To start, let’s review the basics of muscle fibers. There are two main types:
- Type I “Slow twitch”: These Fibers are characterized by their high endurance ability and their low force / power / speed output. They can sustain long-term contractions (as used in posture).
- Type II: “Fast twitch”: These fibers are characterized by high force / power / speed output and their low endurance capability. They are used for short-term contractions. They can be subdivided into:
- Type IIa “Fast Oxidative”: More resistant to fatigue than IIb
- Type IIb “Fast Glycolytic”: Rapidly fatiguing, strongest power output
After understanding the basics, there are a lot of commonly asked questions, such as:
- Why do the different types of fibers have more or less endurance capability?
- The ability of type I fibers to have high endurance is directly related to its ability to support aerobic metabolism. They have high concentrations of mitochondria, myoglobin, and are naturally surrounded by more capillaries. Type II fibers don’t have this ability to utilize oxygen to create energy as efficiently, so they rely on anaerobic metabolism which only lasts for a short duration until it fatigues.
- Why do the different fibers have more or less power?
- The enzyme myosin ATPase is used to catalyze the reaction that ultimately results in a muscle contraction. In type I fibers, there is low enzyme activity, whereas in type II fibers there is high activity. This leads to faster contractile output for the type II fibers, thereby making them better for power.
- Are people naturally one type over the other?
- The average person is roughly a 50/50 combination of type I and type II muscle fibers. However, for more skilled athletes, differences may be apparent. Sprinters and power athletes tend to have higher levels of type II(b) fibers, while distance athletes have more type I fibers. It’s important to remember that aside from high individual differences with fiber proportions, each different muscle itself is naturally a unique combination of the fiber types. So, for example, the hamstrings might be 60% fast twitch and 40% slow twitch, while the quadriceps have 40% fast twitch and 60% slow twitch.
- Can you change fiber types?
- Generally, the fibers are very prone to change from fast to slow (type IIb to type IIa and type IIa to type I). They become more efficient in their ability to utilize oxygen. It’s much more difficult to switch from slow to fast in training, mainly because the type IIb fibers are extremely fatigable and require very short bursts of power/speed and low-volume amount of training. Even elite-level sprinters who are in the peak of their training show a conversion of the type IIb to type IIa fibers. It’s only when they ease down their training that the type IIb starts to come back.
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Written by Dr. Brandon Buchla, DC, CSCS
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