Final answer:
Muscle fatigue occurs when a muscle can no longer contract in response to signals from the nervous system due to factors such as decreased ATP reserves, lactic acid buildup, and imbalances in ion levels. Long periods of exercise can also lead to damage in the sarcoplasmic reticulum and sarcolemma.
Step-by-step explanation:
Muscle fatigue occurs when a muscle can no longer contract in response to signals from the nervous system. The exact causes of muscle fatigue are not fully known, although certain factors have been correlated with the decreased muscle contraction that occurs during fatigue. ATP is needed for normal muscle contraction, and as ATP reserves are reduced, muscle function may decline. This may be more of a factor in brief, intense muscle output rather than sustained, lower intensity efforts. Lactic acid buildup may lower intracellular pH, affecting enzyme and protein activity. Imbalances in Na+ and K+ levels as a result of membrane depolarization may disrupt Ca flow out of the SR. Long periods of sustained exercise may damage the SR and the sarcolemma, resulting in impaired Ca regulation.