Muscle Fibers

Muscle tissue consists of various fibers. Each person has a unique composition of muscle fibers, which does effect athletic performance. There are three major types of muscle fibers. These include type I fibers, type IIa fibers, and type llb fibers. Depending on the sport athletes may have more or less of a certain type of muscle fiber. The human body is an incredible organic machine with different functions. It is pivotal to understand the characteristics of muscle fibers to tailor specific training regimens.

           Muscle fibers are located in the fassicle. The fibers are secured by the endomysium. The muscle almost forms a structure that is similar to a fiber optic cable. It is bundled by a network of wires. These types of wires are supported by both bone and tendons. There is also a network of blood vessels running through the muscle. The epimysium acts as cover for the perimysium of the muscle. The movement of the human body can only happen when tissues contract. When discussing major muscle fiber types, they are mostly regulated to the examination of skeletal muscle.


Sometimes it is referred to as voluntary muscle. These are muscle that respond to the conscious control of the brain. Chemical messages can either direct them to contract or lengthen. When viewed under the microscope they have a striated or striped texture. All of these microscopic fibers work together as one unit in the muscular system.

        Type I muscle fibers are normally called slow twitch. Their speed to contract is lower compared to other fibers. This slow contraction time allows for better performance in endurance events. The force out put from these fibers is low. They do contain high amounts of mitochondria which assist with oxidative metabolism. Slow twitch fibers are more fatigue resistant. The slow twitch fibers are red from the iron in cytochromes. They also contain a large amount of capillaries in their structure. Endurance athletes such as marathon runners,distance cyclists, and cross-country skiers could have up to 90% slow twitch fibers. Athletes with higher amounts of slow twitch fibers will have higher VO2max results. This means an endurance athlete will have higher aerobic capacity.

Mary Keitany
This marathon runner would have more slow twitch muscle fiber composition.

The average person can contain up to 50% slow twitch fibers in their body. Like other fibers they can be built through a specific type of training. Slow contraction is an advantage, because they keep the body going. This gives dedicated swimmers an advantage in endurance. Endurance exercise allows the muscles not to get worn out fast during physical activity. Other benefits include improved coordination, a larger heart which can pump more blood,  and maximizes metabolic efficiency. Oddly enough, it is believed that endurance exercise can improve digestion. Slow twitch fibers are not useful in sporting activities that require explosive power. muscle_fiber_pic   A cyclist or a swimmer attempting to enter weightlifting may struggle if the training regimen is not adjusted. The role of oxygen is critical, due to the fact  it produces energy for muscle contraction. Even though slow twitch and fast twitch muscle fibers are different muscle fibers follow a basic anatomical structure. There are myofibrils, which contain actin and myosin filaments. There is also the nucleus structure and striations present. Myofibrils are divided into sarcomeres. They are by nature contractile. When myfibrils contract sarcomeres shorten. This subsequently results in actin filaments to move past myosin filaments shrinking the H zone. This happens when the muscles contract.

This cyclist when doing this activity is constantly contracting sarcomeres.

 When examining this part of the body, it demonstrates how the body is like an organic machine. The body is constructed of smaller units known as cells which form organ systems that collaborate to keep a life form functioning. Muscle fibers play a role in this process, besides assisting in athletic activity.

       Fast twitch fibers function better in strength sports. They are know for their fast contractions, but can fatigue quickly. Their ability to generate action potentials is very efficient. The higher level of myosin ATPase allows for a larger release of calcium  and uptake by sarcoplasmic reticulum. Fast twitch fibers are suited best for basketball, soccer, hockey, track and filed, and powerlifting. Fast twitch fibers can be divided into two groups: type IIa and type IIb. Type IIa has more muscular force, fatigue faster, and functions on a different energy system. The glycolytic energy results in anaerobic glycolosis which produces ATP. ATP is responsible for supplying a sufficient source of energy for muscular contraction.Type IIb are known for having a high fatigue rate, but have the greatest explosive power. They are the fastest of all the muscle fibers. Physical activities that require a massive burst of power will recruit type IIb fibers. They are summoned last when muscle recruitment is required. Slow twitch fibers go first, followed by type IIa. Finally, when those fibers can no longer sustain pressure type IIb is recruited. This results in the burst of maximal strength. That is the total strength the body can produce. The body contains a general strength reserve. Lifting a carton of juice would not require maximal strength. Lifting a huge weight or a person would.

images (13)
This is the demonstration of type II muscle fibers in use.

