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Skeletal Muscle – Anatomy and Fibre Types

To optimise your clients results your must understand how the different muscle fibres behave so you can prescribe training accordingly. Read this page to understand skeletal muscle from a 'microscopic perspective'

Anatomy of skeletal muscle

Skeletal muscle is made up of thousands of muscle fibres that run the length of the muscle.  Each muscle fibre consists of many contractile units called myofibrils which run the length of each muscle fibre. 

Individual muscle fibres are wrapped with fascia and then further bound together by more fascia into bundles called fascicules.  These bundles of fascicles are then grouped together to form skeletal muscle which is coated by more fascia. 

Fascia is simply a type of connective tissue which provides a protective wrapping.  Fascia gets thicker as it nears the surface of muscle and ultimately coats the entire muscle and eventually becomes the tendon that attaches muscle to bone.

The below image shows the various components that make up a muscle.


As we know myofibrils are the contractile units that make up each muscle fibre.  Myofibrils can be looked upon as the building blocks of muscle and contain units called sarcomeres.

Sarcomeres are the smallest functional unit of skeletal muscle and are composed of two protein filaments; actin and myosin.  Actin is a thin filament and myosin is a thick filament, as shown on the following diagram, and it is the interaction between these two filaments that causes a muscle to contract and shorten.



How do skeletal muscles work to produce force?

Muscles produce force through the binding of the actin and myosin filaments.  These filaments are able to bind because myosin has tiny projections on them that extend towards the actin filaments.  The point where the two filaments bind is known as a cross-bridge and can be seen in the image below.


The number of myosin cross-bridges formed between actin and myosin determines how much force a muscle can produce.  Think of cross-bridges as your hands climbing up a rope.  If you had more than one pair of hands your task would be much easier and your attachment to the rope much stronger.  The more 'hands' (cross-bridges) the better.

There are three types of skeletal muscle fibre and they all produce different degrees of force due to slight differences in the characteristics of how they work.


What are the three types of skeletal muscle fibre?

Skeletal muscles consist of three main fibre types, these are;

personal%20training%20treadmill1. Slow twitch fibres (Type I):  Slow twitch fibres contract slowly but can contract repeatedly over long periods.  They have a good blood supply, hence they are ‘red fibres’, and are suited to endurance activity using the aerobic energy system which relies on oxygen from the blood for the supply of energy.  (Check out the energy system folder at ptdirect to see how the different energy systems function). 

Slow twitch fibres are smaller and develop less force than fast twitch fibres.  Walking or cycling for 30 minutes at low intensity would use mostly slow twitch fibres.


2.  Fast twitch fibres (Type IIa):  Fast twitch IIa fibres have a fast contraction speed and can use aerobic (oxygen dependant) energy sources as well as anaerobic (no oxygen used) energy sources.  Fast twitch IIa fibres are ‘white fibres’ as they are less reliant on oxygen supplied by the blood for energy and therefore fatigue faster than slow twitch fibres.

Fast twitch type IIa fibres are suited to speed, strength and power type activities, such as moderately heavy weight training (8-12 reps) and fast running events such as the 400metres.


 personal%20training%20weight%20lifter3.  Fast twitch fibres (Type IIb):  Fast twitch IIb fibres contract extremely rapidly, create very forceful muscle contractions and fatigue quickly.  Fast twitch IIb fibres are also ‘white fibers’ but unlike IIa fibres they can only use anaerobic energy sources. 

Like type IIa fibres the fast twitch type IIb fibres are also suited to speed, strength and power type activities.  Heavy weight training (1-3 reps), power lifting, and 100metre sprints are examples of activities that predominantly require IIb fibres.


Characteristics of muscle fibre types

As we know there are three types of skeletal muscle fibre and they each have different capacities to produce force, different fatigue rates and rely on different energy systems in order to function.  The reason for these differences is due to their individual characteristics.

The characteristics of the three muscle fibre types are summarised in the following table.


