how those are distributed within the muscle itself depends on the function of the muscle, your genetic makeup, and your training status.

Individual muscle fibers are like long cylindrical tubes. Each tube is a single muscle cell! These cells are pretty unique because they're unusually long compared to most other cells in your body. They can stretch from a few millimeters to several centimeters long.

Now, these muscle fibers are made up of even smaller units called myofibrils. Think of myofibrils as the internal machinery of the muscle fiber. They're like the gears and engines responsible for muscle contractions.

Zoom in even further, and you'll find that myofibrils are composed of smaller structures called sarcomeres. Sarcomeres are the functional units of muscle contraction. They're like the Lego blocks that make up the myofibrils. They contain proteins—actin and myosin—that slide past each other during contractions, creating the muscle's movement.

Around these myofibrils, there's a network of sarcoplasmic reticulum (a fancy term for the muscle's internal transport system) and mitochondria (the powerhouse of the cell) that help keep everything running smoothly.


Alright, let's talk about what these muscle fibers actually do. Their primary job is to contract; that's how they generate force and make your body move. When your brain sends a signal to your muscles telling them to contract, it's these muscle fibers that spring into action via the nerve that controls each motor unit. [1]

Now, how does this contraction happen? When a muscle contracts, these sarcomeres shorten. It's like the actin and myosin proteins in the sarcomeres are doing a tug-of-war, pulling closer together and causing the entire muscle fiber to contract.

When you're running, lifting weights, or even just picking up a pencil, it's these muscle fibers contracting that allow you to perform those actions. They're pretty versatile too! They can contract with varying degrees of force (depending on what type of and how many muscle fibers are recruited) and speed depending on what you're doing. For instance, when you're sprinting, your muscle fibers are contracting rapidly and forcefully to generate that speed. In contrast, during a leisurely stroll, they're contracting more gently to maintain that relaxed pace.

The key thing to remember about muscle fibers is that they contract in an all-or-none capacity. When they receive a signal from your nervous system, a muscle fiber will either contract fully or not at all. It's like flipping a light switch—once the signal comes, they go all-in on the contraction. How much force is produced depends, then, on both the muscle fiber type AND the number of muscle fibers contracting at the same time.


Let's start with the big picture: there are primarily two main types of muscle fibers: slow-twitch (Type I) and fast-twitch (Type II). Slow-twitch fibers are the endurance gurus. These fibers excel at activities that require endurance and stamina like distance running. They're like the Energizer Bunny: contracting repeatedly without tiring quickly, and highly efficient at using oxygen to generate energy. Endurance running heavily relies on aerobic metabolism - the process that uses oxygen to produce energy - and slow-twitch fibers are champions at this; they're well-equipped for aerobic activities and can keep contracting for a long time using this aerobic ("oxidative") pathway. Slow twitch fibers are called "slow" because they contract slowly relative to your fast-twitch fibers (but slow in this context is still around 100 milliseconds, so quite fast compared to how you and I experience things that are slow vs fast!) [2]


Slow-twitch muscle fibers (Type I fibers), possess several distinctive properties that make them particularly well-suited for endurance activities and sustained contractions:

  1. High Oxidative Capacity: These fibers are rich in mitochondria, the powerhouse of the cell responsible for producing energy. Their abundance of mitochondria allows them to efficiently use oxygen to generate ATP (adenosine triphosphate), providing a continuous and steady supply of energy for prolonged periods.

  2. Slow Contraction Speed: Slow-twitch fibers contract at a slower rate compared to fast-twitch fibers. This slower contraction rate allows them to sustain contractions for longer durations without fatiguing quickly, making them ideal for activities requiring endurance, such as distance running or cycling.

  3. High Resistance to Fatigue: These fibers are highly resistant to fatigue due to their reliance on aerobic metabolism and their ability to maintain contractions for extended periods without tiring easily. They're designed to keep working efficiently over extended periods.

  4. Rich in Myoglobin: Myoglobin is a protein that stores and transports oxygen within muscle cells. Slow-twitch fibers have a high concentration of myoglobin, enabling them to efficiently extract oxygen from the bloodstream, enhancing their endurance capacity.

