Understanding Endurance through the Lens of Muscle Fiber

Understanding Endurance through the Lens of Muscle Fiber

In this post we provide a baseline level of understanding of the physiology that allows–or limits–the ability to go far and fast. This is an essential building block toward understanding how a runner’s endurance and speed relate. And if your aim is to get across the finish line as quickly as possible, you need to develop not just the endurance to cover the distance, but the ability to cover it at your goal pace. We present here a modern understanding of the physiology of endurance and speed–one in which we can train them as two sides of the same coin. 
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Training design for running has historically been driven from the extreme ends of the sport. You have the “just finish” plans geared toward building the minimum fitness needed to complete a given race distance, or you get a watered down version of elite training. But most of us--whether we’re training for a 10K PR or a BQ--are not just slower versions of elites (as much as we might like to think so), nor are we simply couch-to-5K’ers with a few more miles on our shoes. Either way, cookie-cutter training plans might work to develop baseline fitness or provide a new stimulus for a single race or season, but they eventually leave dedicated runners with diminishing returns. Regardless of where we each fall on the pace spectrum, those seeking long-term improvement will be limited by factors that change and evolve over miles and years in ways that static, pre-baked plans cannot address. 

Members of the Prospect Park Track Club Wearing their Race Kit

We are here because Bakline believes that speed happens at many paces. In this Find your Fast series we’ll be exploring a range of topics and research on the science of running and endurance and how it applies to training for competitive runners across the pace spectrum. If you’re dedicated to improvement and pursuing your own definition of speed, we want to help you get there. We hope you enjoy this first post in the Find Your Fast Series: Understanding Endurance through the Lens of Muscle Fiber. 

The purpose of this post is to provide a baseline level of understanding of the physiology that allows–or limits–the ability to go far and fast. This is an essential building block toward understanding how a runner’s endurance and speed relate. And if your aim is to get across the finish line as quickly as possible, you need to develop not just the endurance to cover the distance, but the ability to cover it at your goal pace. The traditional model of training for distance posits that endurance and speed are competing qualities, with the assumption that endurance is built on long slow distance that will be diminished by too much “fast” running. There is a growing body of science that challenges that view. We will present here a modern understanding of the physiology of endurance and speed–one in which we can train them as two sides of the same coin. 

The Basic Process that Powers Movement

The runner’s engine is built of components that interact at the macro and micro scales to turn fuel into energy and energy into forward motion. At the organ/systems level, it begins with the lungs’ ability to bring in oxygen and the heart’s capacity to circulate that O2 and various fuels to the muscles. The muscles themselves are composed of fibers, each of which is a single, complete cell. Every skeletal muscle fiber:

  • Has the capacity to process fuel (e.g., fats and carbohydrates) into Adenosine TriPhosphate (ATP: the basic unit of cellular energy), and 
  • Contains the apparatus that uses ATP to generate mechanical contractions that create physical movement. 

Based on these characteristics, fibers are broadly characterized into two types—fast twitch and slow twitch, which are differentiated by their method of energy production and the strength of contraction. 

The fast-twitch (AKA Type II) fibers are the fight-or-flight first responders. They are found in high numbers in muscles like the hamstrings and gastrocnemius (commonly referred to as the “gastroc”) of the calf, and are capable of producing a lot of force, but also tire quickly. (Hill sprints, anyone?) As we will see when we discuss energy production, the major limiting factor of the fast twitch fibers’ high power output is their poor fuel economy. These fibers are race horses–they can go hard and fast but as soon as the race is over, they need to go back to the stable for a good rest.

At the other end of the spectrum are the slow-twitch (AKA Type I) fibers, found in the highest numbers in muscles that work continuously against gravity–things like the postural muscles of the core and the soleus (the deep muscle of the calf). These muscles have to be low-key “on” all the time, so they need to be highly fatigue-resistant. Slow twitch fibers aren’t generally called upon to initiate quick movements requiring a lot of power, however, so they can afford to be very efficient with their fuel and energy use. These are the workhorses of the body, plodding along tirelessly. All. Day. Long. 

And what about all the other horses who are neither Seabiscuit nor a Clydesdale? Some are still naturally faster, even if they can’t win the Preakness. Some are a bit slower and more hard-wearing. Some will inexplicably excel at dressage. But how well they do any of these things will largely be a result of focused training. Similarly, in most runners, a large portion of muscle fiber exists on the spectrum between the fastest and the slowest, with properties that depend on the location of the fiber, the runner’s current conditioning and training load, and even what their athletic history looks like. 

In other words, while Type I and II fibers are discussed in most run coaching courses and training books, the subject is often taught in a way that suggests that they are fairly binary in nature and a runner’s tendency toward one or the other is largely a result of genetics. What’s almost never discussed is how much of our muscle fiber and its functionality is adaptive. Our training is fundamentally training these fibers and could push them one way or the other in a manner that benefits or undermines our training goals. Understanding this, we can design workouts and training blocks that actually change the way our hybrid muscle fibers are recruited, utilize fuel, and ultimately perform on race day.[1] 

Now that we’ve established the types of muscle fiber and how they are classified, we’ll delve further into how each functions at a cellular level and–more importantly–how they work together.


