Interhemispheric inhibition and gait adaptation associations in people with multiple sclerosis.

BACKGROUND: Multiple sclerosis is a neurodegenerative disease that damages the myelin sheath within the central nervous system. Axonal demyelination, particularly in the corpus callosum, impacts communication between the brain's hemispheres in persons with multiple sclerosis (PwMS). Changes in interhemispheric communication may impair gait coordination which is modulated by communication across the corpus callosum to excite and inhibit specific muscle groups. To further evaluate the functional role of interhemispheric communication in gait and mobility, this study assessed the ipsilateral silent period (iSP), an indirect marker of interhemispheric inhibition and how it relates to gait adaptation in PwMS. METHODS: Using transcranial magnetic stimulation (TMS), we assessed interhemispheric inhibition differences between the more affected and less affected hemisphere in the primary motor cortices in 29 PwMS. In addition, these same PwMS underwent a split-belt treadmill walking paradigm, with the faster paced belt moving under their more affected limb. Step length asymmetry (SLA) was the primary outcome measure used to assess gait adaptability during split-belt treadmill walking. We hypothesized that PwMS would exhibit differences in iSP inhibitory metrics between the more affected and less affected hemispheres and that increased interhemispheric inhibition would be associated with greater gait adaptability in PwMS. RESULTS: No statistically significant differences in interhemispheric inhibition or conduction time were found between the more affected and less affected hemisphere. Furthermore, SLA aftereffect was negatively correlated with both average percent depth of silent period (dSP%AVE) (r = -0.40, p = 0.07) and max percent depth of silent period (dSP%MAX) r = -0.40, p = 0.07), indicating that reduced interhemispheric inhibition was associated with greater gait adaptability in PwMS. CONCLUSION: The lack of differences between the more affected and less affected hemisphere indicates that PwMS have similar interhemispheric inhibitory capacity irrespective of the more affected hemisphere. Additionally, we identified a moderate correlation between reduced interhemispheric inhibition and greater gait adaptability. These findings may indicate that interhemispheric inhibition may in part influence responsiveness to motor adaptation paradigms and the need for further research evaluating the neural mechanisms underlying the relationship between interhemispheric inhibition and motor adaptability.

Propulsive Force Modulation Drives Split-Belt Treadmill Adaptation in People with Multiple Sclerosis.

Most people with multiple sclerosis (PwMS) experience significant gait asymmetries between their legs during walking, leading to an increased risk of falls. Split-belt treadmill training, where the speed of each limb is controlled independently, alters each leg's stepping pattern and can improve gait symmetry in PwMS. However, the biomechanical mechanisms of this adaptation in PwMS remain poorly understood. In this study, 32 PwMS underwent a 10 min split-belt treadmill adaptation paradigm with the more affected (MA) leg moving twice as fast as the less affected (LA) leg. The most noteworthy biomechanical adaptation observed was increased peak propulsion asymmetry between the limbs. A kinematic analysis revealed that peak dorsiflexion asymmetry and the onset of plantarflexion in the MA limb were the primary contributors to the observed increases in peak propulsion. In contrast, the joints in the LA limb underwent only immediate reactive adjustments without subsequent adaptation. These findings demonstrate that modulation during gait adaptation in PwMS occurs primarily via propulsive forces and joint motions that contribute to propulsive forces. Understanding these distinct biomechanical changes during adaptation enhances our grasp of the rehabilitative impact of split-belt treadmill training, providing insights for refining therapeutic interventions aimed at improving gait symmetry.