Chapter 5: Functions of Muscles

25. Muscle fiber classification

Muscles are composed of individual muscle fibers that differ in their characteristics and functions. Muscle fibers can be classified into different types based on their contractile properties and energy systems they utilize. The two main types of muscle fibers are:

a) Slow-twitch (Type I) fibers: These muscle fibers are more fatigue-resistant and designed for endurance activities. They contract slowly and can sustain contractions over a longer period. Slow-twitch fibers rely on aerobic metabolism for energy production and are rich in mitochondria and myoglobin.

b) Fast-twitch (Type II) fibers: These muscle fibers contract rapidly and generate more force, but they fatigue more quickly. Fast-twitch fibers can be further divided into two subtypes:

•   - Fast-twitch oxidative-glycolytic (Type IIa) fibers: These fibers possess characteristics of both slow-twitch and fast-twitch fibers. They have a moderate resistance to fatigue and can generate force quickly. They rely on both aerobic and anaerobic energy systems.

•   - Fast-twitch glycolytic (Type IIb or IIx) fibers: These fibers are optimized for short bursts of intense power. They contract rapidly and fatigue quickly. Fast-twitch glycolytic fibers primarily rely on anaerobic metabolism