Muscle Fiber Type in Training

Along with energy systems, muscle fiber type shows that specific training leads to specific results. Strength training and cardio are inherently different activities. It also shows that we have potential for each area based in part on our genes. Other interesting conclusions come about as well.


Different ways exist to categorize fibers. The categorization used here comes from how fast a muscle contracts. Slow-twitch (ST) and fast-twitch (FT) form the two major classifications. Fast-twitch further breaks down to form three types:


  • Type I


  • Type IIA
  • Type IIB

Type I produce the least force and type IIB the greatest. The motor units which control Type II fibers have more fibers to activate. They also have the largest fibers as well.

Fiber type ratios are genetically determined though activities may change the subtypes (A and B). A successful marathoner will have a higher percentage of slow-twitch fibers. A successful weightlifter will have a greater degree of fast-twitch fibers. Both can improve in the other area but will have less success versus their peers. Most people are split about 50/50 among ST and FT fibers for most of their muscles.

Even within a given body, some muscle groups appear mainly one or the other. The soleus of the leg comprises of nearly 85% slow-twitch muscle fibers. The orbicularis oculi of the eyeball comprises of nearly 90% fast-twitch muscle fibers. This shows that certain muscle groups are meant for stabilization and endurance versus speed and strength and vice versa. Stabilizers tend to have a greater proportion of slow-twitch muscle fibers.

As a result of heavy lifting, Type IIB to IIA transformations occurs. This gives them more endurance. Some authorities thought that this indicated that longer exercise would disrupt results from heavy lifting. This is incorrect. It turns out that all fibers become slower with activity, even those built through explosive and heavy activities.

This calls into question the idea of training fast to be fast which draws upon the principle of specificity.

Slow-Twitch (I)

Slow-twitch muscle fibers are smaller compared with fast-twitch fibers. They…

  • have a high oxidative capacity.
  • store less glycogen.

This makes them suitable for long and easier activities. They excite easily and activate first according to motor unit recruitment. This suits aerobic and muscular endurance. This includes something like typing on a keyboard along with a tougher 15-20 minute jog.

ST muscle fibers contract much more quickly relative to the motions for sports which further questions explosive training. They still grow from heavy lifting but contribute less since they produce less force meant for more time. The effort terminates when the fast-twitch muscle fibers fail to produce enough force.

Fast Twitch (IIA and IIB)

Fast-twitch muscle fibers have two main categories:

  • Type IIAs are red muscle fibers with a balance of strength and endurance.
  • Type IIBs are white muscle fibers that function best for explosive or heavy actions but have the worst endurance.


  • have moderate to low oxidative capacity.
  • have higher glycogen storage.

Type IIA can take on the traits of slow-twitch fibers. This gave weight to an argument that you had to focus on either strength or endurance. We now know that all fibers move toward endurance over time when used.

Type II degenerate the most from aging. This explains why everyone can benefit from heavy training. These fibers are not often used during everyday activities and grow weak from disuse.

Muscle Fiber Types Show the Need for Specific Training

Lifting and cardio cause separate changes and both are important for complete fitness. Although we all have different potential for results for each, this does not indicate we should all train differently. You can perform intervals and lift weights with a few basic exercises to get the benefits of both and form a well-rounded program.

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