Mechanisms of short-term training-induced reaching improvement in severely hemiparetic stroke patients: A TMS study

Background. The neurophysiological mechanisms underlying improved upper-extremity motor skills have been partially investigated in patients with good motor recovery but are poorly understood in more impaired individuals, the majority of stroke survivors. Objective. The authors studied changes in primary motor cortex (M1) excitability (motor evoked potentials [MEPs], contralateral and ipsilateral silent periods [CSPs and ISPs] using transcranial magnetic stimulation [TMS]) associated with training-induced reaching improvement in stroke patients with severe arm paresis (n = 11; Upper-Extremity Fugl-Meyer score (F-M) = 27 ± 6). Methods. All patients underwent a single session of reaching training focused on moving the affected hand from a resting site to a target placed at 80% of maximum forward reaching amplitude in response to a visual "GO" cue. Triceps contribute primarily as agonist and biceps primarily as antagonist to the trained forward reaching movement. Response times were recorded for each reaching movement. Results. Preceding training (baseline), greater interhemispheric inhibition (measured by ISP) in the affected triceps muscle, reflecting inhibition from the nonlesioned to the lesioned M1, was observed in patients with lower F-M scores (more severe motor impairment). Training-induced improvements in reaching were greater in patients with slower response times at baseline. Increased MEP amplitudes and decreased ISPs and CSPs were observed in the affected triceps but not in the biceps muscle after training. Conclusion. These results indicate that along with training-induced motor improvements, training-specific modulation of intrahemispheric and interhemispheric mechanisms occurs after reaching practice in chronic stroke patients with substantial arm impairment.