A contraction from a type IIb muscle fiber can last up to 7.5 milliseconds. This is a short period of time for the burst of power to be released.  This means a weight lifter would struggle with an endurance event. The muscles would fatigue at a faster rate. The strength athlete who tries to compete with a runner would find themselves in a real challenge. The runner would most likely have trouble generating a huge bursts of physical power. All these fibers work together producing skilled movement which is directed by electrical impulses in the brain. Diameter of the fibers does effect the force of muscle fiber. Fast twitch muscle fiber has a larger cross sectional area and can therefore generate more force. Slow twitch fibers have limited potential for high levels of muscular hypertrophy.

Here she is using her maximal strength.

There also remains aspect. Sexual dimorphism also does play a role in the distribution of muscle fibers. This does effect athletic performance between the sexes.

Contrary to popular belief, men and women have the same muscle tissue. There are a total of 700 skeletal muscles in the human body which account for at least 40% body weight of the average person. The anatomy of the muscle is the same for both sexes. The difference is that one sex has more of a certain type than the other. The fibers are still the same, but it depends on genetics and which type of exercise an individual engages in. Men have more fast twitch fibers. This is an explanation why men are on average stronger. Women have more slow twitch muscle fibers, which means they are more fatigue resistant .  There is a difference in upper body strength seeing as men have more of a concentration of fibers in that part of the body. Higher skeletal volume means more output. Estimates vary, but a woman of the same height and weight  of a man can be close to 80% of his body strength. That 20% difference reveals that a woman is closer to a man in lower body strength. Testosterone has an enormous impact on protein synthesis. Estrogen forms more fat. Even the most muscular female still carries a higher fat percentage than a man.

   Muscle fibers both fast and slow twitch have even more distinct attributes. The reason for fast twitch muscle fatigue is that lactic acid will accumulate (anaerobic glycolosis process). Acidosis occurs causing fatigue of the muscle fiber. Muscle fibers utilize blood supply in a different manner.Slow twitch fibers have excellent blood supply as compared to fast twitch fibers. Slow twitch fibers get their outlandishly red appearance from the abundance of blood supply. Fast twitch have less blood supply to work with having a white appearance. Lack of blood means a limited supply of available oxygen.  Besides the difference in oxygen, it is fascinating to how it relates to mitochondria. This organelle is important to human life Most eukaryotic cells carry mitochandria. Their total number can vary depending on certain types of activities. They can change shape to be either longer or thinner depending on the circumstance. Mictochandria provide energy where it is needed. Fat cells contain a small number and fat storage does not require energy. Mitochandria are referred to as the “power house” of the cell, seeing as they produce adenosine triphosphate. Slow twitch fibers have more mitochandria, while fast twitch have less. This effects energy reserve for the different types of muscle fibers.There are fuel sources that fibers are reliant on. Slow twitch use glucose and fats, while fast twitch uses ATP derived from glucose. Depending on what sport an individual goes into it is critical to have an understanding of muscle fiber and its histology.

Further Reading

Mader, Sylvia. Biology.New York: McGraw Hill Companies, 2010.

  Clark, Shannon. “Muscle Fibers How They Differ.” N.p., 7 Mar. 2007. Web. 5 May 2016. <;.

Dale, Patrick. “Difference Between Slow-Twitch and Fast-Twitch.”LIVESTRONG.COM. LIVESTRONG.COM, 25 June 2015. Web. 04 May 2016. <;.

Muscle Fibers

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s