Fast and Slow Twitch Muscle Fibre and What This Means.
The natural makeup of your muscle fibres may influence what sports you are naturally good at and how well you respond to training. Fast-twitch muscle fibres contract quickly but tire rapidly. Slow-twitch muscle fibres contract slowly but also tire slowly. Having a greater proportion of fast-twitch fibres can help you excel at activities that require bursts of power and strength and may make you a better sprinter or weightlifter. Having more slow-twitch fibres can help you become an accomplished endurance athlete, such as a distance runner or cyclist.
How Do Muscle Fibres Differ?
Skeletal muscle is made up of bundles of individual muscle fibres called myocytes. Each myocyte contains many myofibrils, which are strands of proteins (actin and myosin) that can grab on to each other and pull. This shortens the muscle and causes muscle contraction.
It is generally accepted that muscle fibre types can be broken down into two main types: slow-twitch (type I) muscle fibres and fast-twitch (type II) muscle fibres. Fast-twitch fibres can be further categorized into type IIa and type IIb fibres (also known as IIx fibres). There are also hybrid fibres with more than one type.
These distinctions seem to influence how muscles respond to training and physical activity, and each fibre type is unique in its ability to contract in a certain way. Human muscles contain a genetically determined mixture of both slow and fast fibre types.
On average, people have about 50% slow-twitch and 50% fast-twitch fibres in most of the muscles used for movement.
The Three Energy Systems
Muscles require ATP to contract. Because the body stores only a small amount of ATP, it must continually replace it through three overlapping energy systems:
The immediate energy system.
The lactic energy system.
The aerobic energy system.
All three operate at the same time. The intensity and duration of the activity determine which system contributes the most.
The Immediate Energy System
The immediate energy system, scientifically known as the ATP–phosphocreatine or phosphagen system, uses ATP and phosphocreatine already stored inside the muscles.
Also known as the Anaerobic energy system, it:
Supplies energy almost immediately.
Produces energy very quickly.
Does not require oxygen.
Does not produce lactate.
Can operate at maximum output for only a short time.
A single shot, discus or javelin throw relies mainly on this system. Its limited stores must then be replenished, which is why suitable recovery between maximum-effort throws and strength exercises is important.
The Lactic Energy System
The lactic energy system, scientifically known as the anaerobic glycolytic system, rapidly breaks down glucose or stored glycogen to produce ATP without using oxygen directly.
It contributes more during:
Repeated throws with short recovery periods.
Longer throwing drills.
Repeated sprints or jumps.
High-repetition strength exercises.
Intense conditioning circuits.
This system produces lactate, rather than a damaging pool of lactic acid. Lactate can be transported and reused as fuel. It is also not responsible for the soreness experienced one or two days after training.
The Aerobic Energy System
The aerobic energy system, scientifically known as the oxidative system, uses oxygen to produce ATP from carbohydrates and fats.
It produces energy more slowly than the other systems but can continue for much longer. It is the main system used during activities such as walking, steady running and cycling.
Although the throw itself is not aerobic, aerobic fitness helps throwers:
Recover between throws and training sets.
Restore phosphocreatine supplies.
Keep moving during long competitions.
Maintain concentration and movement quality.
Cope with lengthy training sessions or multiple events.
A throw may last only seconds, but the complete competition can last several hours.
Muscle Fibre Types and Energy Systems
Every skeletal muscle contains a mixture of different muscle fibre types. The main types are:
Type I: Slow-twitch, fatigue-resistant fibres.
Type IIa: Fast-twitch fibres that combine power with some resistance to fatigue.
Type IIx: Very fast, powerful fibres that fatigue quickly.
Hybrid fibres: Fibres with characteristics of more than one type.
Genetics influences the mixture an athlete has, but training can improve the strength, size and efficiency of the fibres.
Type I: Slow-Twitch Fibres
Type I fibres contract relatively slowly and produce less power, but they can continue working for a long time.
They rely mainly on the aerobic energy system and are useful for:
Posture and joint stability.
Warm-ups and continuous movement.
Longer training sessions.
Recovery between throws.
They do not provide most of the power in a throw, but they help support the athlete throughout training and competition.
Type IIa: Fast-Twitch Fibres
Type IIa fibres contract quickly and produce more force than Type I fibres. They are also reasonably resistant to fatigue.
They use all three energy systems:
The immediate energy system for explosive effort.
The lactic energy system during repeated intense movements.
The aerobic energy system to support recovery.
These fibres are important for repeated throws, strength exercises, sprints and plyometric training.
Type IIx: Fast-Twitch Fibres
Type IIx fibres contract very quickly and produce high levels of force and power. However, they fatigue rapidly.
They rely heavily on:
The immediate energy system during a single maximum effort.
The lactic energy system when intense efforts are repeated with short rest periods.
These fibres are particularly important for explosive activities such as shot, discus and javelin.
Type IIx fibres are sometimes called Type IIb, but Type IIx is the more appropriate term in humans.
Hybrid Fibres
Some fibres contain characteristics of more than one type, such as Type I/IIa or Type IIa/IIx. Their characteristics can change in response to training, inactivity and recovery.
What This Means for Throwers
All three energy systems work together:
A single throw relies mainly on the immediate energy system.
Repeated high-intensity throws and drills increase the contribution from the lactic energy system.
The aerobic energy system supports recovery between attempts and helps athletes cope with long training sessions and competitions.
Fast-twitch fibres produce much of the force and speed required for throwing, but slow-twitch fibres and aerobic fitness help the athlete remain stable, recover and repeat high-quality efforts.
Does Muscle Fibre Type Affect Sports Performance?
Olympic athletes tend to fall into sports that match their genetic makeup. Olympic sprinters have been shown to possess about 80% fast-twitch fibres while those who excel in marathons tend to have 80% slow-twitch fibres.
Research on sports performance within families also suggests that genetic factors may influence about 30% to 80% of the differences among individuals.
However, genetics are just one aspect. Fibre type is part of a great athlete's success, but it alone is a poor predictor of performance. Environment and training matter too. There are many other factors that go into determining athleticism, including mental preparedness, proper nutrition, and hydration, getting enough rest, and having appropriate equipment and conditioning.
Can Training Change Your Muscle Fibre Type?
There is some evidence showing that human skeletal muscle may switch fibre types from "fast" to "slow" due to training or transition from hybrid to pure fibres.2 This is not entirely understood, and research is preliminary and still looking at that question.
One very limited study showed the muscles fibres of a set of twins. One had trained as an endurance athlete for decades and the other was sedentary for decades. Researchers found that the endurance athlete had 55% more slow-twitch fibres than the sedentary twin. Also interestingly, the untrained twin had more fast-twitch and hybrid fibres.
Although genetic differences may be dramatic at the elite levels of athletic competition, following the scientific principles of conditioning can dramatically improve the personal performance of a typical athlete.
With consistent training, muscle fibres can develop more and improve their ability to cope with and adapt to the stress of exercise. With consistent effort, they may even have the potential to shift types to favour your sport over time.
