Modulation of tibialis anterior muscle activity changes with upright stance width

When individuals stand with their feet apart, activation of tibialis anterior (TA) muscle seems to slightly exceed rest levels. In narrow stances, conversely, the stabilization of body lateral sways may impose marked, active demand on ankle inversors/eversors. In this study we investigate how much the modulation in TA activity, associated to center of pressure (COP) lateral sways, changes when stance width reduces. Surface EMG and COP coordinates were collected from 17 subjects at three different stances: feet apart, feet together and tandem. Pearson correlation analysis was applied to check whether the expected greater modulations in TA activity corresponded to a stronger association between fluctuations in EMG amplitude and COP lateral sways. When standing at progressively narrower stances participants showed larger fluctuations in COP lateral sways (p < 0.01) and higher EMG-COP association (p < 0.01); marked increases in TA activity were only observed in tandem stance (p < 0.001). Interestingly, more pronounced modulations in TA activity were observed for subjects showing greater association between EMG amplitude and COP sways in feet together and tandem stance (Pearson R > 0.56, p < 0.02), though not when standing with feet apart (R = −0.22, p = 0.40). These results indicate that the contribution of TA activity to lateral sway control increases for narrower stances.