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Mechanism
Intermediate
9 min read

Muscle fibres

A muscle is not made of one kind of fibre. Two broad families sit side by side, one slow and fatigue-resistant, the other fast and quickly tired — and their proportion is not a choice.
3D scene
The muscle fibre and its stores
The scene shows fibres in section and the glycogen granules they contain. It does not distinguish types: the fibres appear there as one uniform set, which the literature contradicts. Without the 3D, the idea is unchanged — the fibre is a cell that stores its fuel on the spot.
Open in the explorer

What a muscle is made of

The pages in the Body pillar describe whole muscles: their attachments, their path, what they produce. None had yet gone one level down, to the part they are made of.
A muscle is an assembly of bundles, each bundle an assembly of fibres. A muscle fibre is a single cell, several centimetres long, containing many nuclei. Inside, aligned protein filaments slide past one another: it is that sliding which shortens the fibre. What allows it comes from elsewhere: the neuromuscular junction page describes the signal and the calcium that uncover the attachment points.
Sarcomere
The smallest unit able to contract, a few micrometres long. Thousands of sarcomeres end to end form a fibre; their simultaneous shortening produces that of the whole muscle.
contractile unit

Two families, a continuum

Not all fibres are alike. They are usually classed into two broad families, sometimes three, according to how fast they contract and how they produce their energy.

What separates the two broad families

Criterion
Type I, slow
Type II, fast
Contraction speed
Slow
Fast
Fatigue resistance
High
Low
Dominant energy pathway
Aerobic
Anaerobic, then aerobic
Mitochondrial density
High
Lower
Capillary density
High
Lower
Growth potential
Lesser
Higher
That table is convenient and misleading if you stop there. Properties in fact vary continuously, and type II fibres themselves subdivide by endurance. The classification also depends on method: two techniques applied to the same sample do not give exactly the same categories.
Certainty level · Established
Muscle fibres differ in contraction speed, fatigue resistance and the energy pathway they favour.
Descriptive anatomy and reviews of energy system interaction describe the same organisation. What varies between studies is the number of categories retained and the boundaries between them, which depend on the classification technique used.
Standring S (2020) · Gastin PB (2001)

Not every muscle is made the same way

The proportion of each type varies from muscle to muscle, and that variation follows function. Muscles working continuously against gravity — the erector spinae, the soleus of the calves — contain a large share of slow fibres. Those producing brief, intense efforts contain more fast ones.
It also varies between people, for the same muscle. That is where most of the shortcuts creep in.
Caution
What cannot be deduced
A fibre proportion cannot be read from a performance. A fast sprinter may be fast for many reasons other than typology: segment lengths, technique, training, force per unit of cross-section.
It can only be measured by biopsy, on a fragment of sampled muscle. No field test, no questionnaire and no outside observation gives access to it.
It does not determine what is reachable. This page describes an organisation; it says neither what your muscle is made of nor what you should do with it.

What training changes, and what it does not

Training changes a great deal inside a fibre: mitochondrial density, the capillary network, glycogen stores, the quantity of contractile protein. Those changes are the ones described on the page about how muscle grows.
Switching from one type to another is another question, and a more delicate one.
Certainty level · Probable
Training changes the properties of muscle fibres, and transitions within the fast-fibre family are documented.
The review on the signals that trigger hypertrophy describes the pathways by which loading changes the fibre's protein expression. How much conversion occurs between types, particularly between slow and fast, remains debated and depends heavily on the protocols and durations studied.
Wackerhage H, Schoenfeld BJ, Hamilton DL, Lehti M, Hulmi JJ (2019)

