What is muscle fatigue primarily characterized by?

Muscle fatigue is primarily characterized by a loss of force or power output. This phenomenon occurs during prolonged or intense exercise when the muscle's ability to generate force declines, affecting performance. Physiologically, muscle fatigue arises from a complex interplay of factors, including the depletion of energy substrates, accumulation of metabolites, and changes in muscle membrane excitability.

While factors such as lactic acid production can relate to muscle fatigue, they are more indicative of the biochemical processes that accompany fatigue rather than defining its primary characteristic. The decrease in muscle size refers to atrophy, which is a long-term change rather than an immediate effect of fatigue. A temporary drop in heart rate does not describe muscle fatigue directly, as it relates more to cardiovascular responses to exercise rather than the functionality of muscle itself.

Thus, the correct characterization of muscle fatigue is fundamentally about the loss of force or power output, emphasizing the muscle's diminished ability to contract effectively during physical activity.