Fast-twitch muscle fibers, known as Type II muscles, are a category of skeletal muscle fibers in the human body. Muscles consist of fibers, and electric impulses from the brain stimulate the fibers to produce movement in the form of contractions. Fast-twitch muscle fibers contract quickly and help you perform fast, high-intensity activities for short periods, like sprinting, jumping jacks, box jumping, and weightlifting and strength-training workouts. Fast-twitch muscles are a large proportion of the small muscles in hands and eyes for rapid movement.

There are two subcategories of fast-twitch muscle fibers: Type IIA and Type IIB or Type IIX. Type IIB creates the fastest and most powerful muscle contractions, but these are also the least efficient as they tire out the fastest. Type IIA are quicker to recover but not as vigorous as Type IIB.

Slow-twitch, or Type I muscle fibers, are skeletal muscle fibers that slowly contract. Slow-twitch muscle fibers support everyday actions, like standing from a seated position and maintaining normal posture. In fitness and exercise, slow-twitch muscle fibers are necessary for endurance athletes such as marathon runners, long-distance runners, swimmers, and cyclists. Muscle groups like the erector spinae in the lower back and soleus muscles in the legs have a high concentration of slow-twitch muscle fibers. 

Skeletal muscles are made up of a combination of slow-twitch and fast-twitch muscle fibers, but in different proportions. Differences between these two types of skeletal muscle fibers include: 

- __Performance__: Fast-twitch muscle fibers are faster acting, but tire more quickly in comparison to slow-twitch muscle fibers. 
- __Energy__: Fast-twitch muscle fibers produce small amounts of energy rapidly, which works best for [HIIT exercises](https://www.masterclass.com/articles/hiit-workout-guide). Slow-twitch muscles slowly produce larger amounts of energy, supporting endurance activities, like long-distance running.
- __Physiological__: Slow-twitch muscle fibers have more blood vessels and capillaries, so they are more efficient in energy usage in comparison to fast-twitch fibers. 
- __Oxygen__: Fast-twitch muscles are relatively anaerobic, meaning they use little to no oxygen, and instead rely on different sources of bodily energy, such as glucose, which they use to make adenosine triphosphate (ATP). ATP is an essential chemical, present in all living organisms, that acts as a kind of energy shuttle within the cell, making activities possible, such as movement. Type IIA fast-twitch muscles are technically glycolytic, meaning they use oxygen and glucose as fuel. Type IIB, the fastest and most forceful, but also the least efficient, have low oxidative capacity and high myosin ATPase activity (they act very quickly but also tire quickly). Slow-twitch muscle fibers are aerobic and have higher concentrations of mitochondria and myoglobin, proteins that bind oxygen.
Each type of muscle fiber has different functions, and developing them requires different kinds of training and conditioning.

- __Slow-twitch muscle fibers__: To get the most out of your slow-twitch muscle training, concentrate on endurance training activities and slower weight training with higher reps. Running, jogging, hiking, rowing, and swimming are all good ways to strengthen your slow-twitch muscles. With weight training, choose higher numbers of reps, shorter rest periods, and slower motions. When resistance training, try to lengthen the amount of time spent under tension.
- __Fast-twitch muscle fibers__: For building fast-twitch muscle mass and strength, focus on high-intensity, low-duration exercises. If training with weights, complete lower reps as fast as you safely can. Uphill sprints such as stair climbing are great for building this type of muscle, as are [box jumps](https://www.masterclass.com/articles/box-jumps-guide), kettlebell exercises, and jump squats. 
Throw on some athleisure, fire up a [MasterClass Annual Membership](https://www.masterclass.com), and get ready to sweat it out with exclusive instructional videos from Nike Master Trainer and *GQ* fitness specialist Joe Holder. Want to improve your cardiovascular endurance? Give Joe’s HIIT workout a go. Trying to get a little swole? He’s got a strength training workout for that. From fitness tips to nutrition hacks, Joe will have you feeling healthier in no time.
Learn the differences between fast-twitch and slow-twitch muscles in fitness.

Learn the differences between fast-twitch and slow-twitch muscles in fitness.

How to Train Fast-Twitch and Slow-Twitch Muscle Fibers

Fast-twitch muscle fibers, known as Type II muscles, are a category of skeletal muscle fibers in the human body. Muscles consist of fibers, and electric impulses from the brain stimulate the fibers to produce movement in the form of contractions. Fast-twitch muscle fibers contract quickly and help you perform fast, high-intensity activities for short periods, like sprinting, jumping jacks, box jumping, and weightlifting and strength-training workouts. Fast-twitch muscles are a large proportion of the small muscles in hands and eyes for rapid movement.There are two subcategories of fast-twitch muscle fibers: Type IIA and Type IIB or Type IIX. Type IIB creates the fastest and most powerful muscle contractions, but these are also the least efficient as they tire out the fastest. Type IIA are quicker to recover but not as vigorous as Type IIB.

