Author(s): MATTA, P., BAURÈS, R., ZÉNON, A., DUCLAY, J., SILVA, S., REDDY, L., ALAMIA, A., Institution: INSERM , Country: FRANCE, Abstract-ID: 2259

INTRODUCTION:
While the recruitment of motor brain areas during brief contractions is well established (1), the neural mechanisms underlying sustained force over time remain debated, with conflicting evidence on the need for continued activation of motor areas during low-force tasks (2-4). Critically, how motor control and fatigue-related brain activity evolve across repeated sustained contractions remains poorly understood. Here, we combined fMRI and EEG to identify neural signatures of motor control and fatigue within a sustained contraction, and across a prolonged fatiguing task.
METHODS:
Participants (n=26 for fMRI; n=32 for EEG) performed 100 isometric wrist flexions (16s contraction, 8s rest) at 20% of maximal voluntary force. Maximal voluntary force was assessed before and after the task, and perceived effort was recorded throughout. fMRI data were preprocessed in SPM12 (slice-timing correction, motion realignment, normalization to MNI space, and spatial smoothing). EEG data were preprocessed in EEGLAB, including filtering, bad-channel rejection, re-referencing, and ICA artifact removal. EEG spectral power was estimated using wavelets, focusing on beta activity in motor and frontal clusters. Neural correlates of individualized force parameters were quantified using GLMs. Contraction phases (initiation, maintenance, release) and task progression were analyzed using repeated-measures ANOVAs and paired t-tests, and neural measures were correlated with force loss and perceived effort. Multiple comparisons were controlled using FWE correction (p<0.05) for fMRI and FDR correction (p<0.05) for EEG and correlations.
RESULTS:
BOLD and beta power in motor regions reflected the absolute value of the force derivative rather than force. Within contractions, BOLD increased from the early to the late stage of the maintenance phase in motor, cingulate, subcortical, and frontal regions, whereas beta power decreased specifically in motor areas. Across contractions, we observed an overall increase in BOLD during the maintenance phase, alongside a progressive decrease in beta power in motor areas. Motor and cingulate BOLD increases specifically correlated with pre-to-post force loss. Motor beta power decrease correlated with increases in perceived effort. Finally, as perceived effort increased, participants progressively slowed contraction initiation, which correlated with reduced BOLD during the initiation phase. No specific fatigue-related pattern was observed in the release phase.
CONCLUSION:
Using both EEG and fMRI, we show that motor control during sustained submaximal contractions primarily relies on neural encoding of force changes. We also identify a broader fatigue-related network whose contribution evolves dynamically throughout the contraction and across the task, accompanied by behavioral adaptations likely to reduce neural demand.

1. Porro et al., J Neurosci, 1996
2. Kuhtz-Buschbeck et al., Eur J Neurosci, 2001
3. Yang et al., Open Neuroimaging J, 2011
4. van Duinen et al., NeuroImage, 2007