Corticospinal Excitability in Response to Mediolateral Gait Instability

SUMMARY

Unpredictable gait disturbances, particularly in the mediolateral direction, pose a significant challenge to stability and are a common contributor to falls. Although the corticospinal tract is critical for gait and postural control, its response to such instabilities remains unclear. To investigate if corticospinal excitability increases during unpredictable lateral destabilisations, single-pulse transcranial magnetic stimulations were delivered over the primary motor cortex of 15 healthy individuals during steady-state and randomly laterally destabilised treadmill gait. Full-body kinematics were recorded using an optoelectronic motion capture system. Stimulations where the coil was >5 mm from the target location on the head were excluded. Corticospinal excitability was quantified for four upper– and three lower-leg muscles as the amplitude of the motor evoked potential (MEP) and compared between steady-state and destabilised gait. Lateral destabilisation resulted in a wider step width and shorter stride duration with increased variability and greater dynamic instability. Foot placement control during laterally destabilised gait was increased at midstance and there was also an increased average foot placement error in this condition. No differences in corticospinal excitability were observed in the lower-leg muscles. All upper-leg muscles demonstrated greater absolute MEPs in destabilised relative to steady-state gait. After normalising to muscle activity these periods became less pronounced, however, increases in corticospinal excitability during destabilised gait were observed in all but the gastrocnemius muscles. These finding suggest a heightened readiness of the corticospinal tract projecting to upper-leg muscles during destabilised gait, which could reflect general stabilising strategies such as decreasing stride time and increasing step width. Increases in the relative contribution of the corticospinal tract (i.e. normalised MEPs) were primarily noted during periods of low muscle activity, possibly hinting towards greater motor planning.