Updated: 1/22/2018

Types of Muscle Fibers

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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

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