In Standing, Corticospinal Excitability Is Proportional to COP Velocity Whereas M1 Excitability Is Participant-Specific.

Tulika Nandi,Lisanne B M Bakker,Helco G Van Keeken,Iris Kok,Claudine J C Lamoth,Beth E Fisher,George J Salem,Tibor Hortobágyi

doi:10.3389/fnhum.2018.00303

Abstract

Reductions in the base of support (BOS) make standing difficult and require adjustments in the neural control of sway. In healthy young adults, we determined the effects of reductions in mediolateral (ML) BOS on peroneus longus (PL) motor evoked potential (MEP), intracortical facilitation (ICF), short interval intracortical inhibition (SICI) and long interval intracortical inhibition (LICI) using transcranial magnetic stimulation (TMS). We also examined whether participant-specific neural excitability influences the responses to increasing standing difficulty. Repeated measures ANOVA revealed that with increasing standing difficulty MEP size increased, SICI decreased (both p < 0.05) and ICF trended to decrease (p = 0.07). LICI decreased only in a sub-set of participants, demonstrating atypical facilitation. Spearman’s Rank Correlation showed a relationship of ρ = 0.50 (p = 0.001) between MEP size and ML center of pressure (COP) velocity. Measures of M1 excitability did not correlate with COP velocity. LICI and ICF measured in the control task correlated with changes in LICI and ICF, i.e., the magnitude of response to increasing standing difficulty. Therefore, corticospinal excitability as measured by MEP size contributes to ML sway control while cortical facilitation and inhibition are likely involved in other aspects of sway control while standing. Additionally, neural excitability in standing is determined by an interaction between task difficulty and participant-specific neural excitability.

Highlights

Mechanical challenges and sensory manipulations of standing balance increase the spontaneous movements of the center of mass (Prieto et al, 1996; Amiridis et al, 2003)
In this study we aimed to determine the effects of standing task difficulty on peroneus longus (PL) corticospinal and M1 excitability, and the association between excitability and center of pressure (COP) velocity, in healthy young adults
Unlike AP sway (Papegaaij et al, 2014), increasing ML sway correlates with increasing tibialis anterior (TA) (ρ = 0.68; Nandi et al, 2018) and PL (ρ = 0.50) motor evoked potential (MEP)

Summary

Introduction

Mechanical challenges and sensory manipulations of standing balance increase the spontaneous movements of the center of mass (Prieto et al, 1996; Amiridis et al, 2003). Corticospinal excitability and primary motor cortex (M1) inhibition measured by transcranial magnetic stimulation (TMS), increases and decreases respectively (Tokuno et al, 2009; Baudry et al, 2014; Papegaaij et al, 2014, 2016; Nandi et al, 2018) Such neural adjustments tune muscle contractions, adjust COP dynamics and Corticospinal and M1 Excitability in Standing center of mass sway, thereby ensuring that balance is maintained. In contrast to anteroposterior (AP) and direction non-specific manipulations, we recently demonstrated that mediolateral (ML) manipulations of BOS produce correlated changes in the neural excitability of the tibialis anterior (TA) and COP velocity in young adults (Nandi et al, 2018) These findings are in line with EEG observations indicating that active neural control is greater during ML compared to AP sway (Slobounov et al, 2008). We examined, for the first time, M1 GABAb inhibition, which shows distinct task-specific modulation compared to GABAa inhibition (Opie et al, 2015)

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Conclusion