Muscle Fibre Types — Clinical Guide for Athletes

What are the different muscle fibre types?

Skeletal muscle is made up of bundles of fibres comprised of long cylindrical cells that contract to produce force. There are three main types of muscle fibres which vary in their resistance to fatigue, contraction speed and energy systems. Most muscles contain a combination of all three types; however, genetics and training influence the proportions of these fibres in each muscle. The following are the three primary types of muscle fibres:

1. Type I (Slow-Twitch Oxidative)
2. Type IIa (Fast-Twitch Oxidative)
3. Type IIb or IIx (Fast-Twitch Glycolytic)

Type I: Slow-Twitch Fibres

• Contraction speed: Slow
• Fatigue resistance: High
• Energy system: Aerobic (oxygen-dependent)
• Force production: Low

Type I fibres are built for endurance. They contract slowly but can sustain activity for extended periods without fatiguing. These fibres are rich in mitochondria, capillaries, and myoglobin, allowing them to efficiently use oxygen to generate energy.

Athletes with a high proportion of type I fibres excel in aerobic, steady-state activities that require stamina over speed such as long-distance running, long-distance swimming, and cross-country skiing.

Type IIa: Fast-Twitch Oxidative Fibres

• Contraction speed: Fast
• Fatigue resistance: Moderate
• Energy system: Both aerobic and anaerobic
• Force production: Moderate to high

Type IIa fibres are the hybrids in that they contract more quickly than type I fibres and can produce more force, but still have relatively good endurance. They can use both oxygen and glycogen as fuel sources. These fibre types are highly adaptable and play a vital role in multi-modal athletes who need a balance of strength and endurance. An abundance of these muscle fibres is particularly good for middle-distance running (eg. 400 to 800m), functional strength sports including Crossfit and rugby, as well as combat sports like MMA and boxing.

Type IIb (or IIx) – Fast-Twitch Glycolytic Fibres

• Contraction speed: Very fast
• Fatigue resistance: Low
• Energy system: Anaerobic (glycolysis)
• Force production: Very high

Type IIb fibres excel in explosive power. They contract rapidly and produce high levels of force, but they fatigue quickly due to reliance on anaerobic metabolism which produces lactic acid. These fibres are excellent for sprinting, powerlifting, plyometrics and jumping sports such as basketball. Athletes with a high percentage of type IIb fibres tend to be powerful and fast but may struggle with endurance without proper conditioning.

How Muscle Fibres Influence Athletic Performance

Your muscle fibre composition has a major impact on your natural strengths as an athlete. Elite marathon runners often have 70–80% type I fibres, while world-class sprinters appear to have up to 75% type II fibres.

However, ratios of muscle fibre type can be altered. While genetics play a big role, training can modify fibre characteristics. For example:

• Sprint and strength training can increase the size and strength of type II fibres.
• Endurance training can increase mitochondrial density in type IIa fibres, making them more fatigue resistant.
• Deconditioning (e.g., from injury or inactivity) can reduce fast-twitch function, especially in Type IIb fibres.

Training Strategies for Each Muscle Fibre Types

Type I – Train for Endurance

• Goal: Increase aerobic capacity, efficiency, and recovery
• Methods: Long-duration, low-intensity cardio (running, cycling, swimming)
• Reps/sets: High reps, low weight (15–20+ reps)

Type IIa – Train for Strength & Work Capacity

• Goal: Improve both force production and fatigue resistance
• Methods: Moderate to heavy resistance training, circuit training
• Reps/sets: Moderate reps, moderate to heavy load (8–12 reps)

Type IIb – Train for Power & Speed

• Goal: Maximise speed, strength, and neuromuscular coordination
• Methods: Plyometrics, sprints, Olympic lifts, heavy lifting with low reps
• Reps/sets: Low reps, high intensity (1–5 reps at 80–100% 1RM)

Fibre Type Testing: Can You Know Your Ratio?

Direct muscle biopsy testing is rare outside of elite sports science labs. However, you can estimate your fibre dominance through performance indicators:

• If you excel at long, steady exercise, you are likely type I dominant.
• If you thrive in short, intense bursts, then you are more likely type II dominant.
• Functional testing, such as vertical jump height or sprint vs. endurance ratios, can also provide some insight to help determine your dominant fibre type.

Athletic performance is deeply rooted in your muscle fibre composition, but with smart, targeted training, you can develop the full spectrum of fibre types to match your sport or goals.

Whether you’re aiming for a new 10K PB or training for a martial arts competition, understanding and training the right muscle fibres can help maximise your performance.