Video: Types of skeletal muscle fibers

Welcome, welcome, welcome to the Kenhub Race Day! Today, athletes of all shapes and sizes gather for the ultimate showdown. It's a spectacle of sportsmanship and grit. We have three formidable athletes ready to show off their skills today. On your marks, ready…

In Lane 1, we have the Walker, the master of endurance. This athlete is a marathon runner and can endure long distances. In Lane 2, right in the middle, we have an average Runner who is maintaining a steady pace. And in Lane 3, the Sprinter. Look at them go, covering ground in the blink of an eye.

Oh, wow! Look at them go! However, you might be wondering how and why they're running at such different speeds. The secret to their speed lies not just in their legs but in the muscle fibers that power them. These muscle fibers explain why our athletes run at different speeds and we're about to uncover the magic behind them.

Before we dash right into it, let's take a look at what we will cover today. We'll start by defining muscle fibers. Then we'll dive into the criteria used for classifying muscle fibers by looking at the speed of contraction of the fibers and their metabolic properties. Then we'll explore each type of muscle fiber examining their diverse characteristics. Finally, we'll look at how muscles adapt to enhance performance in exercise, providing real world insights into the fascinating universe of muscle fibers.

Let's get started.

Muscle fibers, also known as myocytes, are the fundamental building blocks of human skeletal muscle. These remarkable cellular structures are responsible for executing the complex and coordinated movements that define our physical capabilities. Myocytes come in several distinct varieties, each uniquely adapted to support specific types of muscle contractions. Different proportions of these distinct muscle fiber types can be found in all skeletal muscles. The diversity in muscle fibers allows skeletal muscles to perform a wide range of movements.

Muscle fibers exhibit plasticity. Plasticity means that they can change in size or even convert to a different fiber type to adapt to new functions of the skeletal muscle. The fibers are therefore structured in a way that offers a wide range of functional capabilities and enables us to perform a variety of movements, ranging from slow, sustained endurance activities to rapid, forceful actions.

While the muscle fibers can share a common basic architecture and function, they are not identical in terms of their microstructure and contractile properties. These differences allow us to classify the fibers into three distinct types. But before we delve into the classification itself, we need to understand the two critical characteristics of the muscle fibers that serve as criteria for this classification.

Firstly, we have the speed of contraction – how fast some fibers contract relative to other others; and secondly, the metabolic profile, which basically means how fibers produce ATP using aerobic or anaerobic metabolism.

The different types of skeletal muscle fibers exhibit variations in their contraction speed. This difference in their need for speed is one of the key factors setting them apart. The variation in contraction speed is primarily due to the expression of distinct isoforms and quantities of myosin ATPase.

Myosin ATPase is the energy currency that resides in the head region of myosin. This little enzyme has a big job. It's responsible for catalyzing ATP hydrolysis, the process that fuels muscle contractions. The more myosin ATPase you have, the faster your muscle fibers can contract. However, the amount of myosin ATPase in the muscle fibers is not the only criteria we use to tell the different types of fibers apart. We also use the metabolic profile.

Metabolism plays a pivotal role in determining muscle contraction, and therefore, it's taken into consideration when classifying muscle fibers. Remember we mentioned that muscle fibers can produce ATP through aerobic or anaerobic metabolism? Metabolism encompasses various intricate biochemical processes that we won't delve deeply into today. However, to lay the foundation for understanding muscle fibers, it's essential to grasp the fundamentals. Specifically, we should be aware that the final phase of cellular aerobic respiration is termed oxidative phosphorylation and it occurs in mitochondria, the powerhouse of the cell. This process is dependent on oxygen.

Oxygen is transported to muscle fibers through the finely-branched capillaries. It is then stored by a molecule similar to hemoglobin called myoglobin. Myoglobin acts as a reliable reservoir, containing a constant reserved oxygen supply to myocytes, which consume much more energy than other tissues in the human body. The myoglobin and oxygen give muscles the characteristic red hue that we see in muscle tissue.

