What type of Muscle Fibers does Aerobic Training Chiefly Work?

Written by Ben Bunting: BA, PGCert. (Sport & Exercise Nutrition) // British Army Physical Training Instructor // S&C Coach.


When you exercise, your body uses different kinds of muscle fibers. Some are called fast-twitch, while others are called slow-twitch.

In this article we shall address the primary muscle fibers engaged during aerobic training.

Aerobic Training 

If you want to improve your aerobic capacity, you must know the difference between the different types of muscle fibers. These fibers are responsible for producing force through contractions.

The ratio between each fiber type is dependent on the function of the muscle. Some of the fibers can be trained and strengthened by specific exercises.

Both slow and fast twitch muscle fibers can be found in the skeletal muscles. Each fiber consists of smaller units called myofibrils. They are also composed of mitochondria, which provide energy. 

Fast-twitch muscle fibers are used in short bursts of strength, such as in a sprint. Slow-twitch fibers are engaged during endurance activities. They have more blood vessels, which means they can last for a longer period of time without losing strength.

Fast-twitch muscle fibers contain a large number of mitochondria, which allows them to produce ATP efficiently.

This enables them to burn through fuel quickly and provide quick peak forces. However, they can fatigue easily and need to be replenished by a source of oxygen.

A type of fiber called the type IIA can use both anaerobic and aerobic energy sources. In contrast, type IIX fibers have the lowest oxidative capacity and use only anaerobic metabolism. 

Type IIA fibers are often used for strength training, and they can use anaerobic glycolysis to produce ATP. They are also used in max lifts and sustained power activities.

Cardiovascular training or aerobic exercise is a great way to boost your heart health. You can work out at the gym, running along the street or even activities at home or even in a swimming pool.

This type of workout is important because it is known to prevent and treat high cholesterol, diabetes, obesity and even heart disease. It can also help you increase your V02max.

The intensity of your aerobic training should be between 60 percent and 80 percent of your estimated maximum heart rate.  

VO2max explained

VO2max is the maximum amount of oxygen that your body can utilise during dynamic aerobic exercise.

It is an important metric for many fitness enthusiasts. A higher VO2 max can help you lower your 5K times, improve your endurance during races, and combat memory loss as you get older.

A VO2max measurement is performed through a graded exercise test. During this test, the athlete wears a mask that connects to a metabolic cart.

This cart measures the concentrations of oxygen and carbon dioxide in inspired and expired air. The metabolic cart also calculates the volume of oxygen used during the workout.

Studies show that people with high VO2 max have a lower risk of developing diseases, such as cancer, and have a longer life span. 

If you are a fitness fiend, you probably already know how to measure your basal metabolic rate (BMR) and other fitness markers.

If you're unsure of how to test your VO2max, a certified personal trainer can do it for you. That said, many products such a fitness trackers can calculate it quite accurately.

Muscle Fibers Explained

Muscle fibers are the specialized cells in our body that facilitate the movement of our limbs. They can be grouped into two categories, the fast-twitch (FT) fibers and the slow-twitch (ST) fibers.

Each category is distinct in their functions and characteristics. It's important to understand the differences between these two types so you can train appropriately.

Fast-twitch muscles are used during explosive or strength-based activities. Their speed at breaking down ATP produces short bursts of power. They are also good for stabilizing bones and joints.

Muscle fibers also work together to help generate movement. A muscle spindle is a group of smaller specialized muscle fibers. There are about five chain fibers and two bag fibers in a typical spindle.

Muscles that are arranged in parallel have more force. This happens because the muscles shorten to counter gravity.

The gluteus maximus and quadriceps in particular contract to lengthen to control motion when you sit or stand up.

In addition to controlling physical forces in the body, they can influence your sports performance training goals. 

Depending on the type of energy system that is being used to fuel the contraction, the muscle fibers will perform differently.

For instance, the glycolytic system rely on a slow rate of anaerobic metabolism to produce ATP.

Another energy system is the oxidative cycle. During this process, oxygen converts to ATP. Although a more efficient method, it can cause early fatigue. Luckily, resistance training can increase the muscle's ability to use the oxidative cycle.

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Slow Twitch Muscle Fibers 

Muscle fibers can be classified by how they generate energy. There are slow-twitch and fast-twitch fibers, and each type can be trained with specific exercises. The slow-twitch fibers have mitochondria, which use oxygen to produce ATP, which fuels muscle contractions.

This type of fiber is able to sustain force for a longer duration during exercise, and it does not experience early fatigue like fast-twitch fibers do.

