Chapter 2 – Muscle Fiber Types

Intro to chapter:

How an athlete appears on the outside is just one factor contributing to their success. What’s on the inside of the body, different combinations of muscle fibers spread throughout, should also be taken into account as it plays a very significant role in sport and training performance in combination with body type. Being aware of the pros and cons that your body type comes with is useful but having knowledge on the inside workings of our bodies will prove even more useful.

There are three fiber types in particular which we will be examining:

1. Slow twitch type I fibers (ST)
   • Used for aerobic activity (intaking O2) such as long distance running
   • Do not produce lactic acid
   • Low power produced
   • High resistance to fatigue
   • Slow muscle contraction time
2. Fast twitch II a fibers (FTIIa)
   • Used for long term anaerobic activity such as resistance training
   • Trainable to be used for anaerobic or aerobic activity
   • Contract moderately fast
3. Fast twitch II b fibers (FTIIb)
   • Strictly anaerobic and rely on energy from glycolytic pathway
   • Very fast contraction speed
   • Very high amount of power produced over a short period of time
   • Used for short-term anaerobic activity such as sprinting or high jumping

We learn from this video that muscle fiber type composition is determined genetically, meaning that some people can have advantages in activities in which the muscle fiber type they are dominant in are being used. The dramatic heterogeneity of fiber type composition between people may explain their remarkable variation in exercise performance and (1)

During the 1970s and 1980s, it was popular to determine the muscle fiber composition of athletes from different sports events. These studies revealed that successful endurance athletes have relatively more ST than FT fibers in the trained musculature. In contrast, sprinters have muscles that are composed predominantly of FT fibers Accordingly, the belief that muscle fiber type can predict athletic success gained credibility. In particular, the notion that the proportion of ST fibers might be a factor governing success in endurance events was proposed. (1)

Let’s take an athlete such as Usain Bolt for example.

Just for reference, in untrained individuals, the proportion of ST fibers in the vastus lateralis muscle (the largest of the quadriceps muscles and the most commonly studied muscle in humans), is typically around 55%, with FTa fibers being twice as common as FTb fibers. (1)  

However, Usain Bolt is far from a typical case. A sample of Bolt’s vastus lateralis muscle (largest muscle in the quadriceps group) was found to be composed of 80% FTIIb fiber type, a number which is unusually high compared to the average male whose muscles are generally 50% Fast twitch and 50% slow twitch. His composition gives an edge enabling remarkable power and force and a speed 99.9% higher than the general population. (2)

A study conducted by Fink and colleagues found that the percentage of ST fibers in long-distance runners correlated with their success. He compared the slow twitch muscle fiber types between 14 elite runners, 18 good (but not world-class), and 19 untrained men. He observed that the average slow twitch muscle fiber type composition went down significantly between each class of runners, raising the notion that success is partly attributed to the fiber types which they possess. (3)

Supersets for gymnasts:

Gymnasts require a lot of power in order to execute their movements successfully and be technically sound. To train for power, gymnasts will do supersets; One exercise immediately followed by another, most commonly a resistance training movement followed by a body weight movement. This combination of resistance training exercises followed by body weight movements will increase power. For example a gymnast who needs to jump high will superset squats and box jumps. The squats will place a heavier load on the body, which will then not be present in the box jump, allowing for the athlete to perform the movement, which has similar bio-mechanics as the squat, with much more power, strength and speed.

Slow Twitch Variation Between Long Distance Runners (Fink, et. al)

Elite Runners: 79.0% ST

Good Runners: 61.8% ST

Untrained men: 57.7% ST

However, fiber type alone did not determine and fully explain the performances of the elite athletes. For example, two athletes with similar best times for the 42.2 km marathon distance (approximately 2 hr 18 min) had 50% versus 98% ST muscle fibers. In a following study by Foster and colleagues revealed that endurance running performance was better related to an athlete’s maximal O₂ uptake (VO2 max) (4). From this it can be said that while an athlete’s muscle fiber type is an important morphological component and is related to several contractile (i.e. speed of contraction) and metabolic properties (i.e. how energy is produced), other physiological factors (e.g., VO_2max, maximal cardiac output, and speed/power output at the lactate threshold) are more likely to determine the upper limits of endurance capacity. (1).

To know about the fiber type composition you possess, it’s not very likely you will be able to take a piece of your muscle and analyze it using expensive tools like scientists do, but this doesn’t mean you can’t have some sort of idea about what muscle fibers you might be dominant in – again, you never really know your genetic potential until you’ve pushed yourself to the limit at which you can determine whether or not you are strong and weak in certain aspects of training. It’s all just a matter of finding what you are good at through trial and error and being consistent with it to achieve your goals.

If you excel at swimming or long-distance running or any exercise that is aerobic then you are naturally predisposed to being ST dominant.

If you excel at explosive movements such as weight lifting, sprinting, high jumping, gymnastics. or any physical movements which anaerobically use a lot of energy over a short period of time, you are naturally predisposed to being FT dominant.

Although sports like basketball and volleyball incorporate movements that are heavily reliant on fast twitch muscle fibers, it is also important that athletes in these sports not only train fast twitch but slow twitch as well in order to have energy for the duration of the match. Having a strong slow twitch through long-slow distance training will allow for lactic acid to be converted in to pyruvate very efficiently, resulting in a decrease in lactic acid volume, thus allowing the athlete to perform for a longer duration.

Again, it should not be ignored that one can have an average muscle fiber type composition but possess strengths in other genetic areas, VO2 max for example, and have as much success as someone whose fiber type composition gives them an advantage for the corresponding activity type whether it be aerobic or anaerobic. It’s all just a matter of trial and error; no one’s goals are clear right away. Like anything in life, it just takes some time to develop self-awareness and self-evaluation in order to put yourself on a path that will be most suited for you.

1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC521732/#idm139866823426672title
2. https://theboar.org/2018/10/bolt-legs/
3. Fink WJ, Costill DL, Pollock ML. Submaximal and maximal working capacity of elite distance runners. Part II: Muscle fiber composition and enzyme activities. Ann N Y Acad Sci. 1977;301:323–327. 
4. Foster C, Costill DL, Daniels JT, Fink WJ. Skeletal muscle enzyme activity, fiber in relation to distance composition and VO₂max running performance. Eur J Appl Physiol Occup Physiol. 1978;39:73–80.