Author Identifier

Liam Tapsell: https://orcid.org/0000-0002-8052-196X

Date of Award

2025

Document Type

Thesis

Publisher

Edith Cowan University

Degree Name

Doctor of Philosophy

School

School of Medical and Health Sciences

First Supervisor

Christopher Latella

Second Supervisor

Janet Taylor

Third Supervisor

Anthony Blazevich

Abstract

Interhemispheric inhibition (IHI) is a signal, from one hemisphere to the other, that suppresses activity and can decrease motor output from the target hemisphere. IHI is thought to contribute to balancing forces between sides of the body, evidenced particularly in clinical populations, making it important to investigate changes during bilateral contractions. IHI can be assessed using transcranial magnetic stimulation (TMS). Dual site TMS allows comparison of unconditioned motor evoked potentials (MEPs) to those conditioned by contralateral stimulation, resulting in short- (SIHI) and long-interval IHI (LIHI). SIHI and LIHI are measured using different interstimulus intervals and represent separate inhibitory mechanisms. IHI of muscle activation is also measured as a reduction in electromyography (EMG), known as the ipsilateral silent period (ISP), via stimulation of the ipsilateral hemisphere. This thesis incudes three studies which investigated whether IHI was affected by voluntary contraction intensity or muscle fatigue, and if modulation of IHI affected reaction time.

Study 1. To comprehensively investigate IHI, it is important to first understand how it may change with different levels of voluntary activation. Literature on the topic is conflicting and primarily uses unilateral contractions but uniformly suggests that at least some form of IHI is altered by contralateral contraction. Thus, the first study aimed to assess the effect of contraction intensity on SIHI, LIHI and the ISP. Participants contracted left and right first dorsal interossei (FDI) between 0% and 100% maximal voluntary isometric contraction (MVIC) in different configurations. Varying contraction intensity of the right (conditioning) FDI during steady contraction of left FDI showed weaker SIHI (less inhibition) with the right hand at rest than with it active, no differences in LIHI, and a larger ISP with maximal right-hand contraction than rest or submaximal contraction. When varying contraction intensity of the left (tested) FDI while right FDI steadily contracted, SIHI was stronger during left FDI rest than active conditions, but there was no effect on LIHI. ISP was significantly shorter in the maximal condition than other conditions. These results show that contraction intensity should be considered when measuring SIHI and ISP but does not appear to impact LIHI. This suggests that for future studies involving SIHI and ISP, but not LIHI, need to consider contraction intensity in the aims and implications of the research.

Study 2. If IHI works to balance forces between contralateral muscles, altering this force-balance might result in IHI modulation. Therefore, the second study aimed to assess if muscle fatigue affected IHI. Participants performed bilateral contractions of their FDIs with and without a preceding unilateral fatiguing right MVIC which reduced the force-generating ability of the right FDI. TMS was used to assess unconditioned MEP size, SIHI, LIHI and ISP. The contralateral silent period that follows an MEP was also measured. Muscle fatigue did not appear to have any effect on the unconditioned MEP, nor SIHI, LIHI, or ISP. Fatigue did not alter the silent period following unconditioned MEPs, but silent periods following SIHI[1]conditioned MEPs were shorter with contralateral fatigue. This potentially indicates that fatigue increased IHI onto the inhibitory neurons that subserve the silent period. These results indicate no change to IHI of excitatory neurons under fatigue, suggesting fatigue is not a factor in acute IHI modulation.

Study 3. Knowledge of how IHI affects voluntary movement in healthy populations is limited. Given IHI’s effect on motor excitability, the third study aimed to measure if changes in IHI would affect reaction time. Participants held a target abduction or adduction force with their right index finger and simultaneously performed a reaction time task requiring left index finger abduction as a response. TMS was delivered during the pre-movement phase to elicit MEPs, SIHI and LIHI. Motor excitability at the time of stimulus was higher during right abduction and there was weaker SIHI (less inhibition), but no difference in LIHI. Importantly, reaction time was faster during right abduction. Thus, stronger SIHI brought about by an action of one hand increased reaction time of the other hand, demonstrating an influence of IHI on voluntary movement initiation in a healthy population.

These studies work together in assessing the interplay between motor output and IHI. The present thesis expanded on previous research into the effect of contraction on IHI by taking measures across a wide spectrum of bilateral contractions in unfatigued and fatigued states. These results are relevant to the both the research of IHI and to clinical populations whose motor symptoms appear related to the phenomenon. Furthermore, the impact of IHI on motor output is demonstrated by showing its modulation can alter motor initiation.

DOI

10.25958/bnk2-v709

Access Note

Access to this thesis is embargoed until 1st January 2027

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Available for download on Friday, January 01, 2027

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