Twitch muscle fibers are responsible for muscle contraction. They get their name from the ‘twitching’ movement they produce when they contract. There are two types of twitch muscle fibers: fast-twitch and slow-twitch. As you might expect, each type of fiber has its own unique set of characteristics (8). Fast-twitch muscle fibers are used for activities that require quick, explosive movements, such as sprinting or jumping. They contract more quickly than slow-twitch muscle fibers, but they also tire more quickly. Slow-twitch muscle fibers are used for activities that require sustained muscle contraction, such as long-distance running or cycling. These contract more slowly than fast-twitch muscle fibers, but they can do so for a longer period of time before fatigue sets in. Both types of fibers are important for overall fitness and performance. You can train each type of fiber differently to improve your results. Read on to learn more about fast-twitch and slow-twitch muscle fibers, and how you can benefit from training both.
Twitch muscles are defined by several characteristics:
Let’s take a closer look at each of these characteristics for fast-twitch and slow-twitch muscle fibers (2) (5) (7) (8).
Fast-twitch muscle fibers are used for activities that require quick, explosive movements, such as powerlifting, sprinting, jumping, strength and agility training.
Slow-twitch muscle fibers are used for activities that require sustained muscle contraction, such as long-distance running or cycling, swimming, power walking and aerobic training (9).
Fast-twitch muscle fibers are typically larger than slow-twitch muscle fibers.
Slow-twitch muscle fibers are typically smaller than fast-twitch muscle fibers.
Fast-twitch muscle fibers can contract up to 2-3 times faster than slow-twitch muscle fibers.
Slow-twitch muscle fibers contract more slowly than fast-twitch muscle fibers.
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Fast-twitch muscle fibers fatigue more quickly than slow-twitch muscle fibers.
Slow-twitch muscle fibers can sustain muscle contraction for a longer period of time before fatigue sets in.
Fast-twitch muscle fibers generate more force than slow-twitch muscle fibers.
Slow-twitch muscle fibers generate less force than fast-twitch muscle fibers.
Fast-twitch muscle fibers have fewer mitochondria than slow-twitch muscle fibers.
Slow-twitch muscle fibers have more mitochondria than fast-twitch muscle fibers.
Fast-twitch muscle fibers contain less myoglobin than slow-twitch muscle fibers.
Slow-twitch muscle fibers contain more myoglobin than fast-twitch muscle fibers.
Fast-twitch muscle fibers have a less dense network of capillaries than slow-twitch muscle fibers.
Slow-twitch muscle fibers have a more dense network of capillaries than fast-twitch muscle fibers.
Fast-twitch muscle fibers have a higher level of ATPase than slow-twitch muscle fibers.
Slow-twitch muscle fibers have a lower level of ATPase than fast-twitch muscle fibers.
Fast-twitch muscle fibers have a lower oxidative capacity than slow-twitch muscle fibers.
Slow-twitch muscle fibers have a higher oxidative capacity than fast-twitch muscle fibers.
Now that we know the difference between fast-twitch and slow-twitch muscle fibers, let’s understand the working of each type in more detail.
Fast-twitch muscle fibers are also known as Type II muscle fibers. They are further divided into Type IIa and Type IIx fibers.
Type IIa fibers are also called “intermediate” or “fast-twitch oxidative-glycolytic” fibers. They are the most fatigue-resistant of the fast-twitch muscle fibers and have the greatest capacity for aerobic metabolism (10).
This is because they contain more mitochondria and myoglobin than Type IIx fibers. This enables them to produce more energy and use oxygen more efficiently.
Type IIx fibers are also called “fast-twitch glycolytic” fibers. They have the greatest capacity for anaerobic metabolism and generate the most force. But they also fatigue more quickly than Type IIa fibers (10).
Due to their high myosin ATPase activity, low oxidative capacity and lack of myoglobin, they are inefficient.
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Slow-twitch muscle fibers are also known as Type I muscle fibers. They have a higher oxidative capacity than fast-twitch muscle fibers and can use oxygen more efficiently (10).
That’s because they contain more mitochondria and myoglobin. They also have a more dense network of capillaries, which helps to deliver oxygen to the muscle fibers more effectively.
This combination of factors enables slow muscle fibers to produce more energy aerobically and resist fatigue for a longer period of time.
