An Investigation of Pain and Fatigue During Exercise

Abstract

Exercise performance is governed by a complex interaction of numerous physiological and psychological processes involving the neural, musculoskeletal, cardiorespiratory, and metabolic systems. Fatigue and pain are two distinct, highly prevalent, and intertwined exercise-induced psychophysiological phenomena; though they have the potential to determine performance, their underpinning mechanisms are not well-understood. The possible interactions between fatigue and pain with processes related to sensory feedback, central motor command, and the perception of exercise warrants deeper comprehension. Thus, this thesis sought to investigate how fatigue and pain modulate exercise performance and associated psychophysiological responses. Study 1 (Chapter Three) demonstrated that when three exercise bouts are completed across 24 hours, prolonged depressions in central activation measures occur despite no cumulative impairment in muscle fatigue or perceptual responses. Study 2 (Chapter Four) extended this experimental framework by investigating five consecutive days of interval cycling performed to exhaustion. By the end of the week, peripheral fatigue and metabolic stress were mitigated, while corticospinal excitability was suppressed and perceptual responses were exacerbated. Both these “fatigue” experiments demonstrated that overall performance (time-to-task failure) was retained, indicating this maintenance was due to a counterbalance between exercise-induced physiological and perceptual responses. In Study 3 (Chapter Five), contralateral experimental pain impaired isometric knee extension performance but did not alter any corticospinal or neuromuscular parameters. This study was extended to dynamic exercise in Study 4 (Chapter Six), where cycling performance was again impaired by experimental pain with most corticomotor measures remaining primarily unchanged. These “pain” experiments demonstrated that the blunting effect of pain on performance occurred through an augmentation of perceptual responses and did not modulate neurophysiological factors. The overall findings of this thesis suggest that i) physiological and perceptual responses to exercise are dissociated and modulate performance independently of each other, and ii) performance is ultimately limited by an individual’s perception. Future studies can continue to utilize integrative research approaches to address chronic adaptations to consecutive fatiguing or painful exercise and better elucidate the biological mechanisms of exercise-evoked perceived responses.