Slow vs Fast Twitch Muscle Fibers: Structure and Function
Basic Classification
Muscle fibers are primarily classified into two main types:
- Type I (Slow Twitch)
- Type II (Fast Twitch)
Fast twitch fibers can be further subdivided into Type IIa and Type IIb, but for IB SEHS purposes, we'll focus on the main distinction between Type I and Type II.
Structural Differences
Fiber Size and Appearance
- Type I (Slow Twitch):
- Smaller in diameter
- Red in appearance due to high myoglobin content
- More capillaries surrounding each fiber
- Higher mitochondrial density
- Type II (Fast Twitch):
- Larger in diameter
- Whiter in appearance
- Fewer capillaries
- Lower mitochondrial density
Internal Structure
- Type I fibers:
- More extensive capillary network
- Higher number of mitochondria
- Greater storage of triglycerides (fat)
- Type II fibers:
- Larger glycogen stores
- More extensive sarcoplasmic reticulum
- Higher concentration of enzymes for glycolysis
Functional Differences
Energy Systems
- Type I:
- Primarily uses aerobic metabolism
- Excellent fat oxidation capabilities
- Slower energy production but more sustainable
- Type II:
