What all Athletics Coaches Should Understand About Training and Muscle Fibres
Applied findings for Track and Field coaches based on Plotkin et al., 2021 Muscle Fiber Type Transitions with Exercise Training, Find the full article on Sports
Knowledge of muscle fibre types and how they can change is valuable for personalising training programs for different athletes’ goals. Understanding how different training modalities affect fibre type composition can help tailor programs for endurance, strength, power, or a combination of these qualities.
What are the Main Types of Muscle Fibres?
Human muscle fibres are categorised based on their myosin heavy chain (MHC) isoforms, which determine their speed and fatigue resistance. The main types are:
Type I (slow-twitch): Contract slowly but are highly resistant to fatigue, making them ideal for endurance activities like long-distance running.
Type IIa (fast oxidative glycolytic): Contract faster than type I fibres with moderate fatigue resistance, suited for activities requiring both speed and endurance.
Type IIx (fast glycolytic - also listed as IIb in some sources): Possess the fastest contraction speed but fatigue quickly, ideal for short bursts of powerful movements like sprinting or weightlifting.
However, things aren’t so clear-cut in reality. Hybrid fibres expressing multiple MHC types are common, meaning a single fibre can exhibit characteristics of different types
Can Training Increase the Total Amount of Muscle Fibres?
Yes, studies cited in the review support the view that training can increase the total amount of muscle fibres and change the proportional type of muscle fibres within an individual. This may occur by two different processes:
Activation of satellite cells: Satellite cells are muscle stem cells located on the periphery of muscle fibres. They play a role in muscle growth and repair. Intense training, particularly resistance training, can activate satellite cells, leading to the formation of new muscle fibres.
Splitting of existing muscle fibres: If a hybrid fibre undergoes a significant shift towards a single MHC isoform, there is evidence it may split into two distinct fibres, each expressing a pure form of the dominant isoform, effectively increasing the total number of fibres.
Can Different Forms of Training Change the Proportion of Muscle Fibres?
Yes. Training can evoke an adaptation where the proportion of muscle fibres change in response to the needs of the indiidual. For example, long distance running typically leads to a shift towards the more oxidative, fatigue-resistant Type I fibres.
⏫ Type I Muscle Fibres
Increases the amount and proportion of slow-twitch fibres and Type I/IIa hybrids, making muscles more fatigue-resistant, aligning with the increased oxygen demands of endurance exercises.
Example Case Study: Novice marathon runners saw a significant increase in Type I fibres in their vastus lateralis (quadricep) after 13 weeks of training. (Luden et al, 2012)
⏫ Type IIa Muscle Fibres
⏬ Type IIx Muscle Fibres
When performed with high loads and slower movement, resistance training transforms Type IIx and hybrid fibres into Type IIa fibres and increases the total number of muscle fibres.
Example Case Study: Six weeks of strength training increased Type IIa and decreased Type IIx fibres in the triceps brachii. (Widrick, 2002)
⏫ Type IIa Muscle Fibres
⏬ Type I Muscle Fibres
Results in a greater number and proportion of Type IIa fibres and a reduction in Type I fibres, while maintaining the number of Type IIx fibres.
Example Case Study: Combination training (ballistic, plyometric, and strength exercises) showed an increase in Type IIa fibres and a reduction in Type I fibres, without the loss of Type IIx fibres seen in traditional resistance training alone (Andersen, 1994).
Detraining Effects
While we usually associate detraining with lower levels of performance, it is a little more complicated than that when it comes to muscle fibres! Inactivity generally leads to a shift towards faster MHC expression, meaning a proportional increase in Type IIx fibres.
Interestingly, after a period of training and detraining, there can be a “Type IIx fibre overshoot” where the proportion of these fibres exceeds the baseline level. This is particularly relevant for athletes who want to obtain a greater proportion of Type IIx fibres and coaches may use periods of strategic deloading close to important competitions (tapering), to try and achieve this effect.
Track and Field Requirements
Sprinters and jumpers rely heavily on fast-twitch fibres (Type IIa and IIx) for explosive power and speed.
Middle-distance runners need a balance of fast-twitch (Type IIa/IIx) and slow-twitch fibres (Type I) for a combination of speed and endurance.
Long-distance runners, wheelies, and walkers benefit from a higher proportion of slow-twitch fibres (Type I) for sustained effort and fatigue resistance.
Throwers will generally have a proportionally greater demand for Type IIx fibres.
Individualisation Considerations
Every athlete has a unique muscle fibre type distribution influenced by genetics and training history.
1. Training History:
An athlete's training background influences their current muscle fibre type profile.
Individuals with a history of endurance training likely have a higher proportion of Type I and Type I/IIa hybrid fibres.
Those with a background in strength or power training likely have a greater proportion of Type IIa fibres.
Coaches should consider this history when designing training programmes, building on existing adaptations and gradually introducing new stimuli to avoid drastic shifts that could be detrimental to performance.
2. Training Responsiveness:
Individuals respond differently to training. Some athletes exhibit greater fibre type plasticity, meaning they adapt more readily to training stimuli.
Factors influencing responsiveness include genetics, age, and training status. Younger athletes and those new to training tend to show greater adaptability. Well-trained athletes generally demonstrate lower rates of muscle fibre type shifting. Coaches need to monitor athletes' progress and adjust training plans based on individual responses. This includes assessing performance improvements, fatigue levels, and recovery rates.
3. Genetics:
Genetic predisposition plays a significant role. Some individuals naturally have a higher proportion of fast-twitch fibres, making them predisposed to power and speed events, while others have a higher proportion of slow-twitch fibres, favouring endurance activities. While it's helpful to have an understanding of an athlete's baseline fibre type composition, there are no practical ways of doing this outside of a laboratory, other than using your own observation and intuition about an athlete.
Summary
While understanding a little bit about muscle fibres and how they can adapt through the training process, there is an absence of research looking specifically at the Track and Field context. Athletics coaches need to integrate this scientific understanding with your knowledge of your events, athlete assessment skills, and training program design expertise to address individual factors and maximise your athletes’ development.