Muscle Fibers: How Do They Differ?

Our body is composed of many different groups of muscles and each person has a unique composition of muscle tissue in their body. Read on here to learn more about the 3 different muscle fibers and how they build differently depending on the type of sport.

Our body is composed of many different groups of muscles and each person has a unique composition of muscle tissue in their body. Different types of athletes usually have one predominant type of muscle fiber, that being the kind that suits their sport the best.

The Different Types

The Three Different Types Of Muscle Fibers Are:

  1. Type I fibers
  2. Type IIa fibers
  3. Type IIb fibers

Each one has it's own characteristics and is suited to a particular type of movement. Another way to classify these fiber types is by their contractile and metabolic properties, thereby dividing them into slow and fast twitch fibers.

Type I: Slow Twitch Fibers

The first type of fiber we will look at is the type I, or slow twitch fibers. These fibers are slow to contract (hence, slow twitch), and can sustain muscular contractions for an extended period of time. This factor makes them ideal for endurance type of events where one is exercising for long durations. They also contain large and numerous mitochondria which aid in their oxidative metabolism (the use of oxygen). These types of fibers are fatigue resistant but are only able produce a relatively low level of force output.

Physically, these fibers are red in appearance, due to their iron containing cytochromes, have a small fibers diameter and have many capillaries throughout their structure. For the average sedentary child or adult, slow twitch fibers comprise approximately 50% of their muscular tissues. Endurance athletes, such as marathon runners, cross-country skiers and distance cyclists often possess up to 90% slow twitch fibers. On the other hand, athletes that rely on short bursts of energy possess the lowest levels of slow twitch fibers, often around only 25%.

Athletes with a higher proportion of slow twitch fibers also commonly have the highest VO2 max results, as this is a test primarily of aerobic capacity and these are the most important fiber types in relation to this measurement.

Type IIa: Fast Twitch Fibers

The next category of muscle fibers is fast twitch fibers, divided into type IIa and type IIb. Fast twitch fibers are known for their ability to rapidly transmit action potentials and generate a high crossbridge turnover rate (responsible for quick muscle contractions).

These fibers also possess a high activity level of myosin ATPase and show a rapid rate of calcium release and uptake by the sarcoplasmic reticulum (Katch et al, 2000). Due to these properties, these fibers generate an explosive burst of power for a short period of time. This makes them most suitable to stop and go activities such as basketball, soccer, and hockey, as well as max output activities such as weightlifting, and many track and field events.

These fibers rely heavy on the glycolytic energy system (using the method of anaerobic glycolosis to produce ATP). Type IIa fibers are in the middle of the muscle fiber spectrum, as they are less fatigue resistance, produce more muscular force, and contract at a faster speed than slow twitch fibers.

Type IIb: Fast Twitch Fibers

The type IIb fibers are the most fatigable out of all the fibers but also generate the most power and force, and therefore are the fastest twitch muscles fibers. These types of fibers are recruited in activities that require an all out burst of power and only act for an extremely short period of time, as the total length of their contractions usually last only 7.5 milliseconds.

In terms of general recruitment, they are also the last to be recruited. For example, upon normal activities, slow twitch fibers are recruited first, followed by type IIa when the type I can no longer suffice, and then finally the type IIb, which are recruited to produce maximal strength.

What Is The Difference?

In regards to physical appearance, type IIa are pink in color, have an intermediate diameter, capillary level and mitochondria volume. The type IIb fibers are white in color, have the largest diameter and have a low capillary and mitochondrial volume. Most strength athletes possess a higher % of fast twitch fibers, as do those in short duration, quick moving activities.

The primary reason why fast twitch fibers are not resistant to fatigue is because they rely on anaerobic glycolysis to produce ATP. During this process lactic acid begins to accumulate and a condition called acidosis occurs which brings about muscular fatigue. Due to their low capillary level, they do not make use of oxygen nearly like slow twitch fibers do (which also explains their reliance on anaerobic metabolism).

Although each muscle fiber type has certain characteristics that make it more suited for certain activities, this does not mean that an athlete with a predominance of one type of muscle fiber can only participate in those activities that call for that type.

With proper training, they still can learn many of the skills and techniques used in different sports and can achieve success across a wide variety of activities.

It is common however, that athletes with a predominance in one type of fiber do naturally tend to be drawn to the types of activities more suited to their body as they tend to naturally be better at those actions and therefore often enjoy participating more.

At the elite level of competition, you may find that those athletes who do possess the certain characteristics in their muscle fiber that are required by the sport do tend to be able to push the envelope slightly further, as their body may react better to training methods and will show faster improvement.


For recreational athletes though, commitment to practice and training will play a far more significant role in performance level than muscle fiber type alone will.

So, the three types of muscle fibers are slow twitch (Type I) and fast twitch (Types IIa and IIb). Slow twitch are characterized by having long contraction rates, being resistant to fatigue, relying on oxygen as their main source of metabolism, and are used primarily in endurance type of activities that don't require a great deal of force.

Fast twitch fibers on the other hand have short but powerful contraction rates, are highly fatiguable due to their reliance on anaerobic metabolism that produces lactic acid, and are more suited to activities that are powerful and quick in duration.

So the next time you are watching or participating in an athletic event, try and think of what type of muscle cells are involved and see if you can pick out which athletes have a predominance of that muscle type in their bodies (those with fast twitch tend to have bulkier muscles due to hypertrophy, whereas those with slow twitch are generally longer and thinner).