Fast Twitch and Slow Twitch Muscle Fibres: What Are They and Can We Specifically Train Them?
- Andrew Parks
- Jun 18
- 8 min read
If you’re a sports fan or have a history of exercising yourself, you are likely familiar with the terms “fast twitch fibres” and “slow twitch fibres” when it comes to describing athletic performance. Examples of sports requiring more “fast twitch” outputs would be things like sprinting and Olympic weightlifting. Sports requiring more “slow twitch” output would be sustained endurance activities like cycling and marathon running.
But have you ever wondered what fast and slow twitch really means? Or why some people seem to naturally have more fast twitch fibres than others? Or even wondered if training can improve the function of specific fibre types? In this week’s newsletter I hope to answer some of these questions. I also hope to help you understand the difference between fibre types and why it may be important to use more specificity in our approach to exercise in order to optimize performance, health and longevity.
Understanding the Muscle Fibre Type Spectrum
Human skeletal muscle is composed of various fibre types, commonly grouped into Type I (slow-twitch, oxidative), Type IIa (fast twitch, oxidative-glycolytic), and Type IIx (fast twitch, glycolytic). The word “twitch” refers to the speed and force of contraction that the different fibre types can perform. While these categories are very much simplifications of a much more nuanced reality, they offer a useful lens for understanding muscle function and the foundations for designing specific training programs whether you’re an athlete or just looking to improve your performance in daily life.
Type I muscle fibres are the fibres we use during more aerobic, sustained and endurance activities. These fibres are “slow twitch” meaning they contract slower and with less force, however they have the most fatigue resistance thus allowing them to remain active for extended periods of time. Type I fibres are oxidative in nature (e.g., use oxygen to produce energy), and have the highest mitochondrial density and an elaborate capillary network which allows them to utilize oxygen to produce energy and force. These would also be the primary fibres used in activities of daily life like walking, cooking and fidgeting. They would also be used in sustained activities like running or cycling.
Type IIa muscle fibres are very versatile and adaptable. These are also sometimes referenced as intermediate fibres as they have some characteristics of both fast and slow twitch fibres. They can generate more force than Type I fibres but they also recover faster than their Type IIx counterparts (see below). Type IIa fibres are active in repeated bouts of sub-maximal intensity, such as strength training with moderate loads, or sports requiring both speed and endurance. For example, they would be highly active in sports like hockey, lacrosse, or during high effort interval running or cycling.
Type IIx muscle fibres are the fastest, largest, and most powerful muscle fibre type, but also the most easily fatigued. These fibres are recruited during maximal efforts, like sprinting, jumping, and lifting near-maximal loads. These are the classic “fast twitch” fibres that we think of when we see sprinters, high jumpers and Olympic weightlifters performing fast and explosive movements in their sport. The anaerobic metabolism (e.g., in the absence of oxygen) in Type IIx fibres allows them to deliver large bursts of energy however can only be sustained over short duration.
There are also hybrid fibres which exhibit the characteristics of multiple fibre types, however the ability to quantify the proportion of these fibres has been challenging in past research as not all fibre identification techniques in the research setting are created equally.
Do Muscle Fibres Adapt to Training?
The human neuromuscular system is highly plastic. Reviews discuss how specific types of training can shift the metabolic, structural and molecular properties of muscle fibres which ultimately results in changes in function and performance. With a consistent and specific training stimulus over a given period of time, fibres can shift their characteristics along the continuum from slow to fast. For example, a predominantly sedentary lifestyle often leads to a decline in fast-twitch fibres and a shift toward slower, less powerful muscle profiles as well as a generalized loss of lean mass known as sarcopenia. Conversely, training – particularly with resistance and power-focused movements – can stimulate shifts from more of a slow twitch fibre bias to fast twitch, while also amplifying the effects of existing fast twitch fibres.
It’s important to note that a large degree of fibre composition is genetically predetermined. If you’ve ever watched the Olympics and observed athletes across different domains of sport, you’ve likely seen this first hand. Think about it, have you ever stumbled across the men’s or women’s marathon and thought, “these runners must be built for this to be able to run at this pace, for this long”? Or in contrast when you witness the speed, power and explosiveness seen in 100m sprinters or high jumpers. Are they built for their sport?
An oversimplified answer to this question would be yes, they are built for it. However, it’s also the years of specific training that has prioritized the development and optimization of the fibre types that makes them world class in their respective sport. So really, it comes down to understanding the interplay between genetics and the principle of training specificity – a training principle that states your body and physiology will adapt to the specific training stimulus you expose it to.
But is optimizing one fibre type best for our long-term health and aging?
Aging, Sarcopenia, and the Decline in Power and Strength
As we age, the natural progression of sarcopenia begins to affect our physical performance in athletics but also potentially in our daily lives. Research suggests that we lose approximately 3-8% of muscle mass per decade after the age of 30, with even steeper declines in strength and power after the age of 50.
Sarcopenia and the loss of lean mass is unfortunately a natural process of aging, but the exaggerated loss of strength and power is facilitated by a more rapid loss of type IIa/IIx fibres. These fibres are not recruited in everyday activities like walking or standing and become highly susceptible to atrophy with disuse. Over time, the motor neurons that innervate these fibres withdraw, and unless stimulated, become dysfunctional. This leads to a progressive loss of power – the ability to produce force quickly – which is one of the strongest predictors of falls, frailty, and reduced functional independence in older adults.