It's important to note that aerobic metabolism produces larger amounts of ATP compared to the more hurried efforts of anaerobic metabolism. While aerobic metabolism operates like a well-oiled machine in a spacious well-ventilated kitchen, anaerobic metabolism is the swift and resourceful cook in a tight spot, as it can perform in the absence of oxygen. This process is known as anaerobic glycolysis, which is another intricate biochemical process. However, for today's discussion, we'll focus on grasping the fundamental concepts.

Anaerobic glycolysis is a rapid process that turns out ATP almost 100 times faster than oxidative phosphorylation through glycolysis. Glycolysis produces two ATP per glucose molecule and thus provides a direct means of producing energy in the absence of oxygen. However, this speed comes at a cost. Anaerobic metabolism produces lactate as a byproduct. Lactate leads to metabolic acidosis, resulting in muscle fatigue ability. These differences in the biochemical pathway that muscle fibers rely on to produce the ATP they need to contract are also used as a criterion to classify them into different types.

Now that we understand the criteria we use to classify the muscle fibers, let's take a look at each type of fiber in more detail.

Muscle fibers can either be considered slow twitch or fast twitch fibers. This depends on the activity of ATPase within the myocytes. So as the name suggests, slow twitch fibers have a low myosin ATPase activity while fast twitch fibers have a high myosin ATPase activity. Remember, ATPase quantity and activity correlates positively with contraction speed.

Remember the walking athlete in lane 1 at the Kenhub Race Day who is well known for being able to go for long distances? They were probably predominantly relying on slow twitch fibers, also known as type I fibers. In slow twitch fibers, ATP, the energy currency of our muscles, is produced from aerobic metabolism via oxidative phosphorylation. These fibers have an abundant amount of mitochondria, the powerhouse of the cell, which turns out ATP. Therefore, they are capable of sustained contractions over an extended period without being easily fatigued due to the large amounts of ATP they can produce.

However, due to their relatively low amounts of myosin ATPase, type I fibers have a slower contractile speed. They are also usually the first type of fibers to be recruited during contraction. Furthermore, type I fibers are well supplied by capillary networks relative to their size. The capillaries supply a large amount of oxygen to the myocytes, which use it to produce energy in the form of ATP. Also, these myocytes possess a large amount of myoglobin which stores oxygen within the fibers in a similar way that hemoglobin stores oxygen in red blood cells. All that myoglobin gives these fiber types a reddish color in fresh specimens. Type I fibers also have a small diameter which doesn't make them ideal for generating high levels of tension.

Now, on the other hand, we have fast twitch fibers. Fast twitch fibers can be further subdivided into type IIa and type IIx fibers. A quick side note should be mentioned here. The term type IIb fibers is often mistakenly used as a synonym for type IIx muscle fibers; however, these are two different types of muscle fibers. In this video, we'll only discuss type IIx as type IIb muscle fibers are not usually found in human muscle tissue.

Type IIa fibers are also known as fast oxidative fibers or intermediate fibers. These are the fibers that the athlete in lane 2 was mostly relying on. These fibers can be considered as a transitional type between type I and type IIx muscle fibers, hence, their nickname, intermediate fibers.

Unlike the slow twitch fibers we discussed earlier, type IIa fibers are larger and more numerous than type I. They're quite the all-rounder. They primarily rely on aerobic metabolism but can switch to anaerobic metabolism if the need for energy is urgent. These fibers are resistant to fatigue and can sustain contractions for a prolonged period; however, their endurance isn't as remarkable as that of type I fibers.

Type IIa fibers are also capable of generating faster contractions and higher tension than type I fibers but less than that of type IIx fibers which we'll see next. These fiber types are recruited for contraction after type I but before type IIx muscle fibers.

Type IIa fibers possess a greater number of mitochondria compared to type I fibers as well as moderate amounts of glycosomes which store glycogen or glycolytic enzymes required for anaerobic metabolism. However, when it comes to myoglobin content and capillary density relative to their size, they lack behind type I fibers. This slight difference in composition results in a lighter pink coloration. These characteristics make type IIa fibers intermediate in terms of their properties. They're like the middle ground between endurance and speed.

