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Muscle Fibre Types and the Muscle Contractile process

Writer's picture: jonathan hazelljonathan hazell

Muscle Fibre Types and the Muscle Contractile process

Firstly, let's define muscle fibres. Healthline gives the most straightforward and less jargon-riddled explanation:


Muscle tissue contains muscle fibres, which consist of a single muscle cell. They help control the physical forces within the body and, when grouped, can facilitate the organised movement of limbs and tissues.

Human skeletal muscles have three types of fibres: Type I, Type IIa and Type IIx (also known as IIb).


  1. Type I: Slow oxidative (SO) fibres contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce Adenosine triphosphate (ATP), the sole fuel for muscle contraction. They produce low-power contractions over long periods and are slow to fatigue.

  2. Type IIa: Fast oxidative (FO) fibres have fast contractions and primarily use aerobic respiration. However, because they may switch to anaerobic respiration (glycolysis), they can fatigue more quickly than SO fibres.

  3. Type IIb (or x): known as Fast Glycolytic (FG) fibres, have the fastest contractions of the three and primarily use anaerobic glycolysis. The FG fibres fatigue more quickly than the others.

Muscle Fibre Type Characteristics
Muscle Fibre Type Characteristics

  1. Type I Slow Twitch Oxidative Muscle Fibers

Keeping the description simple, Type I Slow Twitch fibres are red muscle fibres responsible for long-duration, low-intensity activities such as walking or aerobic activity. Type I fibres contract slowly and utilise aerobic respiration, using oxygen and glucose to produce ATP (Adenosine Triphosphate). They generate low-power contractions that last for extended periods and resist fatigue.


ATP is the primary energy source for use and storage at the cellular level. Its structure comprises a nucleoside triphosphate, a nitrogenous base (adenine), ribose sugar, and three phosphate groups linked in series. As an energy-carrying molecule, ATP fuels various cellular functions and is the most abundant energy carrier in the body. ATP is particularly effective in releasing energy quickly, which is why it is often referred to as the "energy currency" of biological systems.


Type I fibres are used for aerobic exercise and endurance movements. They can function for long periods without fatigue. They do not require much energy but don’t produce high tension, so they are not recruited for powerful, fast movements requiring high energy.


Type I is used for long-distance running, low-intensity swimming, low-intensity endurance cycling, hiking, low-to-moderate-intensity dancing, walking, maintaining posture, stabilising bones and joints, cleaning the house, and sitting upright in a chair.


Type I fibres rely heavily on Oxidative Phosphorylation rather than glycolysis for ATP production (energy). They depend on a plentiful supply of oxygenated blood to utilise oxygen to generate the energy necessary for muscle contraction. Slow Twitch fibres have a much better blood supply and ability to receive oxygen than type II fibres. Because these fibres require oxygen to produce ATP, they are more fatigue-resistant.


I always use the analogy of the ‘Tortoise and the Hare’; the Type I fibres are the tortoise and Type II the Hare. Type I don’t produce much power but are heavily resistant to fatigue and can contract for an extended period. They also have a high concentration of mitochondria, which is the cell's powerhouse where aerobic respiration takes place, meaning they can produce more energy over a sustained period, making them better for aerobic activities.  Type 2 (“fast twitch”) fibres, on the other hand, are suited for short, fast bursts of activity that don't require as much oxygen.

 

2.   Type II A & B Fast Twitch Muscle Fibers

Fast twitch muscle fibres are more complicated than Type I Slow Twitch; firstly, there are two types, Type IIa and Type IIx (sometimes referred to as IIb instead of x). Thinking back to the tortoise and hare, your fast-twitch (or type II) fibres are like the hare. They can produce a lot more force and power for a short time, but they get fatigued fast.

Muscle Fibre Types
Muscle Fibre Types

 In general, Fast Twitch muscle fibres are mainly recruited for anaerobic exercise (without oxygen), whereby the muscles are placed under high demand or heavy load for shorter work periods but higher intensity levels.


They can be faster than Type I, producing more force, power, and strength, but fatigue faster than Type I. Any resistance training will use Type I and II fibres; however, training heavier loads (80% or more of 1RM) or lighter loads with explosive plyometric tempos will recruit Type II fibres. Type II fibres also tend to achieve hypertrophy quickly, which can be important for bodybuilders.


