Mixed

Exercise intensity exceeding Critical Power (severe-intensity domain) causes fatigue through the attainment of a consistent, limiting muscle metabolic milieu (low pH, low PCr, high lactate), whereas fatigue at lower intensities (heavy/moderate) is constrained by substrate availability (glycogen depletion) and limitations to muscle activation rather than this specific metabolic threshold.

To optimize performance, recognize that exercising above your Critical Power (CP) triggers a specific metabolic failure (acidosis, phosphocreatine depletion) that limits tolerance to a consistent threshold regardless of exact workload. Below CP, fatigue is driven by different factors like glycogen use. Therefore, pacing strategies must account for this non-linear shift in fatigue mechanisms; you cannot simply scale up moderate-intensity endurance tactics to severe-intensity efforts.