Fatigue and the role of natural stimuli: an integrated approach to performance and perception

Fatigue is a multidimensional concept with physiological and psychological components, comprising two main perspectives: performance fatigability, defined as the decline in objective performance, and perceived fatigability, related to the changing sensations that influence the performer's perception of effort. Performance fatigability depends on muscle contractile properties and neural activation, while perceived fatigability is influenced by homeostasis and psychological state. Transcranial magnetic stimulation (TMS) is a noninvasive tool used to study central and peripheral fatigue. During voluntary contractions, TMS induces a silent period and transient muscle relaxation, offering insights into central contributions to fatigue. Unlike peripheral stimulation, TMS allows assessment of muscle properties under central drive, making it a more sensitive and ecological measure. This dissertation investigates the relationship between TMS-induced muscle relaxation and muscle fibre phenotype in the knee extensors, considering also sex differences in muscle composition and their influence under both unfatigued and fatigued conditions. It also explores how theta burst stimulation (TBS) modulates corticospinal excitability, using the stimulus-response (SR) curve as a measure in the biceps brachii. Additionally, the dissertation examines the effects of virtual nature immersion compared to urban scenarios on physiological responses, cognitive performance, and perceived fatigue during single-limb isometric exercise. Finally, it evaluates whether immersion in a natural virtual environment can modulate corticospinal excitability of the knee extensors, potentially through emotional engagement. The closing section summarizes key findings, highlights methodological strengths and limitations, and outlines directions for future research.