What causes muscle fatigue?

Muscle fatigue primarily occurs due to the depletion of energy sources and the accumulation of lactic acid within muscle fibers during sustained activity. During intense or prolonged exercise, muscles use up their stored adenosine triphosphate (ATP) and creatine phosphate, which are critical for energy provision. As energy stores decline, the body becomes less efficient at contracting muscles.

In addition, when oxygen supply is insufficient for aerobic metabolism, muscles switch to anaerobic metabolism, leading to lactic acid formation as a byproduct. The accumulation of lactic acid can lower the pH within the muscle fibers, which may interfere with muscle contraction and lead to the sensations associated with fatigue, such as muscle weakness and discomfort.

The other options do not accurately capture the main physiological events that lead to muscle fatigue. For instance, while hydration status is important for overall performance, excessive hydration does not cause fatigue directly. Overstimulation of neurons and increased nerve impulses, while they can lead to excessive signal transmission, do not account for the biochemical and metabolic factors that are primarily responsible for muscle fatigue.