Slow twitch and fast twitch muscle fibers

Definitions & Key takeaways

Slow twitch fibers, also known as "red fibers," are characterized by a high endurance capacity and a slow contraction speed. They contain more mitochondria and blood vessels, allowing them to use oxygen efficiently for energy production. Slow twitch fibers are used for activities that require sustained effort, such as long-distance running or cycling. They use oxidative phosphorylation for obtaining ATP.

On the flip side, fast-twitch fibers have a high power output and tire quickly. They contain fewer mitochondria and blood vessels and rely on stored glycogen as a fuel source. They are used for short, intense activities such as weightlifting or sprinting. They use anaerobic glycolysis for obtaining ATP.

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Introduction0:00–0:40

Skeletal muscle fibers are divided into two main types: slow-twitch, which are also called slow oxidative fibers, and fast-twitch muscle fibers.
Fast-twitch muscle fibers are further subdivided into fast oxidative and fast glycolytic fibers. This classification is based on the speed of contraction and the metabolic pathway that’s used to make ATP, a molecule that stores energy for muscle contraction.
Most muscles possess a mix of slow-twitch and fast-twitch fibers, but the predominant one determines the primary function of the muscle.
Alright, now let’s take a look at a muscle cell, or myocyte - and specifically it’s sarcoplasm, which is the cytoplasm of a muscle cell.

Myocyte0:40–1:25

The sarcoplasm is filled with stacks of long filaments called myofibrils. Each myofibril has thick myosin and thin actin filaments that don’t extend through the entire length of the muscle fiber, instead they’re arranged into shorter segments called sarcomeres.
The myosin filaments have these small club-like extensions, which are called myosin heads. The thin actin filament look like a pearl necklace that’s gently twisted.
Each pearl represents one G-actin protein, which has an active site where the myosin head binds to during contraction. Now, before myosin can bind actin, myosin needs to power up.

Power stroke1:25–2:02

Part of the myosin head is an ATPase, meaning that it can cleave an ATP molecule to ADP and phosphate ion and release some energy.
The energy is used to cock the myosin head backwards, into its high energy position. Next, the myosin head binds to the active site, and this triggers the release of the stored energy in the myosin head.
When that happens, the myosin head launches pulling the thin filament along with it. This is called the power stroke.
The combined power strokes of all the myosin heads lead to sliding of the thin filament along the thick filament, and this results in the contraction of the skeletal muscle fiber.

Muscle Contraction2:02–2:33

Now, the speed of contraction depends on how quickly the ATPase enzyme cleaves a molecule of ATP. And there are actually two forms of this enzyme; slow twitch fibers have an ATPase that hydrolyzes ATP slowly, and fast twitch fibers have an ATPase that hydrolyzes ATP quickly.

ATP sources2:33–4:15

Since myosin heads need ATP to reset, a lot of ATP is used by muscle fibers, and the main source for ATP is glucose. Normally, some skeletal muscle cells take up glucose and store it as glycogen - using a process called glycogenesis.
That way, when these muscles need energy, they can break down the glycogen to form glucose again - using a process called glycogenolysis.
To make ATP, glucose is put through glycolysis which is a set of biochemical reactions that take place in the sarcoplasm and results in 2 molecules of pyruvate and 2 ATP.
If oxygen and mitochondria are available, pyruvate is put through a process called aerobic respiration which is where pyruvate is converted into acetyl-COA which enters the mitochondria to produce NADH via the citric acid cycle.
NADH is then used to drive oxidative phosphorylation within the mitochondria, and the result is that each glucose molecule yields a total of 38 ATP.
In the absence of oxygen or mitochondria, there’s only anaerobic respiration, or anaerobic glycolysis, which is where glucose is broken down into pyruvate and that’s about it.
It’s quick, but we only get 2 ATP per glucose and the excess pyruvate gets converted into lactic acid, which goes into the blood, and then to the liver.
The liver can recycle lactic acid into pyruvate and then pyruvate into glucose by using 6 ATP. The new glucose can then be sent to the muscles or other organs.

