Types of muscle fibers
What are muscle fibers?
The muscle fiber is a special class of cell that has the ability to contract and relax in response to various stimuli , which may be chemical, electrical or mechanical. They are spindle cells with abundant mitochondria; they are also called myocytes.
Muscle fibers are essential for life and are present in many organs to allow involuntary contraction that ensures their functionality (for example, in the walls of the stomach, intestine, bladder). In addition, they make up the muscles that enable our voluntary movements.
Classification of muscle fibers:
-Type I (they are also called red or slow twitch fibers): they predominate in the red muscles; they are small in diameter, rich in myoglobin and mitochondria, and poor in glycogen . They are very resistant to fatigue but generate little force , so they are important when doing low intensity activities that require resistance.
They have high oxidative capacity and contribute greatly to aerobic metabolism , so they play an important role during the development of sports activities with high oxygen demand (examples: marathon, cycling).
-Type IIa (another name given to these is intermediate fibers). They are also red fibers that resist fatigue , but not as much as the previous ones. Its power of contraction is greater. They have an intermediate level of oxidative activity and abundant glycogen. Furthermore, they have high ATPase and phosphorylase activity. They are especially demanded during activities where both strength and endurance come into play, such as during skating or swimming.
-Of type IIb (also called the fibers are white, of fast twitch or explosive ). It is the fastest type of muscle fiber. Its content of myoglobin and mitochondria is lower , which is why fatigue occurs much faster in this type of fiber than in the previous ones (they are anaerobic). They are also rich in glycogen. Although these types of fibers are associated with greater efforts, they can sustain you for less time.
They come into play mainly during intense exercises that demand a lot of strength, such as weight lifting. On the other hand, they are the ones with the greatest potential for expansion (up to 4 times), that is why they are the ones that “can be worked on” in disciplines that seek to increase muscle mass, such as bodybuilding.
The table below summarizes the main characteristics of the different types of muscle fibers:
| Type I | Type IIa | Type IIb | |
| Oxidative enzyme activity | high | High average | Come down |
| Glycolytic enzyme activity | Come down | high | high |
| Amount of glycogen | Low | Half | Half |
| Shrinkage time | 60-110 ms | 30-55 ms | 20-47 ms |
| Fatigue resistance | Very high | high | Come down |
| Size (µ 2 ) | 1.70 | 2.90 | 5.30 |
| Irrigation | Big | Big | Limited |
| Mitochondria | Many | Normal | Few |