In this study, we introduced a new index, namely overlap index, to quantify the spatial resolution of cortical activity for muscle coordination based on the measurement of EEG-EMG coherence during a motor task. By applying this index on 4 control and 4 hemisphere chronic stroke subjects we successfully identified that there is a significantly increased overlap between biceps brachii at the elbow and intermediate deltoids at the shoulder when stroke subjects generating a static shoulder abduction torque. Muscles that have increased overlap in cortex are consistent with those that coactivate abnormally in stroke when compared to control subjects. These results not only proof the effectiveness of this index in quantifying the spatial resolution of cortical activity but also point out that the reduced spatial resolution of muscle activity in cortex can be a reason for the abnormal muscle coactivation observed in impaired arms following stroke. Quantification of the cortical overlap index will provide us with new tools to test for the modifiability of the nervous system following clinical interventions. This work will be an important step toward our long-term goal of developing more effective rehabilitation techniques for the treatement of stroke.