Fatigue and Endurance

Muscle fatigue is the progressive weakness and loss of contractility that results from prolonged use of the muscles. For example, if you hold this book at arm's length for a minute, you will feel your muscles growing weaker and

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Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 11 Muscular Tissue 429

eventually you will be unable to hold it up. Repeatedly squeezing a rubber ball, pushing a video game button, or trying to take lecture notes from a fast-talking professor produces fatigue in the hand and finger muscles. Fatigue has multiple causes:

  • ATP synthesis declines as glycogen is consumed.
  • The ATP shortage slows down the sodium-potassium pumps, which are needed to maintain the resting membrane potential and excitability of the muscle fibers.
  • Lactic acid lowers the pH of the sarcoplasm, which inhibits the enzymes involved in contraction, ATP synthesis, and other aspects of muscle function.
  • Each action potential releases potassium ions from the sarcoplasm to the extracellular fluid. The accumulation of extracellular K+ lowers the membrane potential and excitability of the muscle fiber.
  • Motor nerve fibers use up their acetylcholine, which leaves them less capable of stimulating muscle fibers. This is called junctional fatigue.
  • The central nervous system, where all motor commands originate, fatigues by processes not yet understood.

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