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July 17, 2017 By Shelley Harper

Muscle Fiber Types and Strength Training


Muscle Fiber Types.jpeg


Building muscle is essential to all sports. From gymnastics to swimming to basketball, developing muscle will improve strength, stamina, and performance.


However, compare Nastia Liukin to Michael Phelps to Lebron James and you find that their muscle compositions are very different. Each of these Olympians has found a way to build not just muscle, but also the right muscles. When looking to increase muscle mass it is important to make sure athletes are training the correct type of muscle fiber specific to their sport and event. Specificity is key in strength training and vital for success.


Muscle Fiber Types

The chart below shows the differences among the three main muscle fiber types: Slow Oxidative, Fast Oxidative/Glycolytic and Fast Glycolytic.


muscle fibers


The chart reveals that slow oxidative fibers are not as strong as the other two, but can be repeatedly used for extended periods of time without fatiguing, which is essential for endurance-oriented athletes.


Fast oxidative/glycolytic fibers provide a faster twitch and larger force while still maintaining resistance to fatigue, great for extended sprinters such as a 400-meter run specialist.


Lastly, fast glycolytic fibers provide the largest force and fastest twitch speed but are highly fatigue-able and are reserved for high-intensity bursts such as short sprints or maximal lifts.


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Muscle Fiber Types in Elite Athletes 

Athletes at the top of their peak athletic ability, have shown important specialized muscle fiber type characteristics. For example, sprinters usually have predominately Type IIB fast glycolytic muscle fibers, while distance runners have a larger proportion of slow-twitch, high oxidative muscle fibers. Training and building the right muscle fibers will increase athletic development and performance.


Training Muscle Fibers

Myoplasticity refers to the capacity of skeletal muscles to change. Training can increase muscle mass and alter muscle fiber composition depending on the specifics of an athlete’s training. Let’s look at how endurance and strength training each change muscles in different ways.


Endurance Training

Endurance training is when an athlete’s muscles are performing high-frequency, low-force output activity. In other words, the muscles are repeatedly activated for long periods of time at a lesser power. While endurance training does not significantly increase your muscle fiber cross-sectional area, there are many other significant adaptations that improve performance. Most significantly is the increase in mitochondrial mass allowing for an increase in oxidative metabolism in skeletal muscle.


Strength Training

Strength training is when your muscles are performing low-frequency, high-force output activity. Strength training induces hypertrophy, which is the increase of the cross-sectional area of a muscle fiber. Strength training results in the hypertrophy of both type I and type II muscle fibers. It is also important to note that mitochondrial density actually decreases with an exclusive high-intensity strength program. This is important because athletes who only train with resistance are more likely to find a decrease in muscle endurance capacity. 


Recap: 5 Things to Remember

  1. There are three different muscle fiber types: slow oxidative, fast oxidative/glycolytic, and fast glycolytic.

  2. Endurance training has minimal effect on the size of the muscle, however, it does increase mitochondrial mass allowing for increased oxidative metabolism in skeletal muscle.

  3. Strength training induces hypertrophy (an increase of muscle fiber size) of both type I and type II fibers, however, it results in a decreased mitochondrial mass of skeletal muscle.

  4. Combining endurance training with strength training is key to maximizing muscle power and resistance to fatigue.

  5. Specificity in training is key. Exclusively endurance training for a spring event will not provide athletes with the correct muscle adaptions, just as entirely sprint training will not prepare athletes for an endurance event.


For more reading on the importance of maintaining muscle health read this article, and more about muscle soreness and its effects find that information here.


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  • Brooks, George et al. Exercise Physiology: Human Bioenergetics and Its Applications. McGraw Hill: New York. 2005.


About the Author

Shelley Harper

Shelley graduated from UC Berkeley in 2012 majoring in Integrative Biology and is currently applying to nursing school. She competed on the women’s swim team at Cal and contributed to three NCAA Championship team titles in her four years. Shelley’s interest in exercise physiology was sparked after discovering connections between the materials learned in her anatomy lab and her athletic endeavors. It is her goal to share this knowledge and inspire other athletes to make these connections to help them reach their personal goals. After finishing her swimming career in 2012, Shelley is now a triathlete utilizing her background in exercise physiology to aid this athletic transition.

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