Which of the following can lead to muscle fatigue in untrained individuals?

Muscle fatigue in untrained individuals is primarily associated with prolonged high-intensity exercise. This type of exercise places significant demands on the muscles, which can lead to a rapid depletion of energy stores and an accumulation of metabolic byproducts, such as lactic acid. For individuals without a baseline of fitness, the muscles may not be accustomed to such intensity or duration, making fatigue more likely.

During prolonged high-intensity activities, the body struggles to supply adequate oxygen to the muscles, resulting in anaerobic metabolism, which is less efficient than aerobic metabolism. The resulting fatigue is characterized not only by a decrease in muscle performance but also by a feeling of heaviness or weakness in the muscles due to the strain placed on them.

In contrast, short exercise duration and low-intensity activities tend to be manageable for untrained individuals and may not lead to significant muscle fatigue. Light stretching exercises also do not typically induce fatigue, as they are designed to enhance flexibility without imposing high demands on energy stores or muscle endurance.