FAST-TWITCH VS. SLOW-TWITCH MUSCLE FIBER TYPES + TRAINING TIPS

What Is A Muscle Fiber?

Muscle fibers are single muscle cells that help the body perform a specific physical function. Like muscles themselves, not all muscle fibers are the same. We’re focusing on skeletal muscle fibers, but there are also smooth and cardiac muscle fibers.

There are seven primary types of skeletal muscle fibers, including fast-twitch and slow-twitch. They each have different functions that are important to understand when it comes to movement and exercise programming.

Muscle Fiber Type Comparison Chart

*ATP (adenosine triphosphate) is the body’s energy currency. It provides energy for the muscle cell to contract. Type II muscle fibers have more readily available ATP. Type I fibers rely on aerobic respiration (oxygen delivery) to produce ATP in the muscle cells.

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**Oxidative capacity refers to how much oxygen a gram of muscle uses in an hour.

Slow-Twitch Muscle Fibers

Slow-twitch muscle fibers are the muscle cells responsible for endurance movements.

Think of the story of the tortoise and the hare. Slow-twitch or type I muscle fibers are like the tortoise. They don’t produce a lot of power, but they’re also resistant to fatigue and can contract for a long time. Slow-twitch muscle fibers help with a lot of your daily movements, like walking, cleaning your house, or sitting upright in a chair.

Type I muscle fibers get most of their energy (ATP) from aerobic respiration, meaning they need oxygen to function. The oxygen makes the muscle fibers look red, which is why slow-twitch fibers are sometimes called red fibers. Type I muscle fibers have a much better blood supply (and ability to receive oxygen) than type II fibers. They also have a high concentration of mitochondria which is the powerhouse of a cell where aerobic respiration takes place.

Because slow-twitch muscle fibers use oxygen to produce energy, they are more resistant to fatigue. Type I muscle fibers are responsible for endurance activities such as distance running, swimming, cycling, hiking, low-to-moderate intensity dancing, and walking.

Fast-Twitch Muscle Fibers

Fast-twitch muscle fibers are the muscle cells responsible for short, powerful movements.

Thinking back to the tortoise and hare, your fast-twitch (or type II) fibers are like the hare. They can produce a lot more force and power for a short time, but they get fatigued fast.

Type II muscle fibers are broken down into type IIx and IIa.

Type IIx fibers produce force that’s much greater than type I fibers. However, they use anaerobic (without oxygen) metabolic pathways to get their energy (ATP). That means they receive less blood flow and oxygen and can only produce force for short periods of time.

Type IIa muscle fibers are like a hybrid of type I and type IIx. They have elements of both fiber types. For example, they use both aerobic and anaerobic pathways and produce a medium amount of power for a medium amount of time.

When your body moves, it will use slow-twitch muscle fibers first. Then, if those fibers can’t produce enough force, the body will use fast-twitch type IIx and IIa fibers to get more power.

So, if your fitness goals involve strength and power, you’ll want to focus on training type II muscle fibers. Technically, any resistance training will train both type I and type II fibers, but training with heavier loads (at least 70% of 1 RM) or lighter weights with explosive tempos are the best ways to activate and train type II fibers. These fibers also tend to achieve hypertrophy (muscle growth) easily, which can be important for bodybuilders.

What's Your Muscle Fiber Type?

There’s a lot of controversy surrounding this topic. Everyone has a mix of type I and type II muscle fibers, but how much you have of each type depends on the person. Unfortunately, the only accurate way to know which mix of fibers you have is via muscle biopsy, and that only gives you the result for the specific one chunk of muscle sampled.

Most experts agree that the distribution of muscle fiber types depend on the primary function of the muscle in question, as well as the person’s:

• Activity level,
• Age,
• And genetics

Activity Level

Your activity level and the types of activities that you do can affect how much you have of each muscle fiber type. For example, endurance athletes usually have a higher proportion of slow-twitch muscle fibers. And strength or power athletes usually have a higher amount of fast-twitch muscle fibers. But the exact proportion of each fiber type can range from 15 to 85% of one type or the other, and the distribution also highly depends on the muscle.

There’s also a theory that people who genetically have a higher percentage of slow-twitch fibers might be drawn to endurance activities, and people with more fast-twitch are drawn to power-based activities (10).