Type II fibres are for short-duration and higher-intensity activities. Type IIx fibres are built for explosive short-duration activities such as Olympic lifting, 100-meter sprints, or powerlifting. Type IIa fibres are designed for short-to-moderate duration, moderate-to-high intensity work, as is seen in most weight training activities.


Let’s take elite athletes as examples. Marathon runners have up to 80% or more slow-twitch muscle fibres in their bodies, and sprinters have up to 80% or more fast-twitch muscle fibres, making them extremely fast, strong, and powerful but with limited endurance.


However, fast-twitch muscle fibres have fewer blood vessels and Mitochondria than slow-twitch muscles. This is because fast-twitch muscles are anaerobic, which means without oxygen. They use sources of energy that are already present inside your body, such as glucose, to make ATP (energy). Here’s a breakdown of the different types of fast-twitch muscles.

 

Fast Twitch Type IIa

Type IIa fibres have a fast shortening speed and transfer energy from aerobic and anaerobic sources. I call these the ‘Type II Hybrids’. These fibres use aerobic and anaerobic pathways and produce a medium amount of power over a medium amount of time. Type IIa have faster speeds than type I but are less fatigue-resistant. They produce ATP (energy) more quickly than type I, thus allowing them to produce relatively high amounts of tension.


Type IIa muscles are also known as oxidative-glycolytic muscles because they can use oxygen and glucose for energy. These fast-twitch muscles have a higher number of mitochondria than Type IIb. Thus, they are similar to slow-twitch fibre muscles in their ability to use oxygen; however, they also use glucose and fat to burn for energy.


Like slow-twitch fibre muscles, they don’t exhaust as easily as type IIb and can quickly recover from short, intense workouts. Research has also indicated a direct correlation between Type IIa and muscle size. Type IIa muscle fibres are more likely to grow during muscle hypertrophy than type I and type IIb fibres.


Fast Twitch Type IIb (or x)

I will refer to them as Type IIb, not IIx. These are the second type of fast-twitch muscles. They don’t use oxygen, so they are labelled nonoxidative muscles, unlike type I and IIa. Instead, Type IIb rely on anaerobic glycolysis to produce the energy (ATP) needed for activity.


Type IIb is also much larger in diameter and contains high amounts of glycogen, which is used in glycolysis to quickly generate energy (ATP) and produce high tension levels.

Because Type IIb does not use aerobic metabolism, they also have low numbers of mitochondria (mitochondria evoke many functions in skeletal muscle, including regulating calcium and reactive oxygen species levels) and myoglobin, which is why they have a white colour. They are, therefore, sometimes referred to as white-cell muscles and Type I Red-Cell muscles.


Type IIb muscle fibres produce rapid, forceful contractions to make quick and powerful movements. As a result, they fatigue quickly, allowing them to be used for short periods, unlike Type I muscle fibres. Type IIb muscles have faster shortening speed and a much greater anaerobic potential than the other fibres.


Type IIb provide much greater force than type I and type IIa fibres; however, as mentioned, they use anaerobic (without oxygen) pathways to get energy (ATP), meaning they receive less blood flow and oxygen and, therefore, can only produce force for short periods of time fatiguing quickly.

 

Type IIb muscle fibres are commonly recruited for athletes such as:

  • Sprinters

  • Powerlifters

  • Olympic weightlifters

  • Strongman

  • Plyometric athletes


To summarise, anyone who needs quick, rapid force to move heavy loads rapidly or move with power over a short period. For example, a 10k marathon runner would not require Type IIb muscle fibres. However, a 100-meter sprinter would require Type IIb muscles and would not rely on Type I like a long-distance runner would.


Because type IIb muscle fibres can fire rapidly with loads of force, powerlifters who lift heavier loads, as does an Olympic lifter, frequently require these muscle fibres in training. Both sports rely on lower repetition sets, meaning these type IIb fibres fatigue quickly and only produce power and force for a short period of time, which doesn’t matter.


What is the Sarcomere

First, let's define the Sarcomere in its simplest terms. Muscle tissue comprises long, cylindrical cells called muscle fibres containing myofibrils. These myofibrils are further divided into repeating units called sarcomeres, the basic functional units of muscle contraction.