Fast Twitch

Fast twitch (FT) fibers are fairly simple in cellular structure and built for turning fuel into movement as quickly as possible. They produce their energy from sugar (glycogen), which can be stored in limited supply within the fiber itself, or delivered via the bloodstream. Glycogen is a “fast” fuel source because it can be very quickly split into two smaller molecules, releasing a small amount of energy in the form of 2 APT molecules, along with some free hydrogen ions. (The two smaller molecules and hydrogen cannot be further processed within the fiber, and we will come back to their eventual fate in the next section.) The reaction itself is a form of fermentation that requires no oxygen, meaning that provided their glycogen stores are sufficient, the FT muscle fibers can produce fuel independently and on demand. This is an ideal scenario if you’re doing an anaerobic activity for a relatively short period of time. 

Fast Twitch

There are two major limitations to the FT fibers’ energy production process that quickly become relevant to one’s endurance ambitions, however. First, the body’s glycogen stores are finite, and once they run low, FT fibers cannot function efficiently. The second limitation has to do with those hydrogen ions leftover at the end of the fermentation process, which create an acidic environment within the cell. The fiber can keep functioning as acid levels rise—but only to a point. It’s like working out in a sauna… you could do it for a while, but your power output would fall and eventually the unfavorable environment would force you to stop. The same thing happens in the muscle, but luckily, the body has a way to turn down the heat. Remember those two smaller molecules left over from the glycogen fermentation? They come to the rescue by binding with excess hydrogen and forming a new molecule—lactic acid—that can then be shuttled out of the cell and into the bloodstream. The formation and removal of lactic acid helps maintain a favorable chemical balance within working muscle fiber,[2] and as will see later, can be further utilized as a fuel source for slow twitch muscles.

Slow Twitch

And with that, we turn our attention from the live-fast/bonk-quickly fast-twitch muscle and look to the plodding slow twitch (ST) muscle fibers. They produce less power than the FT, but their contractions can be repeated over and over before they tire. Most importantly, ST fibers contain mitochondria (say it with me–the powerhouses of the cell!) which are able to turn a range of fuels (fat, glycogen, and lactate) into ATP in the presence of oxygen. Like the fast-twitch, ST fibers can ferment glycogen for quick energy, but processing fuels in the mitochondria is far more efficient from a fuel-in to energy-out perspective. Following that initial step of fermentation, ST fibers can further metabolize a glycogen molecule in the mitochondria to yield 30 additional ATP. A single fat molecule nets over 100 ATP. Given that fat stores even in the leanest runners far exceed the energy requirements for an ultra, this is essentially a bottomless well of energy for low-intensity running. 

Mile 45 of a 50 Mile Ultramarathon

The limitation here is that the processes involved require a lot of oxygen and are time-intensive. Breaking down a fat molecule from start to finish can take 100 times longer than producing the 2 ATP from glucose–a statistic which actually makes the whole strategy look far less appealing. A process that takes 100 times longer for 50 times the payout is not great in a sport predicated on speed. In fact, all of these energy production processes happen at the same time, but their contributions are balanced based on the energy demand of the activity. 

Muscle Type

Fuel Molecule


Oxygen Environment

Time of Process

Fast Twitch





Slow Twitch





Slow Twitch


30 ATP



Slow Twitch


100 APT


Very Slow


Once the demand for energy output has outpaced the capacity of the mitochondria to supply it, the muscles will increasingly rely on glycogen to supplement the rate of ATP production. It’s important to note here that with the general exception of hypoxia due to altitude, what drives the shift in fuel utilization from predominantly fat to increased glycogen usage is the rate of energy demand,[3] not oxygen availability. This is a critical point because while we can’t speed up the mitochondria to increase the rate of fuel output, we can increase the total number of mitochondria in the ST fibers with training. More mitochondria do demand more oxygen, though, so the number of capillaries serving those fibers must also then increase. Both of these adaptations take time and deliberate training, but together they result in a much greater energy efficiency from the existing muscle mass.[4] This adaptation directly translates into the ability to power higher intensity running while remaining highly fuel efficient, and that means our runner can go faster for longer. 

Hybrid Fiber

There is another resource we can also train to increase a runner’s total aerobic capacity. Recall that so far we’ve only talked about the muscle fibers at the far ends of the twitch spectrum–the fastest and slowest. What about all the fiber in the middle? In untrained individuals, up to 85% of muscle fibers would be classified as hybrid (aka Type IIa), meaning they possess both fast and slow contractile mechanisms and also have the capacity to grow mitochondria. They’re the independent voters of the muscle political world. This makes them sound very versatile and efficient, but without the structure and discipline of training, hybrid fibers revert to mostly fermenting sugar (glycogen) and avoiding making long term career decisions.[1] Untrained hybrid fibers operate essentially as fast twitch, which is part of the reason that we often hear newer runners indicating that they feel comparatively better at shorter distances than longer ones. But with consistent stimuli hybrid fiber mitochondrial density can be increased and their internal machinery adapted to maximize their aerobic capacity until they effectively become slow twitch. This is a massive, massive gain for endurance when 85% of total muscle mass is in play