  5. Red Appearance: Slow-twitch fibers have a reddish appearance due to their rich blood supply and high myoglobin content. This red coloration reflects their reliance on oxygen-rich blood for sustained energy production.

  6. Low Force Production: While they have excellent endurance capabilities, slow-twitch fibers produce relatively low force compared to fast-twitch fibers. They're more suited for activities requiring less force but prolonged effort.

  7. Efficient Fatty Acid Utilization: These fibers are efficient at utilizing fatty acids as a fuel source during aerobic metabolism. This ability to use fats as an energy source is beneficial for endurance activities where glycogen stores may deplete over time.

Then, we've got the fast-twitch muscle fibers: Type II fibers. These fibers are the sprinters—they're all about quick bursts of power, but they fatigue much faster. They're like the cheetahs, exploding into action for short, intense moments but needing more rest afterward.

FAST TWITCH MUSCLE FIBERS Fast-twitch muscle fibers (Type II fibers), possess distinct properties that make them well-suited for quick, powerful movements and activities requiring bursts of strength:

  1. Fast Contraction Speed: These fibers contract rapidly, generating force quickly. Their ability to contract at a high speed allows for swift and explosive movements, making them crucial for activities like sprinting or jumping.

  2. High Force Production: Fast-twitch fibers are capable of generating significant force compared to slow-twitch fibers. This force production is vital for movements that require power and strength, enabling rapid and forceful contractions.

  3. Low Oxidative Capacity: Unlike slow-twitch fibers, fast-twitch fibers have fewer mitochondria and rely more on anaerobic metabolism. They produce energy without using oxygen, making them effective for short bursts of activity but prone to fatigue more rapidly.

  4. Low Resistance to Fatigue: Due to their reliance on anaerobic metabolism and rapid energy production, fast-twitch fibers fatigue relatively quickly compared to slow-twitch fibers. They're designed for intense, short-duration activities rather than sustained efforts.

  5. Low Myoglobin Content: These fibers have a lower concentration of myoglobin compared to slow-twitch fibers, resulting in a paler appearance. This reduced myoglobin content means less oxygen storage and reliance on oxygen for energy production.

  6. Less Capillarization: Fast-twitch fibers have fewer blood vessels (capillaries) surrounding them compared to slow-twitch fibers. This reduced blood supply limits their access to oxygen and nutrients, contributing to their reliance on anaerobic metabolism.

  7. High Glycolytic Capacity: They have a high capacity for glycolysis, a process that breaks down glucose for energy without oxygen. This allows them to generate energy rapidly but results in the accumulation of lactic acid, contributing to fatigue.

  8. Adaptability: Through specific training, fast-twitch fibers can undergo hypertrophy, increasing in size and strength. This adaptation can occur in response to resistance training or explosive, high-intensity activities.

When you start a run, your body recruits slow-twitch fibers first. However, as the intensity of your run increases and/or fatigue sets in, your body might need more muscle power, either to "replace" the output of fatiguing slow-twitch muscle fibers or to increase your overall output by having the fast-twitch fibers work alongside those slow-twitch muscle fibers.

Now, within these broad categories of Slow Twitch and Fast Twitch, there are subtypes that add more nuance to how our muscles work during endurance running: Type IIA and Type IIX (sometimes referred to as Type IIB). Type IIA fibers, i.e. "fast-twitch fatigue resistance", are a bit of a hybrid—they're more fatigue-resistant than typical fast-twitch fibers but can generate more force than typical slow-twitch fibers. Type IIX fibers, on the other hand, are even faster when it comes to generating force, but they fatigue quicker than both Type I and Type IIA fibers.

For endurance athletes, these subtypes play interesting roles. The Type IIA fibers, being somewhat of a middle ground between slow and fast-twitch, can contribute to endurance activities while offering a bit more power. They're versatile all-rounders, helping you maintain pace while having a bit of extra muscle power in reserve when needed.

