The effects of paired associative stimulation on knee extensor motor excitability of individuals post-stroke: A pilot study

Objective: Paired associative stimulation (PAS) modulates bilateral distal lower limb motor pathways during walking. We assessed the effects of inhibitory PAS applied to the vastus medialis (VM) motor pathways of chronic stroke patients. Methods: PAS consisted of 120 electrical stimuli applied to the femoral nerve paired with transcranial magnetic stimulation (TMS) of the lower limb primary motor cortex so that the estimated arrival of the afferent volley occurred 8. ms after delivery of the magnetic stimulus. Stimulus pairs were delivered to the non-paretic VM motor system of 11 chronic stroke patients and the right limb motor system of 11 non-impaired subjects at 0.19. Hz. The effects of PAS on VM motor pathway excitability and muscle activity were assessed during pedaling. TMS-induced motor evoked potential (MEP) amplitudes and the percent of VM activity in the flexion phase of active pedaling (%FLEXVM) was examined before and after PAS. Results: Inhibitory PAS reduced VM MEP amplitudes in the target limb (p< 0.05) of both groups, while post-PAS paretic VM MEP amplitudes increased for some patients and decreased for others. Group mean paretic limb %FLEXVM was not altered by inhibitory PAS. Conclusions: These results indicate PAS can be used to manipulate motor cortical excitability in proximal lower limb representations, however the sign of induced modulation was unpredictable and cyclic muscle activity was not modified. Significance: The study has important implications for the development of therapies involving non-invasive brain stimulation to modify abnormal motor behavior following stroke.