Muscles: how they work, and how to work them.

Muscles: Fast-Twitch and Slow-Twitch Fibers.

Paul Bester



Surely many have seen world class athletes competing and a commentator mentions how amazing a particular performer’s fast-twitch muscle fibers are so well developed. This is usually observed when the individual shows significant acceleration and agility while competing.


When it comes to muscles fibers or sarcomeres (a unit of muscle fiber) there are two particularly distinct groups namely; Slow-twitch and Fast-twitch muscle fibers.


Slow-Twitch or Type I Muscle Fibers:

Sometimes referred to as “red” fibers due to having a darker appearance because of more blood carrying myoglobin being present. Slow-twitch muscle fibers are recruited first when muscle contraction happens. This is due to having lower action potential thresholds (amount of stimulus needed to contract fibers) If not enough force is available, fast-twitch fibers will engage. Slow-Twitch muscle fibers are predominantly found in tonic muscles responsible for maintaining posture and balance (core stabilizing muscles). Even though slow-twitch fibers are not able to generate significant force they are symbiotic (self-sufficient) due to creating their own source of fuel enabling them to withstand a force for a prolonged period of time. Adenosine Triphosphate (ATP) is the chemical compound used for fuel in contractions of muscles, slow-twitch muscles contain mitochondrial organelles that rely on oxygen to create ATP as energy. Therefore slow-twitch muscles are termed aerobic as they need oxygen to function. Many endurance athletes utilize steady-state training techniques to increase their mitochondrial density allowing them to push harder and longer due to improved efficiency in the mitochondria generating ATP from oxygen.



Fast-Twitch or Type II Muscle Fibers:

Fast-twitch muscle fibers can be further classified into two subcategories; Fast-twitch Type IIa which are fast oxydative glycolytic cells allowing them to use oxygen to assist in converting glycogen into ATP. Fast-twitch Type IIb fibers rely on stored ATP within the muscle to facilitate contraction. As discussed above fast-twitch fibers will only be recruited once their action potential stimulus threshold has been met. This is only if the slow-twitch muscle fibers can not provide enough force than is required. The larger the fast-twice muscle fiber, the shorter the time it takes to reach peak contraction force, this force compared to slow-twitch is exponentially higher. Even though fast-twitch muscle takes less time to produce a force, they cannot withhold that force for prolonged times as fatigue is very prominent in fast-twitch fibers. Higher densities of fast-twitch muscle fibers are found in and around the phasic muscles of the body “those initiating movement.”


To optimize fast-twitch muscle fiber function it has been proven that strength and power training can increase the number as well as size of fast-twitch fibers needed for specific movement requirements. Fast-twitch muscles fibers are referred to as the “white” fibers due to containing less vascular anatomy and are solely responsible for the outward muscle definition appearance that is seen in bodybuilding athletes.




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