Muscle Fibers · Histology

Type IIa muscle fibers are often referred to as 'intermediate' because they:

  1. Have fast contraction with moderate aerobic capacity
  2. Are found only during early fetal muscle development
  3. Depend on anaerobic glycolysis as their energy source
  4. Are a type of fiber found in cardiac muscle tissue
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Correct answer: Have fast contraction with moderate aerobic capacity

Type IIa fibers are fast oxidative-glycolytic fibers. They possess the high myosin ATPase activity of fast-twitch fibers but also maintain a significant number of mitochondria and myoglobin to resist fatigue better than Type IIb fibers.

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