If you love athletics, you can clearly notice the difference in muscle appearance between world-famous 100m, 200m sprinters, and marathon runners. Most probably, in the Olympic games, some countries have won the same titles over the last one or two decades. how does it happen? Among many other causes obviously, their muscle fiber types influence the performance massively. In general, All humans have all muscle fiber types in their skeletal muscles. But the ratio makes the difference. Let us focus on these variations of muscle fibers.

Skeletal muscle and its fibers

First and foremost, we just take some time to discuss skeletal muscle structure. Skeletal muscle is contained of long and cylindrical cells. these muscle cells are commonly known as muscle fibers. normally, Skeletal muscle cells are quite large than normal human cells since its length. these are multinucleated cells.

A skeletal muscle fiber is covered by a plasma membrane called the sarcolemma. A muscle fiber is composed of actin and myosin myofilaments, which give the striated appearance for the muscle. This actin and myosin arrangement of muscle fibers contribute to create the sarcomere, which is the functional unit of skeletal muscle.

Exercise increases the mitochondrial content of muscle in a fiber type-specific manner. As well as mitochondrial content is reduced when we don’t use the particular muscle prolong time. It really affects the muscle endurance and metabolism of the muscle fibers then whole muscle.

What are the three types of muscle fibers?

Scientists who have done their studies around this topic classify the muscle fibers into 2 major groups. Those are called Fast-twitch white fibers and Slow-twitch red fibers. Let’s see some distinctive features of these muscle fiber types.

Slow twitch fibers (Type I)

These are the smallest muscle fibers out of all three in diameter. Sometimes you may hear that these slow-twitch muscle fibers are also called red fibers. It is true that these muscle fibers have a bright red color since they got good capillary density which means these fibers are highly vascularized. On the other hand, slow-twitch fibers are comprised of a large number of mitochondria. These powerhouses produce ATP by using oxygen.

Though these fibers have enough oxygen supply and mitochondria, they make ATP by Aerobic cellular respiration. As the name suggests, these fibers generate low force when we compare it to the other two types. because of the low myosin ATPase activity, contractile speed is very slow. But the fibers can make contractions for more than 30 minutes without getting rest. By the way, they have great myoglobin level and myoglobin can store oxygen for contractions. So, Type I fibers show the greatest resistance for fatigue.

Fast twitch fibers (Type IIA)

Type IIA is the largest muscle fibers in humans and these fibers have good capillary density. However, fibers show a reddish-pink color since the diameter. These fast fibers contain a moderate number of mitochondria and glycogen stores. That’s why Type IIA can switch between aerobic and anaerobic respiration. And also, creatine phosphate molecules and creatine kinase enzymes are also identified in muscle fibers.

Due to fast myosin ATPase activity, these fibers contract pretty fast and make a decent amount of muscle power. It is important to notice that they can only continue contractions for less than 30 minutes and show moderate fatigue resistance. Myoglobin level is quite low and it also contributes to enhancing the color of muscle fiber other than oxygen storing.

Fast twitch white fibers (Type IIB)

Type IIB fibers are also known as type x fibers. Researchers have found that these are the largest muscle fibers in some animals. but it is not true for animals. This is the type that contains the least capillary density and intermediate fiber diameter. So, these fibers are a white or pale pink color. Furthermore, the mitochondria amount is really low. So, they don’t try to produce ATP by aerobic respiration. But, Type IIB fibers have a large number of glycogen stores.

These large glycogen stores are converted into glucose and through a process called glycolysis, glucose becomes ATP. Creatine phosphate also involves in the ATP production after binding into the creatine kinase enzyme. So, these two anaerobic processes are really fast and fibers have the ability to make fast contractions since the highest amount of myosin ATPase activity. these contractions are powerful and easily become fatigued.

Energy For muscle contractions

It is clear that without ATP, Adenosine Tri Phosphate skeletal muscles can’t contract. you are able to control muscle contraction voluntarily but if muscle contraction does not occur without sufficient amounts of ATP. There is a small amount of ATP stored in the muscle, it is only sufficient for a few seconds when you start to contract the muscle repeatedly. There are three main mechanisms to create ATP for muscle contractions.

Creatine phosphate metabolism

This is the simplest form of energy systems. Cells store energy in two forms,

1. Directly as ATP (very small amount)

2. As Phosphocreatine (PCr)

This can occur in the presence of oxygen, but the process does not require oxygen. ATP-PCr system can sustain the muscle energy requirement for only 5s -15s, hence provides energy for short, quick bursts of activities like short sprinting, weight lifting, throwing, jumping, etc. obviously, this system acts as a major source of energy during the first 30 s of intense exercise.

Anaerobic glycolysis

ATP is produced by the liberation of energy through the breakdown of glucose. This breakdown of glucose entails through a pathway that involves a sequence of glycolytic enzymes. Lactic acid is produced as a by-product. Anaerobic glycolytic system is capable of supplying energy needs for activities lasting not more than 2 min. This is the major source of energy from the 30s- 90s of activities like swimming, long sprinting.

Aerobic /oxidative system

This is the most complex system which occurs through a process called cellular respiration. This is an aerobic process, which requires oxygen. Cellular respiration occurs within mitochondria. Here, Glycogen, Fats, and Proteins are fuel sources utilized relative to their availability and the intensity of the exercise. This system mainly predominates over the other two energy systems after the 2nd minute of exercise.

How do the types of muscle fibers affect your performance?

In general, we have all these three muscle fibers in our skeletal muscles. But genetically, the ratio of muscle fiber types is different. That is why we see some African countries win every long-running event in the Olympics. this genetic factor uses to achieve the highest part of a player’s performance. however, recent studies have shown some evidence for the capability of changing muscle fiber ratios by training. So, it is good to get some knowledge about how to improve your performance by using this strategy.

As I mentioned earlier, Slow twitch muscle fibers can make contractions over 30 minutes without facing muscle fatigue. So, these fibers are good for marathon runners. Fast-twitch type IIA fibers helpful for walking, sprinting (more than 1 min)activities. If you are a weight lifter or 100m, 200m sprinter, Fast twitch Type IIB fibers are the best muscle fiber type for your performance.

Credits for featured image: Image by tookapic from Pixabay

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