Muscle Fibre Type

Muscle Fibre Type

This article is to complement the Working with Weights series in order to better understand muscle composition and muscle fibre type. What is meant by this?

We have different muscle fibre types for different activities & natural composition of these fibres varies in each individual, just like how you’re blessed with a set number of muscle fibres. Before we look into muscle fibre type lets quickly look at how muscles are supplied with the energy to make us move.

Each skeletal muscle is served by a nerve and blood supply that allows voluntary contraction and efficient nutrient exchange. Microscopically, each muscle is composed of muscle fibres (picture this like a thick rope) and a single unit of a muscle fibre (each thread of the rope) is a myofibril (the term myo- indicates the term refers to “muscle”). Myofibrils are packed in a muscle fibre along with mitochondria and other organelles. Mitochondria are the power plants of a cell providing ATP. ATP (adenosentriphosphate) is the high-energy compound that supplies energy to contract the myofibrils. Mitochondria are more plentiful where cells have to work more – such as in skeletal muscle, kidneys and the liver. ATP can be synthesized from

  • creatine phosphate (CP) reacting with adenosinediphospate (ADP)
  • glucose
  • pyruvic acid
  • free fatty acids from adipose tissue
  • amino acids from protein breakdown

Muscles only have about 4-6 seconds worth of ATP and must be renewed just as quickly as it’s expended.

Three pathways for ATP synthesis are

  1. Direct phosphorylation – doesn’t require oxygen (anaerobic), produces 1 ATP, provides 15 seconds of energy
  2. Anaerobic pathway – anaerobic, produces 2 ATP and lactic acid, provides 30-40 seconds (slightly more) of (ENERGY SOURCE – Glucose)
  3. Aerobic pathway – oxygen required, produces 32 ATP/CO2 & H2O, provides hours of energy
    (ENERGY SOURCE – pyruvic acid, free fatty acids, amino acids)


Now we know about muscle structure and how energy is supplied to them we can delve into muscle fibre types. There are

  1. Slow Twitch/ Type I
  2. Fast Twitch/ Type II (further sub-divided in Fast Twitch A, or Fast Twitch B)


  • slow to contract
  • can power on for long periods of time (endurance)
  • high mitochondria density
  • high capillary density
  • darker red in colour
  • aerobic pathway

Slow twitch muscle fibres are used for daily activities (walking, cooking, gardening, swimming and prolonged aerobic/marathon style workouts), muscle tone and posture.


  • fast to contract
  • fatigue’s moderately easily
  • high mitochondria density
  • medium capillary density
  • medium pink in colour
  • anaerobic pathways

Fast twitch muscle fibres are intermediate fibres used in prolonged stop and go sports such as tennis, soccer and dancing as well as maximum efforts such as weightlifting and track and field events such as 400 meter sprint. Lactic acid build up indicates use of these fibres.


  • fast to contract
  • fatigue’s very easily
  • low mitochondria density
  • low capillary density
  • lightest in colour
  • anaerobic pathways (direct phosphorylation)

Fast twitch muscle fibres are utilised for highly explosive, maximal effort movements and tire very quickly. Sprinters and weight lifters have a higher percentage of fast twitch muscle fibres.

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Knowing this we can determine the type of training you need to do in order to develop a specific muscle group. Keep in mind not everyone’s muscle group composition will be the same as aforementioned. Some ways to determine muscle fibre type are to do a needle biopsy (most painful and expensive), a more accessible way would be to head to the gym and use some weights to see how many repetitions you can do with a weight that’s at 80% of your 1RM. If you fatigue at 5 – 6 reps muscle fibre composition of that muscle group will be more fast twitch fibres and if you can do 10+ reps muscle fibre composition will be predominantly slow twitch fibres.

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With this information on hand you can determine what kind of training (low or high reps, more or less sets) will stimulate certain fibre types increasing growth, strength, or endurance.

From a basic viewpoint the first fibers to be stimulated are the slow twitch fibers, followed by fast twitch type A and lastly fast twitch type B.

Muscles predominantly composed of slow twitch fibers a stimulated best with fewer sets and predominantly fast twitch muscle fibers respond with more sets. Muscles must be stimulated for specific amounts of time for maximum strength and size gains which the following diagram can aid in determining.

Determine your workout
Determine your workout


1 – Marieb, E. & Hoehn, K., ‘Human Anatomy and Physiology’ Pearson Education

2 – Poliquin C., ‘The Poliquin Principles’, Dayton Writer’s Group, 1997

3 –


I'm currently studying a Bachelor in Health Science - Nutritional Medicine with a background in dance and fitness. I believe sharing knowledge is the key. I hope my love for food, fitness and well-being along with my studies helps you in finding something useful. Enjoy!

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