Fast-Twitch Versus Slow-Twitch Muscle Fibers

Are you destined for sprints or marathons? The answer is in your muscle fibers.

Ever wondered why you’re better at long-endurance exercise versus sprints? Or maybe you’re explosive out of the gate but burn out fast. Whether you tend toward a turtle or a hare depends on what muscle fiber type is predominant in your body. If you were to take a microscopic look at a slice of your bicep muscle, you’d see a mix of two general types of muscle fibers: slow-twitch (type I) and fast-twitch (type II). Here, we break down both types and share how to train each type with targeted workouts.

For more exercises targeting slow-twitch and fast-twitch muscle fibers, look no further than Aaptiv.

Slow-twitch vs. Fast-twitch


Slow-twitch muscle fibers contain little powerhouses of energy called mitochondria that use oxygen to fuel muscles. Because oxygen is the fuel, it’s considered aerobic, and this type of oxidative fueling can power a moderate level of force for a sustained time. In general, slow-twitch fibers have a slow contraction speed, low force, and low glycolytic capacity (using stored glycogen for fuel). They’re high in endurance, capillary density (lots of blood flow to the fibers), and oxidation capacity. These muscle fibers are the first ones activated during a contraction, but if the power needed is greater than what can be generated with slow-twitch fibers, the fast-twitch fibers are recruited.


Fast-twitch muscle fibers are recruited when the force needed exceeds what type I fibers can provide. They’re the ones that provide short, high-intensity force, but they don’t have staying power because their fuel source depends predominantly on stored muscle glycogen. When it runs out, it takes time to build back stores. Fast-twitch fibers have a higher activation threshold but, when activated, reach maximum force more quickly than slow-twitch fibers. These attributes enable type II fibers to provide the fast, peak forces needed with weightlifting, sprinting, and other high-intensity, short-duration activities.

Looking for one or all of the types of workouts listed above? Aaptiv has you covered.

We can further break down fast-twitch muscle fibers into type IIa and IIb. Type IIb can produce a moderate level of force and endurance, have a high contraction speed and glycolytic capacity, and have a medium oxidative capacity and capillary density. This type is the intermediate between type I and type IIb because it uses a mix of oxidative (aerobic) and glycolytic (anaerobic) energy production. Because type IIa fibers are a mix of aerobic and anaerobic, they can provide more force more quickly than type I muscle fibers.

Type IIb fibers are the racehorses of muscle fibers. They have the fastest contraction speed and highest force—all powered by a high glycolytic capacity. The capillary, mitochondrial, and endurance levels are low in this type of fiber. They’re good for a fast, intense effort but burn out their fuel very quickly. When trained, fast-twitch fibers, especially type IIb, increase the size and definition of a muscle.

Endurance vs. Speed

Your genetics dictate whether you have a higher percentage of type I or type II. A quick assessment of what type of exercise you prefer can give you a clue. If you dread long, moderate-level exercise bouts but love HIIT workouts, you probably favor fast-twitch. In a study in the Journal of Applied Physiology that observed a world-class sprinter’s thigh muscle biopsy composition, the researcher found it was composed of 71 percent fast-twitch fibers. That gave that sprinter the ability to achieve impressive power and speed.

On the other hand, if you’re always the last person across the finish line in a sprint but can out-hike most of your friends, slow-twitch is your dominant type. There are also differences between men and women in muscle composition. A 2015 study in Physiology that looked at gender differences in muscle fiber composition reported that women have a higher percentage of type I and type IIa muscle fibers than men. Age factors in as well, with age-related muscle mass loss consisting of more type II fibers lost than type I, leading to a reduction in strength and muscle size.

Looking to gain more muscle mass? Check out the strength training classes on Aaptiv.

How to Train Both Muscle Types

Although your body may tend toward one muscle type, you can improve either your endurance or power by doing workouts that target the fibers recruited for the activity. Below are some tips for training both types of muscle fiber.

Type I Muscle Fiber Training

Type II Muscle Fiber Training

Now that you know all about slow-twitch and fast-twitch muscles, train them properly with the help of Aaptiv.



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