Seeing a fibre in section

The muscle fibre and its stores

The scene shows fibres in section and the glycogen granules they contain. It does not distinguish types: the fibres appear there as one uniform set, which the literature contradicts. Without the 3D, the idea is unchanged — the fibre is a cell that stores its fuel on the spot.
Current step
1. The store inside the fibre
On the left, a muscle fibre opened along its length. The dark spheres spread inside stand for glycogen granules, the form in which muscle stores carbohydrate.
Scene description
Comparative diagram: two identical muscle fibres, opened lengthwise like slices, sit side by side on pale plinths. The left one, with full stores, holds about ten dark spheres — the glycogen granules — each ringed by three small spheres standing for the water stored with it; above it, a six-segment gauge is completely filled. The right one, with low stores, holds only three granules and their water, and its gauge shows just two solid segments followed by four hollow ones. Between the two fibres, an upper arrow pointing right represents the effort that empties the store, and a lower arrow pointing left represents the dietary refuelling that rebuilds it. Sizes and counts are symbolic: nothing here is to scale.
Visible structures
  • Glycogen granule
    Glycogen is the form in which muscle stores carbohydrate. It is gathered into granules spread through the fibre, ready to be mobilised quickly during effort.
  • Associated water
    Glycogen is stored together with water, shown here as the small satellite spheres. When the store falls, that water leaves with it and the number on the scale follows, which says nothing about fat mass.
  • Fibre in section
    The fibre is drawn as a slice, as if it had been opened along its length. The section only serves to show what it contains.
  • Low stores
    After a long effort, few granules are left. The fibre still works, but the intensity it can sustain drops.
  • Refuelling
    Dietary carbohydrate rebuilds glycogen, and the associated water comes back with it. This is one of the reasons the number on the scale moves from one day to the next.
Guided steps
  • 1/5
    On the left, a muscle fibre opened along its length. The dark spheres spread inside stand for glycogen granules, the form in which muscle stores carbohydrate.
  • 2/5
    Each granule is ringed by small spheres: the water stored alongside it. Glycogen and water travel together, which explains part of the day-to-day swings on the scale.
  • 3/5
    During sustained effort the fibre breaks down its glycogen to produce energy. The upper arrow shows the move from one state to the other, and the group of granules shrinks.
  • 4/5
    On the right, the same fibre after the effort: few granules, little associated water, an almost empty gauge. The number on the scale has dropped, but that particular drop comes from glycogen and its water, not from fat mass.
  • 5/5
    Dietary carbohydrate rebuilds the store and the water returns with it: the lower arrow brings the fibre back to its full state. This back-and-forth repeats continuously, independently of fat mass.
Model licence · Shapier — Propriétaire — usage interne ShapierLab

What this changes for the other pages

Typology ties several pillars together. It explains why the aerobic system dominates sooner in some muscles than in others, why fatigue does not set in at the same rate everywhere, and why two muscles trained the same way do not respond identically.

Sources

Main sources

  • Standring S (2020). Gray's Anatomy: The Anatomical Basis of Clinical Practice, 42nd edition. Elsevier.
  • Wackerhage H, Schoenfeld BJ, Hamilton DL, Lehti M, Hulmi JJ (2019). Stimuli and sensors that initiate skeletal muscle hypertrophy following resistance exercise. Journal of Applied Physiology.
  • Gastin PB (2001). Energy system interaction and relative contribution during maximal exercise. Sports Medicine.
Put it into practice in Shapier

See the squat exercise page

Open the squat page in Shapier, the reference exercise for loading the quadriceps.
See the squat exercise page
Body Lab explains; Shapier lets you act and track.

Check my understanding

Is classifying fibres into two families, slow and fast, an exact description?
Does a very fast sprinter prove that their muscles contain mostly fast fibres?
What is known about the effects of training on fibre types?
Choose an answer

Read next

  • Motor command
    A muscle does not contract as one block. It contracts in units, recruited in an order that is not left to chance — and that recruitment improves before the muscle gets any bigger.
    With a 3D scene
  • How muscle grows
    What actually makes a muscle grow? This page follows the chain of events that links a set of exercise to a thicker muscle fibre, and separates what is established from what is still debated by research.
    With a 3D scene
  • The aerobic system
    The pathway that supplies most of the energy as soon as an effort lasts. It burns carbohydrate and fat with oxygen, produces a great deal but slowly, and never switches off.
    With a 3D scene

Available offline

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Trust and method

Author
equipe-editoriale-shapier
Editorial review
Thanh Chau
Scientific review
Pending
Published on
August 2, 2026
Reviewed on August 2, 2026
Next review due August 2, 2027
Limits of this page
  • Classifying into two or three types is a simplification: properties vary continuously, and different classification methods do not yield the same categories.
  • A person's fibre proportion can only be measured by biopsy, on a fragment of muscle: it is neither visible from outside nor deducible from a performance.
  • No individual conclusion follows from this page. It describes an organisation; it does not say what your muscle is made of or how you should train.
Sources
  • Standring S (2020). Gray's Anatomy: The Anatomical Basis of Clinical Practice, 42nd edition. Elsevier.
  • Wackerhage H, Schoenfeld BJ, Hamilton DL, Lehti M, Hulmi JJ (2019). Stimuli and sensors that initiate skeletal muscle hypertrophy following resistance exercise. Journal of Applied Physiology.
  • Gastin PB (2001). Energy system interaction and relative contribution during maximal exercise. Sports Medicine.
Educational content. Body Lab does not diagnose and does not replace professional advice.
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