Type IIa fibers are particularly useful for prolonged movements that require more tension than what type I fibers can generate such as running or swimming. They provide the necessary endurance and moderate force production for these activities, making them an invaluable asset for our average runner in lane 2.

Lastly, we have the fibers that we saw in action in lane 3 of the Kenhub Race. This sprinter was mostly reliant on the type IIx fibers. These muscle fibers are all about speed and power and they achieve this by primarily relying on anaerobic metabolism, specifically, glycolysis, for energy production. This metabolic profile equips them for fast and high-intensity contractions. However, there's a trade-off for all this speed. They tend to fatigue more quickly compared to other muscle fiber types. This happens because the lactate produced as a byproduct of anaerobic metabolism causes a local acidosis, increasing muscle fatigue ability.

Type IIx fibers also have a substantial glycogen depot, like a nitrous boost for a race car. This reserved glycogen supply allows for the rapid release of glucose, providing the energy needed for quick and forceful contractions. Morphologically, these fibers are larger in diameter and appear whiter in color. This unique appearance is attributed to their limited reliance on oxidative phosphorylation, resulting in a lower density of mitochondria, capillary structures, and myoglobin content.

So type IIx fibers are the go-to choice for explosive short bursts of anaerobic effort. Activities like weightlifting, sprinting, and jumping predominantly recruit these muscle fibers. They are the muscle workhorses responsible for generating rapid and powerful muscular contractions, just like our sprinter in lane 3, leaving nothing but a streak of speed in their wake.

As the Kenhub Race Day comes to an end, let's quickly take a look at how the type of exercise performed by skeletal muscles leads to adaptations in muscle fibers that are intended to enhance performance.

Firstly, let's take a look at aerobic exercise, often associated with endurance activities like running or swimming. These activities work as a catalyst for enhancing oxidative metabolism across all muscle fibers. This enhancement comes from two significant changes: first, an increase in the number of mitochondria within muscle fibers, and second, the development of a denser network of blood vessels around these fibers, known as capillarization.

On the flip side, anaerobic exercises focus on shorter, more intense activities like weightlifting or sprinting. When you engage in this type of exercise, your muscles respond by not only getting stronger but also by altering their muscle fiber characteristics. Specifically, anaerobic exercise stimulates the expression of specific myosin isoforms that are predominantly found in type II muscle fibers.

It's essential to recognize that these adaptations don't happen overnight. To see noticeable changes in muscle performance and fiber characteristics, one needs to commit to a consistent training regimen. So whether it's aerobic or anaerobic exercise, the key is patience and persistence to reap the benefits, kind of like learning physiology.

So let's recap what we learned today from the Kenhub Race.

We defined muscle fibers as the fundamental blocks of skeletal muscle with the ability to change and adapt. We explored the criteria for their classification including the critical factors of contraction, speed, and metabolic profile. Regarding the speed of contraction, we saw that the variation is primarily associated with quantities of myosin ATPase. Meanwhile, learning about the different types of metabolic pathways, namely, aerobic and anaerobic, shed light on the crucial role of oxygen in energy production.

Then we looked at oxidative phosphorylation and anaerobic glycolysis and the pivotal role they play in determining muscle contraction, and therefore, speed as well as in contraction sustainability. Classifying muscle fibers was our next step, delving into the characteristics of type I slow oxidative fibers known for their endurance and sustained contractions; type IIa fibers are intermediate fibers with versatility in various activities; and type IIx, the fast glycolytic fibers, showing us their explosive power for quick, high-intensity movements.

Finally, we explored how different exercises lead to adaptations in muscle fibers over time.

So hopefully the next time you're taking a leisurely walk, maintaining a steady pace, or sprinting, you remember the magic of the tiny, yet mighty, muscle fibers.

Until our next Kenhub Race Day! Bye for now!