How are motor units classified?

Updated: Oct 16, 2019
  • Author: Friedhelm Sandbrink, MD; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE  more...
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The 3 main types of motor units, which have different physiologic and staining properties, include the following:

  • Type I or type S (slow) - Slow twitch, fatigue-resistant units with smallest force or twitch tension and slowest contraction; contain oxidative enzymes

  • Type IIa or type FR (fast, resistant) - Fast twitch, fatigue-resistant units with larger forces and faster contraction times; contain oxidative and glycolytic enzymes

  • Type IIb or type FF (fast, fatigable) - Fast twitch, easily fatigable units with largest force and fastest contraction; contain glycolytic enzymes

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