Single-limb Stance Increases Cortical Excitability To The Lower-leg Muscles In Able-bodied Adults

Abstract

Postural control is a crucial component required for maintenance of joint stability. Recent evidence has linked alterations in the cortex with sensations of instability as quantified through motor evoked potentials (MEPs) from transcranial magnetic stimulation. However, no studies have quantified the effect of single-limb stance on cortical excitability of the lower-leg muscles. PURPOSE: To determine how single-limb stance on stable and unstable surfaces alters cortical excitability to the tibialis anterior (TA), peroneus longus (PL), and soleus (SOL) muscles. METHODS: MEPs were elicited over the primary motor cortex of 15 able-bodied individuals (22.4±3.6yrs; 174.1±12.9cm; 71.4±21.7kg) under 4 conditions: seated and relaxed (REL); seated and facilitated (FAC); single-limb standing on a stable surface (STA); and single-limb standing on an unstable surface (UNS, Biodex Stability System Level 12). Peak-to-peak MEP’s (%max) were measured at stimulus intensities equal to 90, 110, and 150 percent of TA resting motor threshold (RMT). A 4x3x3 ANOVA compared conditions, intensities, and muscles. RESULTS: A significant 3-way interaction effect was observed (F=1.91, p=0.03). Generally, standing trials increased excitability at all intensities compared to seated trials, while FAC was typically greater than REL. At 90% RMT, only PL excitability increased greatly in the UNS condition. At 110% RMT, greatest increases in UNS trials were observed for the TA (REL=0.02±0.01; FAC=0.08±0.02; STA=0.09±0.03; UNS=0.13±0.04, p<0.05) & SOL (REL=0.017±0.01; FAC=0.03±0.01; STA=0.07±0.02; UNS=0.10±0.03, p<0.05) muscles. At 150% RMT, no differences were observed between standing and unstable conditions. CONCLUSION: Cortical excitability to the lower-leg muscles increases with single-limb stance on stable & unstable surfaces. Increases from STA to UNS were observed across all muscles; however, these appear to be stimulus-dependent indicating altered modulation of excitability across muscles. Further research might investigate how injury affects the cortical modulation of single-limb stance.