Fatigue in muscles or other systems can limit exercise. Muscle fatigue is a decrease in force production during intense exercise, when ATP supply does not keep up with ATP demand. Cellular mechanisms decrease ATP utilization (leading to decreased force) to maintain ATP levels and cellular homeostasis. An accumulation of inorganic phosphate and hydrogen ions in muscle fibers interacts with contractile proteins to reduce force production. During prolonged exercise, neuromuscular fatigue may result from a failure of excitation-contraction coupling, which involves decreased release of calcium from the sarcoplasmic reticulum. Arousal in the central nervous system also affects the state of fatigue by facilitating neural motor outputs, as can increased motivation.

Fatigue will also occur if VO2 is limited, as discussed previously. Similarly, metabolic substrates can limit exercise and cause fatigue. Phosphocreatine is the most important substrate for intense exercise that lasts only a few seconds. For exercise lasting less than 3 minutes, most energy comes from anaerobic glycolysis, so lactate buffering to control muscle and blood pH is also important. During aerobic exercise lasting more than an hour, glycogen stores in the muscle and liver are important. Thermoregulation and maintaining body fluids are also important for preventing fatigue during long-term exercise.

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