Dissertation Defence: The Effect of Repetitive Maximal-Effort Concentric Elbow Extensions on Cortical and Motoneuronal Excitability

Phuong (Lisa) Lan Ha, supervised by Dr. Brian Dalton, will defend their dissertation titled “The Effect of Repetitive Maximal-Effort Concentric Elbow Extensions on Cortical and Motoneuronal Excitability” in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Kinesiology.

An abstract for Phuong (Lisa) Lan Ha’s dissertation is included below.

Examinations are open to all members of the campus community as well as the general public. Registration is not required for in-person exams.

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Abstract

Although fatigue is known to be task-dependent, the neuromuscular mechanisms underlying fatigue from repetitive, dynamic contractions remain less understood than those associated with isometric tasks. This dissertation examined the effects of fatigue on cortical and motoneuronal excitability using three types of maximal-effort, repetitive, dynamic contractions of the elbow extensors: unconstrained velocity with a resistance of 30% maximal voluntary isometric contraction (MVC) torque (Chapter 2), as well as 40°/s (Chapter 3) and 240°/s (Chapter 4) isokinetic contractions. Fatigue was characterized by reductions in peak power and MVC torque. Neuronal excitability of the triceps brachii was assessed using surface electromyography: motor evoked potentials (MEPs), cervicomedullary motor evoked potentials (CMEPs), and maximal compound muscle action potentials (Mmax). Cortical excitability (MEP/CMEP), motoneuronal excitability (CMEP/Mmax), and peripheral excitability (Mmax area) were evaluated pre-fatigue (PRE), during the fatigue task and at task termination, as well as throughout 10 min of recovery. Despite differences in contraction type, velocity and experimental design, fatigue consistently resulted in substantial power and MVC torque loss post task termination compared to the pre-fatigue values. Cortical excitability was often elevated immediately following the fatiguing task (Chapters 2) or during recovery (Chapters 3 and 4), whereas motoneuronal excitability was either unchanged (Chapters 2) or reduced (Chapters 3 and 4), particularly during recovery (Chapters 3 and 4). Peripheral excitability remained mostly unchanged across protocols (Chapters 2 and 3). These findings indicate that fatigue induced by dynamic, repetitive elbow extensions resulted in divergent time courses of cortical and motoneuronal excitability adaptations, and fatigue-related reductions in motoneuronal excitability may limit power recovery despite increased cortical excitability. Collectively, this dissertation highlights the complexity of fatigue-related alterations in neuromuscular function and neuronal excitability for dynamic contractions. Additionally, this dissertation emphasizes the importance of considering the task-dependent nature of fatigue, especially as it relates to contraction type, velocity, and recovery when evaluating and interpreting corticospinal excitability assessments.