Fast Twitch and Slow Twitch Muscle Fibers

Exercise can be broken down into many different forms. There is aerobic fitness, weight lifting, sports specific fitness, biking, swimming, hiking, and the list goes on. The reality is that there are many different ways to exercise but each area has a multitude of physical components that is quite mind blowing when you sit down and think about it. 

Each body has a unique skeletal muscular system which main purpose is to provide movement of the body. Your muscular anatomy is a complex system that is the only tissue of the body that can contract creating the ability to generate force. Inside of your muscle tissue lays a very intricated set of muscles fibers which are categorized into slow and fast twitch types. These are further broken down into type I slow twitch, and fast twitch type IIa and IIx. Muscles contain both type I and type II muscle fibers but your genetic makeup and age will pre determine which type of fibers you have more of. One could argue that a mesomorphic body type would house more type II muscle fibers and an ectomorphic body type would contain more type I muscle fibers. If you want to become more efficient at developing either set of muscle fibers you can as you are not limited to what your genetic make-up has pre-determined. 

Type I (oxidative) red/slow twitch fibers live up to their name because they contract much more slowly than fast twitch fibers making them more useful over a longer duration of time. They require oxygen for energy and are highly efficient at burning fat. They have the highest ability of fiber recruitment and are typically the first to be used for force production but interestingly do not generate much force at all. 

Activities that recruit type I fibers can include maintaining posture, walking, light resistance training and long duration aerobic work such as long-distance running.  Slow twitch fibers are great for muscular endurance but do not provide the same benefits as Type II in terms of strength, explosiveness, and size. Marathon runners or long-distance cyclists tend to have a higher ratio of slow twitch fibers, whereas sprinters, power lifters, baseball players, and boxers have a higher percentage of fast twitch type II. 

Type II white/fast twitch muscles fibers are broken down into two categories, type IIa and type IIx. Type IIa (oxidative-glycolytic), hold a more moderate ability of fiber recruitment which means that once the slow twitch fibers cannot keep up with the level of activity the type IIa fibers will take over. These fibers rely on both aerobic and anerobic energy systems to fuel exercise lasting up to two minutes, but because they produce lactic acid fatigue much more quickly than type I. 

Type IIx (non-oxidative) is considered the most powerful muscle fiber but also is the quickest to fatigue. For this reason they have the lowest level of fiber recruitment. Type IIx fibers are anerobic so they do not require oxygen for energy and are only useful for very short duration activities that require explosiveness and strength like the 100m sprint. Interestingly these specific fibers are often found in sedentary people for the purpose of a fight or flight situation. 

If you imagine suddenly you are in a situation where you are running from wild animal you can see how each of these fibers are recruited. The first 10-12 seconds of all an all out sprint would activate the type IIx fibers, once those fibers tire out type IIa would kick in and active for up to 2 minutes. Once those fibers cannot keep up the type I will take over and be recruited as your full out sprint has declined into a slow jog. 

Understanding muscle fibers types is important to development in sports specific training. If you are someone who wants to be a marathon runner then you must work to develop the type I muscle fibers. Ways to do this include long distance cardio, body weight exercises or light resistance training using slower movement tempos with higher rep ranges. Circuit training or sometimes referred to as “cardio with weights” where you would take little to no rest, preforming multiple sets of different exercises is also a very effective way to engage slow twitch fibers.

To stimulate type II fast twitch muscle fibers it is recommended to preform fast explosive movements such as heavy resistance training, Olympic lifting, sprinting, both non weighted and weighted plyometric exercises. To put it bluntly the more muscle fibers recruited in an exercise, the bigger and stronger the muscle will get.

After the age of 30 humans slowly begin to lose muscle mass each year, as much as 3-5%, for this reason working to develop type II muscle fibers is crucial to reverse the aging process, however working to develop both types of muscle fibers is shown to have many benefits for health and longevity. 


Bree Hunter

Bree Hunter is a Calgary fitness model and personal training professional. Since entering the world of fitness modelling, Bree has achieved an extraordinary and unparalleled year-round physique and she is excited to help her clients reach the same levels of success. Bree's drive and determination saw her achieve high levels of success on the. . .

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