What Are Fast-Twitch Muscle Fibers?

Slow-twitch, or Type I muscle fibers, are skeletal muscle fibers that slowly contract. Slow-twitch muscle fibers support everyday actions, like standing from a seated position and maintaining normal posture. In fitness and exercise, slow-twitch muscle fibers are necessary for endurance athletes such as marathon runners, long-distance runners, swimmers, and cyclists. Muscle groups like the erector spinae in the lower back and soleus muscles in the legs have a high concentration of slow-twitch muscle fibers.

What Are Slow-Twitch Muscle Fibers? 

Skeletal muscles are made up of a combination of slow-twitch and fast-twitch muscle fibers, but in different proportions. Differences between these two types of skeletal muscle fibers include: <ul class="mc-m-4 list-styled"><li class="">Performance: Fast-twitch muscle fibers are faster acting, but tire more quickly in comparison to slow-twitch muscle fibers. </li><li class="">Energy: Fast-twitch muscle fibers produce small amounts of energy rapidly, which works best for <a href='https://www.masterclass.com/articles/hiit-workout-guide' target='_self' class='mc-text--link' data-mc-segment-trigger='true' data-mc-segment-event-type='Article Page Click' data-event-obj='{"textTag":"p","href":"https://www.masterclass.com/articles/hiit-workout-guide","linkCopy":"HIIT exercises","category":null}'>HIIT exercises</a>. Slow-twitch muscles slowly produce larger amounts of energy, supporting endurance activities, like long-distance running. </li><li class="">Physiological: Slow-twitch muscle fibers have more blood vessels and capillaries, so they are more efficient in energy usage in comparison to fast-twitch fibers. </li><li class="">Oxygen: Fast-twitch muscles are relatively anaerobic, meaning they use little to no oxygen, and instead rely on different sources of bodily energy, such as glucose, which they use to make adenosine triphosphate (ATP). ATP is an essential chemical, present in all living organisms, that acts as a kind of energy shuttle within the cell, making activities possible, such as movement. Type IIA fast-twitch muscles are technically glycolytic, meaning they use oxygen and glucose as fuel. Type IIB, the fastest and most forceful, but also the least efficient, have low oxidative capacity and high myosin ATPase activity (they act very quickly but also tire quickly). Slow-twitch muscle fibers are aerobic and have higher concentrations of mitochondria and myoglobin, proteins that bind oxygen. </li></ul>

Fast-Twitch vs. Slow-Twitch: What’s the Difference?

Each type of muscle fiber has different functions, and developing them requires different kinds of training and conditioning.<ul class="mc-m-4 list-styled"><li class="">Slow-twitch muscle fibers: To get the most out of your slow-twitch muscle training, concentrate on endurance training activities and slower weight training with higher reps. Running, jogging, hiking, rowing, and swimming are all good ways to strengthen your slow-twitch muscles. With weight training, choose higher numbers of reps, shorter rest periods, and slower motions. When resistance training, try to lengthen the amount of time spent under tension. </li><li class="">Fast-twitch muscle fibers: For building fast-twitch muscle mass and strength, focus on high-intensity, low-duration exercises. If training with weights, complete lower reps as fast as you safely can. Uphill sprints such as stair climbing are great for building this type of muscle, as are <a href='https://www.masterclass.com/articles/box-jumps-guide' target='_self' class='mc-text--link' data-mc-segment-trigger='true' data-mc-segment-event-type='Article Page Click' data-event-obj='{"textTag":"p","href":"https://www.masterclass.com/articles/box-jumps-guide","linkCopy":"box jumps","category":null}'>box jumps</a>, kettlebell exercises, and jump squats. </li></ul>

How to Train Fast-Twitch and Slow-Twitch Muscle Fibers 

Throw on some athleisure, fire up a <a href='https://www.masterclass.com' target='_self' class='mc-text--link' data-mc-segment-trigger='true' data-mc-segment-event-type='Article Page Click' data-event-obj='{"textTag":"p","href":"https://www.masterclass.com","linkCopy":"MasterClass Annual Membership","category":null}'>MasterClass Annual Membership</a>, and get ready to sweat it out with exclusive instructional videos from Nike Master Trainer and GQ fitness specialist Joe Holder. Want to improve your cardiovascular endurance? Give Joe’s HIIT workout a go. Trying to get a little swole? He’s got a strength training workout for that. From fitness tips to nutrition hacks, Joe will have you feeling healthier in no time.

Want to Dive Deeper Into Your Wellness Journey?

Learn the differences between fast-twitch and slow-twitch muscles in fitness.