What physiological change occurs in muscles as they experience fatigue?

The correct answer is that decreased energy production occurs in muscles as they experience fatigue. When muscles are actively engaged in prolonged or strenuous activity, they require a significant amount of energy to continue functioning. This energy is primarily generated through processes such as aerobic respiration, which relies on the availability of oxygen and nutrients. As fatigue sets in, the efficiency of these processes diminishes, leading to a decline in energy production.

Additionally, as muscles tire, the buildup of metabolic byproducts such as lactic acid can occur, further impairing their ability to regenerate energy efficiently. This reduction in energy production is a primary factor contributing to the sensation of fatigue, as muscles become unable to sustain their normal level of performance.

The other options do not accurately reflect the physiological changes associated with muscle fatigue. Increased contraction force would not be expected, as fatigue typically leads to weaker contractions. Heightened oxygen intake may occur temporarily during intense exercise but does not sustain as fatigue develops, nor does it necessarily correlate with the actual energy production capacity of the muscles. Enhanced nutrient delivery might occur through increased blood flow during exercise, but ultimately, the reduced energy production is the critical factor leading to muscle fatigue.