Table of Muscle Fibre Characteristics

Fibre Type

Type I fibres

Type II A fibres

Type II B fibres

Contraction speed (milliseconds)

Slow (90-140 ms)

Fast (50-100 ms)

Very Fast (40-90 ms)

Size of motor neuron (bigger motor neurons allow nerve impulses to operate more quickly)



Very Large

Resistance to fatigue




Activity Used for

Endurance type activities

Short (less than 2minutes) high intensity activities

Very short (1—30 seconds max) maximal intensity activities

Force production



Very High

Efficiency (think fuel consumption here – high efficiency is like a scooter – doesn’t burn much fuel or produce much force.  A V8 produces heaps of force but is definitely not ‘efficient’)




Number of Mitochondria (Mitochondria are located in muscle fibres and produce energy aerobically (with oxygen))




Capillary density (Capillaries transport oxygen and nutrients to muscles and remove waste products)




Oxidative capacity (This refers to the capacity of the muscle fibre to use oxygen for the production of energy)




Myoglobin content (Myoglobin is a pigment that binds to oxygen giving the fibre a red colour)




Glycolytic capacity (refers to the fibres capacity to store and breakdown glycogen for use as a high intensity energy source)




ATPase levels (this is an enzyme that controls the breakdown and synthesis of ATP for energy)




What determines how many of each muscle fibre type an individual has?

An individual’s muscle fibre type is determined by three factors, these are: 

personal%20training%20genetic%20mother%20and%20daughter1. Genetics:  You are genetically programmed to having a certain percentage of each muscle fibre based on your parents’ genes.  It is thought that the average person is born with around 60% fast twitch and 40% slow twitch fibres, however some individuals can be born with larger amounts of fast twitch or slow twitch fibres and may therefore be more suited to either high force or long duration activities.


 2. Hormone levels within the blood:  The amount of hormone in the blood will affect the fibre type of an individual and how big the fibres are.

Hormone levels naturally fluctuate throughout a person’s lifetime, so some change in fibre type and distribution can occur as we grow and mature.  Males and females also have different levels of certain hormones produced and the type of exercise (i.e. light vs heavy weights) we do will affect the level of hormones released as well. personal%20training%20hormones%20%26%20muscles%20-%20arnie Some of these hormones are ‘catabolic’, that is they stimulate muscle breakdown, while others are ‘anabolic’, that is they stimulate the growth and repair of muscle tissue. 

So depending on our age, gender and the type of training we do, we can cause an increase or decrease in the production of certain hormones.  The result of this is either an increase or decrease in the slow or fast twitch muscle fibres.


personal%20training%20fibre%20type%20-%20taylor3. Training undertaken:  Fibre type and the ability to change fibre type is a common area of debate amongst exercise physiologists.  There is no evidence as yet to show that fibre type can be changed, however there is evidence to show that fibres adapt to the type of training they are exposed to. 

For example, if a person with predominantly slow twitch ‘endurance’ fibres trained predominantly with heavy weights their slow twitch fibres would overtime begin to behave more like fast twitch fibres. 


Muscle fibre and exercise outcomes

We now know about the different muscle fibre types and the factors that determine how many of each muscle fibre type an individual has, so let’s take a closer look at what that means in terms of exercise and sporting outcomes.

In the sporting world a lot is often made out of fibre type, some people believe that knowing an individuals fibre type will predict how successful they will be in a given sport.  Fibre type has been used to explain how certain ethnic groups seem to be over-represented in some sports, for example african americans in basketball, pacific island people in rugby union or rugby league.

As a result some scientists have tried to prove that certain ethnic groups have a higher percentage of fast twitch fibres (african americans) than other ethnic groups (europeans), and that this gives them an advantage in sports relying on fast twitch fibres such as basketball. 

Such assumptions are both scientifically and ethically dubious, for the following reasons;

1. Variation within populations:  All populations whether based on ethnicity, gender, age or athletic success etc, have massive variations within them.  For example no two rugby players are identical (in fibre type and physical appearance); just as no two people of the same ethnicity are identical.  Because of the massive variation within populations, fibre type cannot be isolated as the one single factor that affects performance and determines success more than others.

2. Muscle fibre behaviour:  As discussed previously, muscle fibres can adapt to the training they are exposed to.  So an athlete who may not have a predominance of fast twitch fibres but is determined to be a great rugby union forward (very strong and powerful) can certainly become one…if they train appropriately, eat well, sleep well, avoid lots of alcohol…etc!

3. Success is multi-factor:  Success in the sporting world is a mix of ideal training, ideal recovery, ideal nutrition, desire and many other factors (being tall is probably an advantage for basketball).  No one factor is more or less important than the others and no-one individual is born a successful athlete; success for everyone comes with dedication and hard work, regardless of fibre type.

What can we take from this into the fitness domain?  A lot actually!  It is a key reminder that no-one has a genetic advantage that will guarantee them quick results with minimal effort.  Just like success in the sporting world, success for your personal training clients is multi factor.  An ideal (and individual) combination of factors such as healthy eating, the correct frequency, intensity, type of training, rest, recovery, support, encouragement and individual drive is required by each and every client to ensure their success.


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