3. Muscle Fiber Types

Muscle tissue → Muscle Cells → Muscle Fibers

Two Types of Muscle Fibres

Feature	Slow-Twitch Fibres (Type I)	Fast-Twitch Fibres (Type II)
visual representation
Colour	Red	White/Pale
Also Called	Type I / Slow Oxidative Fibres	Type II / Fast Glycolytic Fibres
Contraction Speed	Slow	Very fast
Time to Maximum Contraction	40–50 milliseconds	~5 milliseconds
Force Production	Low force	High force & power
Fatigue	Highly fatigue resistant	Fatigue quickly
Primary Energy System	Aerobic (uses oxygen)	Anaerobic (without oxygen)
Energy Source	Oxygen and fats (primarily), also carbohydrates	Glucose (glycogen), phosphocreatine (PCr)
Myoglobin	High myoglobin content	Low myoglobin content
Capillary Supply	Large number of capillaries	Low number of capillaries
Mitochondria	Large number of mitochondria	Fewer mitochondria
Aerobic Capacity	High	Lower
Anaerobic Capacity	Low	High
Cross-Sectional Area	Smaller	Larger
Motor Neuron Activation	Smooth and gradual	Rapid and powerful
Typical Activities	Posture, walking, endurance exercise, Pilates, long-distance running	Sprinting, jumping, heavy lifting, explosive movements
Intensity	Low-intensity, long-duration	High-intensity, short-duration
Tensile Strength	Lower	Higher
Training Adaptations	Aerobic capacity can improve, but anaerobic capacity changes very little	Both aerobic and anaerobic capacities can improve with training

Fast-Twitch Fibre Subtypes

Revision Notes

Slow-Twitch (Type I)

• High levels of myoglobin, giving the fibres their red colour.

• Depend on a continuous oxygen supply from the blood.

• Contain many capillaries and mitochondria.

• Produce ATP mainly through aerobic metabolism.

• Primarily use fat (and carbohydrates) as fuel during endurance activities.

• Contract slowly and smoothly.

• Best suited for low-intensity, long-duration exercise.

• Fatigue resistant.

• Generate relatively little force.

• Aerobic endurance can be improved with training, but anaerobic power changes very little.

Fast-Twitch (Type II)

• Lower myoglobin, giving the fibres a white/pale appearance.

• Fewer capillaries and mitochondria.

• Larger muscle fibres with greater cross-sectional area.

• Produce ATP mainly through the anaerobic energy systems.

• Use glycogen (glucose) and phosphocreatine (PCr) for fuel.

• Produce lactate (lactic acid) during intense exercise.

• Contract extremely quickly (around 5 ms).

• Produce high force and power.

• Best suited for short, explosive, high-intensity activities.

• Fatigue quickly.

• Both aerobic and anaerobic energy production can improve with training.

Lactic Acid (Lactate)

• During very intense exercise, fast-twitch fibres produce lactate.

• Lactate accumulates when it is produced faster than it can be removed.

• This contributes to fatigue and reduced muscle performance.

• During moderate or recovery activity, lactate is cleared from the muscle and normal ATP production can continue.

Muscle Fibre Distribution

• Postural muscles contain mostly slow-twitch fibres because they contract continuously to maintain posture.

• Power muscles contain a higher proportion of fast-twitch fibres for explosive movements.

• The proportion of fast- and slow-twitch fibres is largely determined by genetics, which partly explains why some people naturally excel in endurance sports while others perform better in power or sprint events.

Training Adaptations

• Type IIA fibres are highly adaptable and can become more endurance-oriented or more power-oriented depending on training.

• Muscle fibres cannot completely change from one type to another (e.g. Type I cannot become Type II).

• Endurance (aerobic) training increases the number and efficiency of mitochondria, improving oxygen use and aerobic energy production.

• Strength and power training improve the force and anaerobic performance of fast-twitch fibres.