By decreasing CMC excitability, ULI holds the potential for being explored as a stroke model in healthy individuals for developing rehabilitation strategies ( Furlan et al., 2016 ). Determining the minimum effective restriction time is critical for optimising the immobilisation paradigm and facilitating its application.
How does CMC excitability change over a period of 9 h of ULI?
Healthy individuals will have their right (dominant) upper limb immobilised for 9 h. CMC excitability will be assessed with TMS immediately before and after 3, 6, and 9 h of immobilisation. The TMS coil will be positioned over the hot spot of the right FDI muscle. Frameless stereotaxy will be used to keep the position of the coil constant across all TMS assessments. Fifteen MEPs will be recorded from the target muscle during each TMS assessment by using a fixed suprathreshold stimulation intensity (sSI). IO curves will also be obtained at each assessment by using 50, 70, 90, 110, and 130% of sSI.
Fig. 1 shows preliminary MEP data from 5 participants. At the group level there was a depressant effect of immobilisation on CMC excitability. CMC excitability decreased to 63 and 54% of the baseline value after 3 and 6 h of immobilisation, respectively. However, after 9 h, excitability levels increased to 84% of the baseline value, suggesting that CMC excitability might follow a U-shaped curve during ULI. At the individual level there was great variability in CMC excitability among participants over the course of immobilisation, particularly in terms of the position of the deflection point of the excitability curve.
Our data is in line with previous studies reporting inter-individual differences in CMC plasticity after ULI ( Rosenkranz et al., 2014 ). Importantly, our study shows how these differences develop during ULI. This information should be given consideration when seeking for the ideal length of the immobilisation protocol.
Source: P311 Individual differences in contralateral motor cortex (CMC) plasticity during short-term upper limb immobilisation (ULI) in healthy individuals – A transcranial magnetic stimulation (TMS) study – Clinical Neurophysiology