Age

Muscle fiber type is also heavily influenced by the aging process. The percentage of type II muscle fibers tend to decline with age. People usually reach peak muscle mass is typically achieved by the age of 30, which means they have a higher percentage of type II muscle fibers.

Women experience a rapid decline in muscle mass post-menopause. Men have a more gradual decline in muscle mass during and after their 40s. That means that as most people age, they have a higher number of slow-twitch muscle fibers. However, humans still need to have some muscle strength as they age, which is why most experts recommend that older people continue to do strength training exercises (11).

Can Muscle Fibers Change?

In short, yes – but it’s complicated. Whether or not muscle fiber types can change is currently a hot (and controversial) area of research. As we mentioned, experts agree that age, activity level and type, and genetics all influence muscle fiber type distribution. People can only really control one of these factors: activity level and type.

Recent research suggests that people can change their muscle fiber ratio a bit through training. For example, endurance training can change the distribution to favor slow-twitch muscle fibers over fast-twitch muscle fibers. And strength or power training can change the distribution to favor fast-twitch muscle fiber types. However, there is no specific training program or periodization scheme that is researched and proven to make this shift.

It’s hard to draw solid conclusions from current research because many of these studies have small sample sizes, short study durations, differences in genetics between individuals, and other issues. However, it is important to bear in mind that fiber type change may be possible, so it is a good idea to train with your specific goals in mind.

Slow-Twitch Muscle Activities

Endurance-based activities require muscles to use a low amount of power over a longer time, so they typically use slow-twitch (type I) muscle fibers. The activities include:

• Walking
• Hiking
• Running
• Swimming
• Cycling (low to moderate intensity)
• Running (low to moderate intensity)
• Dancing

Slow-Twitch Muscle Exercises

During resistance training, lift lighter loads (below 70% 1RM) at a higher repetition (12-20 reps) to primarily recruit slow-twitch muscle fibers. Any low-speed resistance training exercise will do, because all muscles have a mix of type I and II fibers.

Some examples of slow-twitch stimulating exercises include:

• Slow tempo goblet squats
• Banded kettlebell hinges
• Box step up to balance
• Eccentric push ups
• Single leg/single arm row
• Blackburn T/Y

Fast-Twitch Muscle Activities

Strength- and power-based activities typically use more type IIx and some IIa muscle fibers. These activities require a large amount of force to be produced at once with little need for fatigue resistance. Some activities that use type II muscle fibers include:

• Sprints
• Olympic lifting
• Powerlifting
• Plyometrics

Fast-Twitch Muscle Exercises

To stimulate fast-twitch muscle fibers, lift higher loads (more than 70% 1RM) at lower repetitions (1-12) or use lighter weight with explosive tempos. Some examples of fast-twitch stimulating exercises include:

• Heavy barbell squat
• Heavy barbell bench presses
• Medicine ball slams
• Chest pass
• Box jumps

Hybrid Muscle Fibers and Tapering

Most people have high numbers of hybrid muscle fibers (type IIa) that produce a medium amount of power and have medium fatigue resistance. These fibers tend to be influenced more by training because they operate as fast-twitch fibers in untrained people and slow-twitch fibers in endurance-trained people. Rather than specifically trying to target type IIa fibers with training, train for your sport or activity and allow these fibers to adjust automatically (12).

Fast-twitch fibers can also recruit slow-twitch fibers: endurance training at high-intensity intervals can be effective in improving aerobic power (2), (6).

Tapering during training programs (reducing volume and intensity), can also improve the strength and power of type IIA fibers without decreasing type I performance (9).

One study investigated muscle fiber changes in recreational runners training for a marathon. After 13 weeks of increasing mileage and a three-week tapering cycle, not only did the functions of type I and type IIa fibers improve, but type IIa continued to improve significantly during the tapering cycle (9).

Personal trainers get paid to help people stay active and achieve their goals. National Academy of Sports Medicine’s Certified Personal Trainer course will teach you science like this so you can create informed, customized training plans for your clients.

3 Sports Performance Blogs To Check Out

• How Hardgainers Can Add Size and Weight - NASM
• 10 Effective Shoulder Pain Exercises: Correct Your Shoulder Today! (nasm.org)
• Blood Flow Restriction Training: Effective or Not? [Dec 2021] (nasm.org)

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