Multimodal neuroimaging of fatigability development.

Fatigability refers to the inability of the neuromuscular system to generate enough force to produce movements to meet task challenges. Fatigability has a central and a peripheral component linked via the neuromuscular system, but how these two components interact as fatigue develops lacks a complete understanding. The effects of fatigability are experienced in healthy humans but also accompany various disorders, often exacerbating their symptoms. We studied how fatigability develops in the neuromuscular system using multimodal neuroimaging. We recruited healthy participants to perform a fatiguing grip force task, while recording force, electromyography of forearm muscles (EMG), electroencephalography (EEG), and functional magnetic resonance imaging (fMRI) in 30-second blocks of grip task alternating with 30 seconds of rest. The task entailed maintaining 50% of the maximum force. We combined EMG and EEG to compute corticomuscular coherence and combined EEG and fMRI to compute EEG-informed fMRI. We selected eight task blocks specific to each participant to represent how the neuromuscular system adapted from pre-fatigability to actual fatigability. Those included five blocks for pre-fatigability in which participants could generate enough force to match the required 50% of maximum force and three blocks when the force fell below that limit. Across blocks of the grip force task, we observed changes in the neuromuscular system that preceded grip force changes. We found that electromyography of arm muscles shifted from high to low frequency, EEG in the channel covering the contralateral sensorimotor area increased steadily up to the fifth block and then plateaued, and fMRI signal also increased in the cerebellum. Corticomuscular coherence increased within each of the 30-second blocks of the grip task. EEG-informed fMRI revealed areas of the brain that the traditional regression did not, including the bilateral sensorimotor cortex, temporal-parietal junction, and supplementary motor area. Thus, as fatigability developed, the neuromuscular system experienced changes earlier than the actual behavior. While we found evidence for fatigability of central and peripheral origins, peripheral fatigue seems to occur first.