Do All Paths Lead to Mental Fatigue or Is It Merely a Self-Fulfilling Prophecy?

Fatigue, long recognized as a limiting factor in physical performance, has traditionally been conceptualized as comprising both objective (performance fatigability) and subjective (perception of fatigue) dimensions. In sports science, increasing attention has been paid to "mental fatigue", a state induced by mental efforts, owing to its presumed negative effects on subsequent physical performance. However, this opinion paper argues that the dominant focus on mental fatigue as a singular construct is overly narrow and potentially misleading. Many studies rely on mental tasks (e.g., Stroop test) to induce mental fatigue, assuming that a decline in task performance or self-reported fatigue only equates to mental fatigue. Yet such declines are neither universal nor easily attributable solely to fatigue. Factors such as boredom, mind-wandering, motivation, task automation, and affective states also influence performance and perception during mental tasks. Our research demonstrates that subjective experience during mental tasks is dynamic and multidimensional, with performance decrement and perceived fatigue varying widely across individuals and time. We advocate for a shift from the mental fatigue paradigm to a broader framework, which more accurately captures the interplay between cognitive demands and subjective states. Using tools such as the Temporal Experience Tracing (TET) method, we highlight the complex array of responses during mental tasks that go beyond fatigue alone. In doing so, we challenge current assumptions and call for more nuanced methodologies to assess how mental states affect subsequent physical performance.