What primarily causes muscular fatigue during 20 minutes of intense exercise?

Muscular fatigue during intense exercise is primarily associated with the build-up of lactic acid. When the body is engaged in vigorous physical activity, the muscles rely heavily on anaerobic metabolism for energy. This process occurs when the demand for energy exceeds the oxygen supply available to the muscles, forcing them to produce energy through anaerobic pathways.

During this anaerobic process, pyruvate, which is produced from glucose during glycolysis, is converted into lactic acid. As exercise continues and increases in intensity, lactic acid accumulates in the muscles, leading to an increase in acidity. This change in acidity can inhibit muscle contractions, impact enzymatic functions, and create a painful sensation, all of which contribute to feelings of fatigue.

The other options present scenarios that may contribute to fatigue but are not the primary causes during the 20 minutes of intense exercise. Inadequate glycogen stores might influence endurance over a longer duration, low oxygen levels would be relevant, but anaerobic glycolysis allows for short bursts of energy despite limited oxygen, and depletion of protein typically affects muscle repair and growth rather than immediate fatigue during exercise. Thus, the build-up of lactic acid remains the most significant factor in causing muscular fatigue in this context.