Jung E Park1, Katharine Alter2, Mark Hallett1. 1. Human Motor Control Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland. 2. Functional and Applied Biomechanics Section, Rehabilitation Medicine, Clinical Center, National Institutes of Health, Bethesda, Maryland3Eunice Schriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Mar.