Modulation of Corticospinal Excitability and Muscle Synergies During Visuomotor Locomotor Task in Individuals With and Without Cerebral Palsy: A TMS and EMG Study

Abstract

Studies using TMS and EMG suggest that disrupted functional corticospinal connectivity significantly contributes to difficulty in initiating and controlling voluntary movements such as walking. In individuals with Cerebral Palsy (CP), the corticospinal tract (CST) may therefore be identified as a potential target for improving gait control. Increasing corticospinal excitability may enhance voluntary control of lower-limb muscles and improve selective activation patterns during gait. It remains uncertain whether this pathway can be further activated given the damage caused by the brain lesion.Moreover,muscle synergies, a cooperative activation of groups of muscles,play an essential role in efficient and adaptive locomotion. Disrupted CST projections may reduce the specificity and strength of descending commands,which can lead to the fusion or splitting of muscle synergies.This impaired descending modulation could explain the reduced number of synergies and lower variance often reported in people with PC. The objective of this study was to assess whether a visuomotor walking task promotes an increase in CST excitability and a modulation of muscle synergies compared to a simple walking task in individuals with CP. 16 individuals with CP were initially recruited, muscle synergy analyses were conducted in 14 participants and TMS-based corticospinal excitability assessments in 11 participants, due to contraindications to TMS or technical issues. 14 controls took part in this study.Each participant performed a simple walking task and a visuomotor walking task at comfortable speed, in counterbalanced order.TMS were delivered during walking at late stance phase. Muscle synergies were extracted from full gait cycles recorded throughout each condition. Motor evoked potentials (MEPs) in the tibialis anterior muscle were induced using TMS. Muscle synergies were extracted from EMG signals acquired from six key lower-limb muscles during both tasks. In the visuomotor task, MEPs increased by 59.4% in the CP group (p=0.01) and 113.8% in the controls(p=0.01). An increase in the number of synergies was observed during visuomotor task in CP.These results suggest that performing a visuomotor task allows to enhance the CST excitability in both CP and controls. Moreover, CP showed that either the number or the structure of synergies are modulated by the visuomotor task, in comparison to control subjects.