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

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.

The Potential Role of Neurophysiology in the Management of Multiple Sclerosis-Related Fatigue

“…By contrast, Hunter et al (51) likewise found a reduction of SICI, while ICF decreased during a sustained submaximal voluntary muscle contraction. Besides that, changes of ICF or SICI seem to depend also on the type of fatiguing motor task used in the experimental protocol-for instance, being different during handwriting compared to isometric finger abduction (52).…”

Moving forward: methodological considerations for assessing corticospinal excitability during rhythmic motor output in humans

“…Using a combination of the above-mentioned methods for setting stimulation intensities, it is now well established that corticospinal excitability is influenced by the motor task (25,(73)(74)(75). As such, task-dependent changes in corticospinal excitability are important to consider when selecting stimulation parameters for a given study.…”