Training for Energy Efficiency

We know that the real superpower of ST fibers—and the reason we’d like to have as many as possible—is to increase the mitochondrial workforce and utilize their flexibility in processing fuels. This is where the real magic happens. Remember those two leftover molecules of lactic acid that had to be carted out of the FT fiber to allow it to continue functioning (the turning down the heat in the sauna metaphor)? These can be further processed by the mitochondria in a slow twitch muscle fiber.[5] Here we can wring out another 30 ATP from each lactate molecule as it’s reduced down to H2O and CO2. This is truly a situation of one fiber’s trash being another’s treasure. The substance that quickly becomes a rate-limiter in fast twitch muscle becomes a valuable fuel resource when there is sufficient mitochondrial density to receive and consume it. Hopefully we can all stop hating on lactic acid now that we understand it’s actually a hidden fuel source. 

Developing and increasing this capacity is where training the hybrid fiber really pays off. The point at which total lactate production outpaces the body’s ability to manage it is known as the lactate threshold. Working above this intensity, circulating lactate will continue to rise even while the pace remains constant. Once LT is crossed, the runner has a limited time frame of endurance (commonly estimated to be an hour) at that pace until forced to slow down or stop. The pace and length of time that can be sustained before we cross that line is determined by:

  • How fast you can go while still burning fat
  • How efficiently you can shuttle the lactate produced into other working muscles once your reliance on glycogen stores increases
  • How many mitochondria you have to process that lactate
  • How much circulating lactate you can tolerate in the bloodstream before performance declines

Different race distances and course demands will determine which of these attributes dominates, but the more a runner improves all of them, the faster they will be (i.e., don’t pay so much attention to your VO2 MAX because there’s so many other factors that can be trained). This leads to a few inescapable conclusions regarding endurance training:

  • Maximizing ST fiber and aerobic capacity is essential for efficient energy production regardless of the fuel source
  • Training slow twitch fibers to utilize fat and lactate allows the fast twitch muscle to work longer and harder
  • The ability to sustain a faster pace is the product of balancing reliance on the FT fibers for speed with the necessary ST and hybrid fiber capacity to support and manage lactate output

Fuel Source




  • Body has more than enough reserve to fuel any workout
  • Highest energy yield of any source (~100 ATP)
  • Very slow process
  • Requires a lot of oxygen
  • Only possible in fibers with mitochondria
  • Cannot be utilized by FT fiber

Aerobic Fermentation of Glycogen (Carbohydrate)

  • Stored within ST and Hybrid fiber
  • Medium energy efficiency (32 ATP)
  • Glycogen stores are limited 
  • Slow process
  • Requires oxygen
  • Only possible in fibers with mitochondria
  • Cannot be utilized by FT fiber

Anaerobic Fermentation of Glycogen (Carbohydrate)

  • Stored within muscle fiber
  • Very fast production of energy
  • Does not require oxygen delivery
  • Fuels powerful FT fibers
  • Glycogen stores are limited 
  • Yields only 2 ATP from each glycogen molecule
  • Byproducts which must be removed to allow fiber to continue functioning


  • Uses byproduct of FT energy production
  • Fastest fuel to process in the mitochondria
  • Yields 30 ATP
  • Only fuel source that can be moved between working muscles
  • Can only be processed in fibers with mitochondria
  • Tolerance for circulating lactate levels varies and must be trained
  • Requires oxygen

In the next installment, we’ll look at how we train these adaptations, how we measure them, and what those numbers do (and don’t) tell us. 

Let us know what you think, and DM us on instagram @baklinerunning or email coach@bakline.com with your training questions!

  1. Plotkin, Daniel L., Michael D. Roberts, Cody T. Haun, and Brad J. Schoenfeld. 2021. "Muscle Fiber Type Transitions with Exercise Training: Shifting Perspectives" Sports 9, no. 9: 127. https://doi.org/10.3390/sports9090127
  2. Cairns, S.P. Lactic Acid and Exercise Performance. Sports Med 36, 279–291 (2006). https://doi.org/10.2165/00007256-200636040-00001
  3. Holloszy JO, Kohrt WM, Hansen PA. The regulation of carbohydrate and fat metabolism during and after exercise. Front Biosci. 1998 Sep 15;3:D1011-27. doi: 10.2741/a342. PMID: 9740552.
  4. Shaw CS, Swinton C, Morales-Scholz MG, McRae N, Erftemeyer T, Aldous A, Murphy RM, Howlett KF. Impact of exercise training status on the fiber type-specific abundance of proteins regulating intramuscular lipid metabolism. J Appl Physiol (1985). 2020 Feb 1;128(2):379-389. doi: 10.1152/japplphysiol.00797.2019. Epub 2020 Jan 9. PMID: 31917629.
  5. Brooks GA. The Science and Translation of Lactate Shuttle Theory. Cell Metab. 2018 Apr 3;27(4):757-785. doi: 10.1016/j.cmet.2018.03.008. PMID: 29617642.