A sarcomere is the fundamental contractile unit of muscle fibre. Each sarcomere comprises two primary protein filaments: actin, which is the thin filament, and myosin, which is the thick filament, as seen in Figure 14. These filaments are the active structures that facilitate muscular contraction. The most widely accepted model that explains this process is the sliding filament theory, also known as the cross-bridge theory.


  • Actin Filaments (Thin Filaments): These are primarily composed of the protein actin and are thinner than myosin filaments.

  • Myosin Filaments (Thick Filaments): These are made up of the motor protein myosin, which has long, tail-like regions and globular heads.


The I band, also known as the isotropic band, is the light band of the sarcomere. It is between two thick myosin filaments and contains only the thin actin filaments from two neighbouring sarcomeres. The I band can be seen in below Figure 14. It can be seen located at both ends of the sarcomere surrounding the Z Disc line. During muscular contraction, the I band shortens; as the muscle fibres contract, the thin filaments slide over the thick filaments, pulling the Z lines closer together and reducing the size of the I band. 


The Z disc bisects the I band and is an anchor point for the adjacent actin filaments. Two Z discs that sandwich the contractile proteins, primarily actin and myosin, define each sarcomere. The Z disc is an anchor point for the thin filaments (actin).


During muscle contraction, these filaments slide over the thick filaments (myosin), shortening the sarcomere and overall muscle contraction. The Z discs in adjacent sarcomeres are connected by titin (a large protein), which provides elasticity and contributes to the muscle’s passive resistance to stretching. The Z discs also play a role in signalling pathways involved in muscle contraction and other cellular processes.


An A-band contains the entire length of a single thick filament. Within the A-band is a lighter region known as the H-zone. The A-band corresponds to the area where the thick filaments (myosin) overlap with the thin filaments (actin). It encompasses the entire length of the thick filaments and includes the overlapping sections with the thin filaments. The A-band comprises thicker myosin filaments that form the central part of the sarcomere. The overlapping areas with the thin filaments (actin) contribute to the darker appearance of this band.


The H zone, from the German word 'helle' meaning 'bright', is the central region of the A band within the sarcomere, which does not contain thin (actin) filaments and consists solely of myosin filaments. The H-zone is located in the centre of the A-band and spans the region where the thick filaments do not overlap with the thin filaments.


The width of the H-zone can change during muscle contraction and relaxation. During muscle contraction, the H-zone decreases in width as the thin filaments slide over the thick filaments, shortening the sarcomere. When the muscle is relaxed, the H-zone is wider since there is less overlap between the thick and thin filaments.


Sarcomere Diagram
Sarcomere Diagram

Sliding Filament Theory (Cross-Bridge)

The sliding filament or cross-bridge theory explains how muscle contraction occurs at the molecular level. Developed in the mid-20th century, it has become the foundational concept for understanding how muscles generate force.


The sliding filament model explains how muscles contract through a cycle of repetitive events, causing actin and myosin myofilaments to slide over one another. This sliding shortens the sarcomere and generates tension in the muscle.


When a muscle is stimulated by a nerve signal, calcium ions are released, causing myosin heads to bind to active sites on the actin filaments, forming cross-bridges. This action is facilitated by the presence of ATP, which is required for the myosin heads to attach and contract.


Once the cross-bridge is formed, the myosin heads pivot, pulling the actin filaments inward toward the centre of the sarcomere. This movement is referred to as the power stroke. During this phase, the myosin heads change shape and move, shortening the muscle fibre and generating force.


After the power stroke, ATP binds to the myosin heads, causing them to detach from the actin. The ATP is then hydrolysed to ADP and inorganic phosphate, re-cocking the myosin heads to their original position, ready to attach to the actin again and repeat the cycle.

Sliding Filament Theory Process
Sliding Filament Theory Process

This cross-bridge formation, power stroke, and release process occurs cyclically as long as calcium ions and ATP are available. As many myosin heads attach to and pull on the actin filaments simultaneously, the actin filaments slide past the myosin filaments, causing the entire sarcomere to shorten, leading to muscle contraction.


The cumulative effect of many sarcomeres contracting in sequence across the muscle fibre results in the overall shortening of the muscle, allowing movements such as lifting, walking, or any other physical activity. The sliding filament theory explains how muscles generate force and work during contraction.

 

 

 

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