The Task at Hand: Fatigue-Associated Changes in Cortical Excitability during Writing.

Kezia T M Cinelli,Lara A Green,Jayne M Kalmar

doi:10.3390/brainsci9120353

Kezia T M Cinelli, Lara A Green + Show 1 more

Open Access

https://doi.org/10.3390/brainsci9120353

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Journal: Brain sciences	Publication Date: Dec 2, 2019
Citations: 4	License type: CC BY 4.0

Affiliation: Wilfrid Laurier University

Abstract

Measures of corticospinal excitability (CSE) made via transcranial magnetic stimulation (TMS) depend on the task performed during stimulation. Our purpose was to determine whether fatigue-induced changes in CSE made during a conventional laboratory task (isometric finger abduction) reflect the changes measured during a natural motor task (writing). We assessed single-and paired-pulse motor evoked potentials (MEPs) recorded from the first dorsal interosseous (FDI) of 19 participants before and after a fatigue protocol (submaximal isometric contractions) on two randomized days. The fatigue protocol was identical on the two days, but the tasks used to assess CSE before and after fatigue differed. Specifically, MEPs were evoked during a writing task on one day and during isometric finger abduction to a low-level target that matched muscle activation during writing on the other day. There was greater variability in MEP amplitude (F (1,18) = 13.55, p < 0.01) during writing compared to abduction. When participants were divided into groups according to writing style (printers, n = 8; cursive writers, n = 8), a task x fatigue x style interaction was revealed for intracortical facilitation (F (1,14) = 9.90, p < 0.01), which increased by 28% after fatigue in printers but did not change in cursive writers nor during the abduction task. This study is the first to assess CSE during hand-writing. Our finding that fatigue-induced changes in intracortical facilitation depend on the motor task used during TMS, highlights the need to consider the task-dependent nature of CSE when applying results to movement outside of the laboratory.

Highlights

Single-pulse transcranial magnetic stimulation (TMS) elicits motor evoked potentials (MEPs) that are recorded from the muscle of interest using surface electromyography (EMG)
MEPs are used as a measure of corticospinal excitability (CSE) that reflect the excitability of the pathway from the site of stimulation to the site of recording, such that both cortical and spinal mechanisms contribute to changes in the MEP evoked using single-pulse TMS
In the pre-fatigue trials, there was no effect of task on the amplitude of the test MEP (F(1,18) = 0.60, p = 0.45, η2p = 0.03, Figure 5A), intracortical facilitation (ICF) (F(1,18) = 1.69, p = 0.21, η2p = 0.09, Figure 5C), or CSP (F(1,17) = 1.14, p = 0.30, η2p = 0.06)

Summary

Introduction

Single-pulse transcranial magnetic stimulation (TMS) elicits motor evoked potentials (MEPs) that are recorded from the muscle of interest using surface electromyography (EMG). Intracortical mechanisms that may contribute to CSE are assessed using conditioned MEPs elicited via paired-pulse TMS. Most TMS protocols are conducted with the muscle at rest or during submaximal isometric muscle contractions to standardize levels of muscle activation and control for joint position, muscle length, movement, and other factors. These laboratory tasks are intended to minimize variability, they do not reflect activities of daily living to which the results may be extrapolated.

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