Library · Histology
Muscle fiber types
Make it clearer
Not all skeletal muscle fibers are alike. They fall into three types: I, IIa and IIb.
Type I fibers are the slow ones. They contract slowly, resist fatigue, and are the ones that let you keep going: standing upright all day, running a marathon.
Type I and type IIa fibers are called red, and the color is not decoration. It comes from myoglobin, a protein similar to hemoglobin that stores oxygen inside the muscle. With oxygen on hand they draw energy through oxidative phosphorylation, the efficient method, which is why they are packed with mitochondria, the cell's energy factories. Type IIa fibers sit in between: fast, but for medium-length efforts such as walking or cycling.
Type IIb fibers are the white ones, with little myoglobin and few mitochondria. They draw energy through anaerobic glycolysis, that is, without oxygen, which is fast but runs out quickly. It is the difference between a marathon runner and a sprinter: the same body, two engines on different fuel.
In brief
Skeletal muscle fibers fall into three types: I, IIa and IIb.
Type I fibers are slow, resist fatigue and handle endurance work, such as holding posture.
Type IIa fibers are fast with intermediate fatigue, for walking or cycling; with type I they are red from myoglobin, packed with mitochondria and oxidative.
Type IIb fibers are white, run on anaerobic glycolysis and tire quickly: sprinting and lifting weights.
A marathon runner and a sprinter: the same body, two engines on different fuel.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingPhysiology, Skeletal Muscle, StatPearlsPublished
This page explains the topic in general terms; it does not tell anyone what treatment to follow.
Updated