Your body is composed of both fast-twitch and slow muscle fibers. But the distribution of these fibers varies from person to person. And this can impact your athletic performance. There are a few ways to determine if you have more fast-twitch or slow muscle fibers.
If you’re a highly active person, chances are you have more fast-twitch muscle fibers. That’s because these fibers are better suited for the explosive activities that require quick bursts of energy. For example, power athletes have a higher ratio of fast-twitch to slow-twitch muscle fibers (6).
On the other hand, if you’re not very active, you’re likely to have a 50/50 distribution of fast-twitch and slow-twitch muscle fibers. But if you lead a sedentary lifestyle, you may have a higher percentage of slow muscle fibers.
Your genes also play a role in determining the distribution of fast-twitch and slow-twitch muscle.
According to research, the type of muscle fibers you have is mostly determined by your genes (3). So if your parents were endurance athletes, you’re likely to have more slow-twitch muscle fibers. And if they were power athletes, you’re likely to have more fast-twitch muscle fibers.
As you age, the number of fast-twitch muscle fibers in your body decreases. That’s because these fibers are more susceptible to damage, wear and tear. Furthermore, loss of lean muscle mass is a natural part of the aging process (1) (4).
As you get older, you’re likely to have more slow muscle fibers. Because fast twitch muscles are larger and more metabolically efficient, their loss leads to age-related metabolic changes.
You can modify your workout routine to target specific muscle fibers. This can help you improve your athletic performance and reach your fitness goals.
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If you want to develop slow muscle fibers, you need to do endurance training. This type of training increases your body’s ability to sustain prolonged effort. It enables your muscles to use oxygen more efficiently and resist fatigue for a longer period of time (12).
Endurance training can be performed through a variety of activities, including running, cycling, rowing, and swimming.
Some of the key characteristics of endurance training are:
The goal of endurance training is to improve your body’s ability to sustain long periods of exercise, so the resistance used is usually lower than what you would use for other types of training.
In order to improve your endurance, you need to train your muscles to be able to handle long periods of work. This means that the number of repetitions you do will be higher than what you would do for other types of training.
Endurance training sessions are typically longer than other types of workouts, as they need to be able to train your body to sustain exercise for long periods of time.
The intensity of endurance training is usually lower than other types of training, as the goal is to sustain exercise for a long period of time rather than to achieve maximum effort.
During endurance training, it is important to include active recovery periods, where you keep moving but at a lower intensity. This helps to prevent your muscles from becoming too fatigued and allows you to sustain your exercise longer.
If you want to develop fast-twitch muscle fibers, you need to strength train.
Strength training refers to any type of exercise that helps to build muscle and increase strength. This type of training puts a greater demand on your muscles, which leads to an adaptation where your muscles become stronger (11).
Strength training can be performed through a variety of activities, including lifting weights, bodyweight exercises, and resistance band exercises.
Some of the key characteristics of strength training are:
The goal of strength training is to build muscle and increase strength, so the resistance used is usually higher than what you would use for endurance training.
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In order to build muscle and increase strength, you need to train your muscles to handle heavy loads. This means that the number of repetitions you do, will be lower than what you would do for endurance training.
Strength-training sessions are typically shorter than endurance-training sessions. Lifting for 30-45 minutes is usually sufficient to build muscle and increase strength.
The intensity of strength training is usually higher than endurance training, as the goal is to lift heavy weights and achieve maximum effort.
In order to lift heavy weights, your muscles need time to recover. This means that you will rest for 1-2 minutes between sets of strength-training exercises.
Yes, you can develop both twitch muscle fibers. Resistance training triggers hypertrophy (muscle growth) in both types of muscle fibers. However, the type of training you do will affect the relative amount of growth in each type of muscle fiber.
Aside from resistance training, endurance training at high intensity intervals, commonly known as HIIT, has also been shown to trigger hypertrophy in both slow-twitch and fast-twitch muscle fibers.
The predominant type of muscle fiber that is recruited during HIIT exercises is the fast-twitch muscle fibers.
In the process, fast-twitch muscle fibers are also converted into slow muscle fibers. This conversion process is known as myofibrillar remodeling.
Both slow-twitch and fast-twitch muscle fibers are important for optimal performance. However, the type of training you do will affect the relative amount of growth in each type of muscle fiber. Other factors that affect muscle fiber type are age, genetics, and gender.
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