Slow Oxidative Fibers4:15–6:03

Now we have three different types of muscle fibers, slow oxidative, fast oxidative, and fast glycolytic twitch fibers. Let’s start with slow oxidative fibers, which are also called type I or slow-twitch muscle fibers.
In slow oxidative fibers, “slow” is for having the ATPase that hydrolyze ATP slowly, while “oxidative” stands for the aerobic respiration pathway for metabolizing glucose.
These slow twitch muscle fibers are relatively small, and produce the weakest contractions because they have fewer sarcomeres.
But they’re well-supplied with blood vessels that bring them a plentiful supply of oxygen. And within their sarcoplasm, there’s a high concentration of a red pigmented protein called myoglobin, which binds and stores oxygen.
Myoglobin is similar to hemoglobin, which is the oxygen-carrying protein in the blood, except that myoglobin binds oxygen even more strongly than hemoglobin.
High levels of myoglobin and an extensive network of blood vessels make these fibers appear red, thus, they’re also called “slow red muscle fibers”.
In addition, they have a large number of mitochondria in their sarcoplasm which support the slow but efficient aerobic metabolism that generates 38 ATP per glucose molecule.
Now, these fibers contract slowly so they use up less ATP slowly combined with the fact that we are creating a lot of ATP, there’s usually no need for more glucose so it has a low glycogen storage.
So, even though they’re the weakest fiber, their ability to produce large amounts of ATP through aerobic metabolism means that they’re able to sustain muscle activity for long periods of time, like when running a marathon for example.
Next there are fast oxidative muscle fibers, which are also called type IIa muscle fibers, “fast” is for having the ATPase that hydrolyze ATP quickly, while “oxidative” stands for the aerobic pathway which is mainly used for generation of ATP.

Fast Oxidative Fibers6:03–7:21

These fibers are larger than the slow oxidative fibers, and they have a greater number of sarcomeres, which means that they’re also stronger.
Just like the slow oxidative fibers, fast oxidative muscle fibers have an extensive network of blood vessels, are rich in myoglobin and filled with mitochondria because they primarily rely on aerobic metabolism.
This makes these fibers appear red, just like the slow oxidative fibers, but in contrast to slow ones, these are termed “fast red muscle fibers”.
Now, since these fibers contract quickly, they use up ATP faster. However oxidative phosphorylation in the mitochondria takes longer to generate ATP, so these fibers can also utilize anaerobic glycolysis which is 100 times faster.
The problem is we only get 2 ATP for each glucose so there’s some storage of glycogen in these muscle fibers to provide the additional glucose, but because it’s used up quickly, the fast oxidative fibers fatigue relatively quickly.
Then there are the fast glycolytic fibers, which are also called type IIx, “fast” is for having the ATPase that hydrolyze ATP quickly, while “glycolytic” stands for the anaerobic pathway using glycolysis, that’s used for generation of ATP.

Fast Glycolytic Fibers7:21–8:35

Compared to the previous two types, these fibers are the largest and strongest, because they have the most sarcomeres. Because these fibers rely on the anaerobic glycolysis, they need less oxygen and therefore have fewer blood vessels.
Within their sarcoplasm, they also have low levels myoglobin and few mitochondria. The lack of myoglobin and blood vessels makes these fibers appear white, thus, they’re also called “white muscle fibers”.
In contrast to the previous two, within the sarcoplasm of the fast glycolytic fibers, there’s a lot of glycogen reserves, which ensures that there is plenty of glucose available for anaerobic glycolysis metabolism during contraction.
Due to the fact they mainly use anaerobic glycolysis, these reserves are also used up quickly causing the fibers to fatigue the fastest.
These characteristics make glycolytic oxidative fibers useful in short, rapid, forceful activities, like getting through a sprint race for example.
Alright, now let’s imagine you’re representing your country in the Olympic games on a 1000m running race. When you hear the starting gun, you start sprinting and you activate all three types of muscle fibers.

Case Study8:35–9:18

After the first 100m, your speed gradually falls, because the fast glycolytic fibers, which are the strongest, have run out of glycogen reserves.
After 400 meters, your fast oxidative fibers start running out of ATP since they are “fast” and use up ATP more quickly, and you start slowing down even more.
And for the rest of the race you depend on the slow but reliable slow oxidative fibers to carry you the rest of the way and bring you the gold medal.
Alright, as a quick recap. Slow oxidative fibers are characterized by slow contractions, aerobic metabolism, and red color due to their high levels of myoglobin and extensive network of blood vessels.

Review9:18–9:51

Fast oxidative fibers are characterized by fast contractions, aerobic metabolism with some anaerobic metabolism, and red color.
And fast glycolytic fibers are characterized by fast contractions, anaerobic metabolism and white color, due to their low levels of myoglobin and limited network of blood vessels.