Muscle fatigue

Muscle fatigue is defined as the inability of a muscle to maintain a particular degree of contraction over time. The onset of fatigue is quite variable and influenced by several factors, including:

• Intensity and duration of contractile activity

• Utilization of aerobic vs. anaerobic metabolism for energy

• Composition of the muscle

• Fitness level of the individual

Although the exact mechanisms leading to muscle fatigue remain somewhat unclear, several factors have been implicated:

• Depletion of energy reserves

• Accumulation of lactic acid

• Increase in inorganic phosphate

Depletion of glycogen stores within contracting skeletal muscles fibers is associated with the onset of fatigue. Interestingly, this occurs even though the muscle may be utilizing fatty acids as its primary energy source. The accumulation of lactic acid lowers the pH within the muscle and the change in pH may ultimately alter the activity of enzymes involved with energy production as well as crossbridge cycling. The breakdown of creatine phosphate causes an increase in concentration of inorganic phosphate. Fatigue associated with elevated inorganic phosphate may be due to slowed release of Pi from myosin and therefore a decreased rate of crossbridge cycling. It may also involve decreased sensitivity of the contractile proteins to calcium, which would also impair crossbridge cycling.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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