In order to prevent this, we must continue to stimulate these fast twitch fibres via faster and more explosive forms of exercise. This calls for a paradigm shift in how we approach exercise as we age, as we tend to neglect things that maintain our power, strength and stability. Intentionally exposing ourselves to fast more dynamic movements that challenge and stimulate these fast twitch fibres and the neurons that control them will not only improve our physical and neurological health, but may also significantly reduce the downstream burden of reduced independence, falls, hip and pelvic fractures, and generalized frailty and poor health.
Training for power doesn’t have to be complex or intimidating though.
How do we Train for Specific Fibre Type? Practical Applications
As I mentioned above, the principle of specificity states that adaptations are specific to the stimulus applied. If we want to preserve endurance, we train for endurance. If we want to maintain strength and power as we age, we must deliberately train for these qualities.
Unfortunately, it is often assumed that we shouldn’t be training these qualities as we age, as it may be dangerous. Blasphemy! I would argue that it’s actually much more dangerous to neglect these qualities as in doing so we will lose the ability to catch ourselves if we trip, get up without assistance, climb stairs and main our independence. Reductions of these functions lead to an increased risk of falls, isolation and deterioration of our mental and cardiometabolic health. Not good. So, we must be thinking more downstream when it comes to our exercise and its effect on our health.
Type I Fibre Training
Type I fibres respond best to prolonged, low-to-moderate intensity efforts with high “time under tension” and frequent activation. This includes aerobic activities such as:
Zone 1-2 cardiovascular exercise (e.g., cycling, rowing, running at conversational pace)
Steady-state hikes or walks
Low-load, high-repetition resistance training with short rest intervals
Generally speaking, if your training intent is to improve cardiovascular health, mitochondrial function, and recovery, you are also likely providing an adequate stimulus to the Type I fibres in the muscle being used.
Type IIa Fibre Training
Type IIa fibres thrive on a combination of mechanical load and metabolic stress. They are best developed through moderate-to-heavy resistance training with moderate rest periods, such as:
Traditional strength training (e.g., 6–12 reps at 65–85% 1RM)
Circuit training with controlled rest
High-intensity interval training (HIIT)
Threshold based cardiovascular work like cycling or running
This type of training promotes muscular hypertrophy (cellular growth), enhances glycolytic capacity and metabolic by-product clearance, and improves force production capacity. It is a great way to improve your higher effort performance as well as long-term metabolic and musculoskeletal health, particularly when paired with proper recovery.
Type IIx Fibre Training
Training Type IIx fibres requires high-to-maximal intensity, explosive movements that activate the highest-threshold motor units. This includes:
Sprinting (e.g., short hill sprints or cycling sprints)
Olympic lifts or dynamic barbell movements (e.g., cleans, snatches)
Plyometrics (e.g., box jumps, drop jumps, bounding)
Heavy lifts performed with maximal intent
Fast-twitch fibres innervated by the largest motor units require maximal effort to be recruited, as per the Henneman Size principle. They respond best to low volume, high-intensity efforts with long rest intervals. While Type IIx fibres are the most vulnerable to age-related decline and atrophy, they are arguably the most trainable when exposed to the appropriate stimulus. So, don’t forget to add some fast, dynamic and explosive movements into your routine. It doesn’t need to be complex, you can do this just with your body weight!
Neutralizing Sarcopenia: A Clinical and Performance Perspective
Sarcopenia – the age related loss of lean mass – is not inevitable. Studies show that older adults – even those who are previously sedentary – can significantly improve muscle mass, strength, and function through progressive resistance training as they age. When it comes to strength vs. power, reviews suggest that power-based training may offer even greater benefits for older individuals than traditional strength training alone as they are more strongly associated with functional outcomes like leg strength, walking distance, rising from a chair and others.
This has real-world implications. For aging individuals who want to hike, play tennis, ski, or simply enjoy an active lifestyle, training for both strength and power is not optional—it’s essential. It’s also essential for the basic tasks and demands of daily life that afford us out independence and ability to safely navigate with purpose.
Putting It All Together: Specificity with Purpose
Every training program should be built on the principle of specificity: train the fibres you want to preserve. A well-rounded, performance-focused program should therefore include:
Low-to-moderate intensity aerobic training (to target Type I fibres and support cardiometabolic health and recovery)
Strength training using moderate-to-heavy loads (to target Type IIa fibres and improve lean mass, joint health and resilience)
Power and speed work with high-to-maximal intent (to recruit and preserve Type IIx fibres, support coordination, delay neuromuscular atrophy and improve neurological health)
The blend and emphasis of these components will vary based on goals, age, and training history—but each has a role in maintaining our overall health and preventing age related decline like sarcopenia, frailty and neurodegenerative disease.
Conclusion
Training for muscle fibre specificity is not just about building bigger muscles or improving sport performance – it’s a cornerstone of lifelong health and movement variability as we age. Understanding how muscle fibres differ, how they change over time, and how to train them effectively allows us to build targeted programs that will help extend out active years.
Remember that as we age, it is power – not just endurance and strength – that declines and impacts functional ability the most. By integrating fibre-specific training strategies to maintain power, we can mitigate sarcopenia, preserve high-threshold motor units, and continue to move, play, and live with intensity for decades to come.
Here's to continuing to push ourselves during exercise well into our adulthood!
Yours in good health,
Andrew
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