Types of Muscle Fibers

Introduction

• Skeletal muscle consists of two types of muscle fibers
  • type I
  • type II
    • type IIa
    • type IIb
• Muscle fiber types vary across several characteristics
  • resistance to fatigue
  • color
    • determined by an amount of myoglobin
  • type of metabolism
    • aerobic vs. anaerobic
    • determined by relative amounts of mitochondria and glycogen
• Non-athletes will have approximately equal amounts of type I and type II muscle fibers
• Athletes will have different ratios of type I and type II muscle fibers depending on their predominant activities

Type I Muscle Fibers 

• Also known as slow twitch muscle fibers
• Color
  • red
    • more myoglobin
• Fatigue tolerance
  • high
  • able to maintain sustained contractions
    • higher proportion in endurance runner’s muscles
• Type of metabolism
  • aerobic
    • contains more mitochondria to carry out oxidative phosphorylation 
    • contains less glycogen
• Function
  • postural control
  • walking
• Mnemonic
• “One Slow Red Ox” = type I muscles are slow twitch, red, and use oxidative metabolism

Type II Muscle Fibers

• Also known as fast twitch muscle fibers
• Color
  • white
    • less myoglobin
• Fatigue tolerance
  • low
  • best suited for short high-intensity contraction
    • higher proportion in sprinter’s muscles
• Type of metabolism
  • anaerobic
    • contains less mitochondria
    • contains more glycogen to carry out glycolytic metabolism
• Subtypes
  • type IIa
    • “fast twitch oxidative”
    • aerobic and anaerobic metabolism
    • intermediate contraction strength
  • type IIb
    • “fast twitch glycolytic”
    • anaerobic metabolism only
    • highest contraction strength
• Function
  • power activities
    • sprinting
    • powerlifting