Type IIX fibers, though they're faster at generating force, won't be something you rely on heavily as a distance runner, although they are still important! They fatigue faster, so they might not sustain the demands of long-distance efforts as effectively as the other subtypes. Instead, they often come into play during intense bursts or short, high-intensity segments within a run, like surges or a sprint finish. [3]

Slow Twitch Type I

Fast Twitch Type IIa

Fast Twitch Type IIx

Motor Neuron Size




Nerve Conduction Velocity




Contraction Speed




Fatigue Resistance




Force Production




Capillary Density




Mitochondrial Density




Myoglobin Content




Fiber Diameter




Anaerobic Enzyme Content




Primary Energy System




But wait, I thought there were more subtypes than just those 3?

Correct! But that level of complexity isn't necessary for understanding our topic today. "Because these delineations are based on qualitative analysis of stained fibers, it remains possible that more fiber types will be identified in the future. In summary, the 7 human muscle fiber types, as identified by myosin ATPase histochemical staining are (from slowest to fastest): types I, IC, IIC, IIAC, IIA, IIAB, and IIB. These divisions are based on the intensity of staining at different pH levels, and, as such, any given fiber could be grouped differently by different researchers. Furthermore, not all studies use all 7 fiber types. Some researchers place all muscle fibers into just the original 3 fiber types." [4]


Now, let's explore a concept called "Henneman's size principle" because this is an important thing to understand with regard to muscle fiber recruitment, i.e. WHEN each muscle fiber type is recruited and in what order. This principle dictates that motor units (a motor neuron and the muscle fibers it connects to) are recruited in a specific order, from smallest to largest, depending on the demand of the activity.

The recruitment of these different muscle fibers is a smart strategy employed by your body to manage energy consumption efficiently. By starting with the most efficient (i.e. slow twitch) fibers and then gradually bringing in the more powerful ones when necessary, your body optimizes its resources. If the very first thing your muscles did at the start of a long easy effort run was to recruit and burn through all your fast twitch muscle fibers, that would be highly inefficient!


Slow Twitch Type 1 -> Fast Twitch Type IIa -> Fast Twitch Type IIx

When you're cruising through easy runs and longer runs, your body primarily relies on those trusty slow-twitch fibers: your aerobic monsters. Think of them as the engine driving you forward mile after mile (and this is where the basis of your "endurance' comes from, how well-developed your slow twitch muscle fibers and aerobic system in general are). Easy runs and long runs can increase the number and efficiency of slow-twitch fibers, improving their endurance capabilities and fatigue resistance. This means your body becomes more efficient at using oxygen, allowing you to sustain a higher output (read: pace) for longer periods without relying heavily on the faster fatiguing fibers.

As you fatigue during your run, hit a challenging section like hills, or start to increase the pace, your body might call more upon a combination of muscle fiber types. Those Type IIA fibers, with their mix of endurance and power, might chip in more to support the effort. They provide that extra bit of power when you're pushing through hills or trying to maintain a faster pace.

And of course, some workouts intentionally target Type II fibers, because you want those to also be strong and efficient for when you DO need them in workouts and ultimately in races!

Workouts and activities that target fast-twitch Type IIx fibers include...

  1. Strides

  2. Hill sprints

  3. Plyometrics

  4. Heavy strength training


While you can't drastically change your muscle fiber type composition, training can modify their characteristics. For instance, consistent endurance training might lead to Type IIA fibers adapting to act more like slow-twitch fibers, enhancing their endurance capacity. [5] However, it's essential to remember that each person's muscle composition can vary. Some individuals might naturally have a greater proportion of slow-twitch fibers, giving them a natural advantage in endurance activities. Others might have a more balanced mix or even a higher proportion of fast-twitch fibers, which could make them better suited for shorter, more explosive activities like sprinting.

Additionally, while optimizing the efficiency of your Type I (slow twitch) muscle fibers is key for endurance performance, and is partially why so much of your training is in that "low and slow" conversational easy effort zone, becoming an efficient runner means optimizing ALL muscle fiber types and energy systems, which is why a good training plan will include not just easy effort aerobic runs, but strides and hill sprints along with moderate and high intensity workouts as appropriate for your experience, training load, and goals!


Running Explained is here to help runners learn the what, how, and why of running to become their best running selves! Listen & learn weekly with new episodes of the top-rated The Running Explained Podcast, dive into comprehensive education on individual topics in training and racing with Masterclasses and the Foundations Series, and explore the training plans runners around the world are raving about in the Training Plan library!

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