Training these fibers is beneficial for your cardiovascular health and helps build endurance. You can target them through low-intensity aerobic training.

Different types of exercise also affect the distribution of muscle fiber types. During endurance exercise, AMPK is increased.

This increases the activity of intracellular calcium and the AMPK transcription factor NFAT. Therefore, blocking AMPK during exercise will prevent the shift from fast to slow-twitch fibers.

Although muscle fiber composition is genetically determined, training can activate a higher proportion of a specific type of fiber. For example, by training for 100 meters, an individual recruits more fast-twitch glycolytic fibers. This leads to an improvement in sprint performance and muscular efficiency.

Fast-twitch vs slow-twitch muscles

As previously mentioned, there are two types of muscle fibers. These are slow twitch and fast twitch. It is important for athletes to understand their differences so they can maximize their training.

Slow twitch muscle fibers are used for endurance and long-term muscle contractions. They help burn fat and build endurance during activities such as running. Their ability to provide energy from their own source of fuel, oxygen, allows them to last longer.

Fast twitch fibers are the ones responsible for the rapid contractions needed for sprints. They are also the type of fibers used during high-intensity workouts. High-intensity exercises recruit all of the muscle fibers in the body.

Both types of fibers are critical for daily life. The difference between the two types of muscle fibers is that slow twitch muscles are much more fatigue resistant than fast twitch ones.

During intense exercise, both types of muscle fibers fatigue quickly, but slow twitch fibers are more likely to sustain the exercise for a longer period.

Fast-twitch vs slow-twitch training

Slow twitch fibers are ideal for aerobic exercises. This type of training is often associated with running and cycling.

They are also used in activities that require slow, sustained movements such as walking and posture maintenance.

Fast twitch fibers are best suited for power-based or fast-movement activities. These muscles contract faster, produce more force, but they fatigue quicker. 

Fast-twitch vs slow-twitch fatigue

Fast-twitch muscle fibers are known for their ability to generate force quickly. They are also more efficient in generating ATP fuel. Athletes such as power lifters often rely on fast-twitch muscle fibers.

On the other hand, slow-twitch muscle fibers are more able to sustain long-term activity.

They are fueled by triglycerides, which provide an abundance of ATP for their muscles. Slow-twitch fibers can also help you improve endurance. You can do low-intensity workouts to strengthen your slow-twitch muscle.

In general, both types of muscle fibers are essential for a variety of sports, but they operate in varying ways.

For example, athletes who participate in rock climbing often use a combination of fast-twitch and slow-twitch fibers to help them increase their overall muscle mass.

Slow-twitch fibers can sustain force for an extended period of time 

The aerobic process is a more efficient process than anaerobic respiration, and requires large amounts of oxygen.

However, it can become rate-limiting if the cardiovascular and respiratory systems cannot keep up with the metabolic demand.

This means that the faster the aerobic rate, the more oxygen is required to keep the muscles pumped. Slow-twitch muscle fibers require a constant supply of oxygen and blood. 

Fast-twitch fibers rely on anaerobic respiration

Anaerobic respiration is the process of burning stored carbohydrates, known as glycogen, in the muscle to produce energy.

This process is less efficient than aerobic respiration, and is best suited for quick bursts of movement. However, it also produces lactic acid, which slows down the muscles' ability to produce ATP.

Fast-twitch fibers deplete energy quickly, making it imperative to give them enough rest after a workout to allow the motor units to regenerate and replenish their ATP. 

The transition from aerobic to anaerobic metabolism involves rapid activation of several systems in the muscle.

It also involves redistribution of blood flow and removal of by-products of aerobic respiration. The process also involves the synthesis of pyruvic acid and glucose, which can be useful for aerobic respiration.

However, if the muscle does not receive enough oxygen, these molecules are converted into lactic acid and contribute to muscle fatigue.


Slow twitch muscle fibers are associated with prolonged endurance activities. These include running, swimming, and cycling.

When compared to fast twitch muscles, slow twitch muscles are slower to fatigue and can handle more repetitions. They can also be strengthened by weight training. However, you'll need to pay more attention to them during workouts.

The exact ratio of slow and fast twitch fibers in your body depends on your training and genetics. For example, some people are born with a 50-50 mix of slow and fast twitch fibers.

But even if you're not naturally a slow twitch dominant athlete, you can train your muscles to become stronger and more efficient.

In addition to endurance-based exercise, you can also target your slow twitch muscles with shorter, more intense workouts.

Performing resistance-training exercises with higher repetitions can also engage your slow twitch fibers. You can also challenge your muscles by adding circuit training to your workout routine

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