Muscle fibers : Types and functions

Generally, there are three types of muscle fibers: type I and type IIx being the main type. The third is known as type IIb. (Photos: Unsplash)
In the realm of sports and exercise, great emphasis is often placed on physique as an indicator of muscle mass and fat content. When we exercise, play sports, or lift weights, our muscle mass naturally increases to meet the higher demand of the action. اضافة اعلان

Usually, however, not much thought is put into the type of muscle being built. By building the right type of muscles for your needs, you can maximize the benefit of your exercises and how they serve you in your area of training. 
Types of muscle fibers
Generally, there are three types of muscle fibers: type I and type IIx being the main type. The third is known as type IIb. Colloquially, type I is referred to as slow-twitch muscle fibers, type IIx as fast-twitch, and type IIb as intermediate. 

Slow-twitch and fast-twitch are the most dissimilar and serve near opposite functions, whereas intermediate tends to have characteristics from both main types.

Fast-twitch vs slow-twitch
1- Function

Slow-twitch muscle fibers are predominately used in activities that require prolonged, steady contraction. We unconsciously use our muscles to stay upright in a chair or while standing, and even while resting. 

The muscles in our legs, back, and neck contract to varying degrees. Slow-twitch muscle fibers sustain contraction for long periods but at the cost of speed. Although seemingly insignificant to human perception, slow-twitch muscle fibers in the leg can take as long as 100 milliseconds to reach maximum tension. Fast-twitch muscle fibers, on the other hand, only take 7.3 milliseconds to reach maximum tension, making them useful in situations where a rapid response is needed. 

While their tensions are stronger than those held by slow-twitch muscle fibers, these contractions are not sustainable and fatigue quickly. 

2- Energy use

The human body’s main energy source comes from a molecule known as adenosine triphosphate (ATP). Slow-twitch fibers use this energy source at a slower and steadier pace than fast-twitch fibers, which use more energy over a shorter period. 

This allows fast-twitch fibers to produce higher tension than slow-twitch fibers.

3- Need for oxygen

ATP is produced in the body by two different processes: aerobic and anaerobic metabolism. Aerobic metabolism requires oxygen to convert sugars into energy (ATP). Although slow, it is sustainable and produces more ATP per sugar. This process is used in slow-twitch and determines function, energy use, and contraction speed. 

Fast-twitch fibers use anaerobic metabolism, which does not require oxygen, to produce ATP at a much faster rate but far less efficiently. It also produces lactic acid as a by-product which contributes to muscle fatigue. 

4- Blood supply

There is a vast network of blood supply in place to deliver constant and adequate amounts of oxygen to slow-twitch muscle fibers. Slow-twitch muscle fibers have a high concentration of the protein myoglobin, which improves oxygen delivery to them. 

Due to myoglobin’s color, slow-twitch muscle fibers are also called red fibers. Since fast-twitch fibers do not have a high oxygen demand, the blood supply to them is considerably less, worsening the effects of lactic acid since accumulations cannot be removed quickly.
Intermediate muscle fibers
Intermediate muscle fibers are the happy medium between slow-twitch and fast-twitch fibers. By classification, they are a type of fast-twitch fiber that uses aerobic metabolism. The rate at which intermediate fibers produce ATP is much higher than slow-twitch fibers. This allows them to have high tension, speed, and resist fatigue. This type of muscle fiber is mostly used in general movements like walking.      

How to grow certain fibers
When we exercise, the high demand placed on our muscles causes tiny tears in the fiber. These tears typically go unnoticed or may present themselves as soreness. After a workout, the body repairs or replaces damaged muscle fibers. In doing so, the number of fibers increases and thickens, growing and developing the muscles.

People are usually born with a relatively even amount of slow-twitch and fast-twitch fibers, but it can vary slightly based on genetics. By engaging in certain exercises, you can significantly improve the specific types of muscle fibers you lack.

1- Slow-twitch muscle exercises

Jogging or running — especially for long distances — is ideal for building slow-twitch fibers. As you build these fibers, you will be able to go further and build your stamina without feeling fatigued. Other exercises include yoga, Pilates, biking, and swimming at a steady pace. 

2- Fast-twitch muscle exercises

Exercises that require bursts of energy utilize fast-twitch muscles. A common example is weightlifting, requiring short, quick contractions between each repetition. Similarly, engaging in sprinting and boxing can build these muscles. 
Which type is better?
The answer to which muscle type is better is purely subjective. From a practical standpoint, slow-twitch fibers tend to have more applicable uses since daily activities, such as walking and posture, are heavily reliant on them.

For athletes, the choice of fiber is dependent on their chosen sport. Long-distance runners, yoga practitioners, and avid walkers should opt to improve their slow-twitch muscles. On the other hand, sprinters, weightlifters, and boxers need fast-twitch muscles more. 

There are certain sports like soccer that require a combination of both: players must run long distances, which uses slow-twitch fibers, and also sprint, which requires fast-twitch. It would be reasonable for the average person to engage in activities that improve both types. Building slow-twitch fibers in the legs and back while simultaneously building fast-twitch in the legs and arms will create the most practical outcomes. 

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