title: Reply.
authors.affiliation: Rehabilitation Medicine Department, Clinical Center, National Institutes of Health, Bethesda, MD(∗).
authors.forename: Jay
authors.initials: J
authors.lastname: Shah
authors.affiliation: Bioengineering Department, George Mason University, Fairfax, VA(†).
authors.forename: Siddhartha
authors.initials: S
authors.lastname: Sikdar
authors.affiliation: Center for the Study of Chronic Illness and Disability, George Mason University, Fairfax, VA(‡).
authors.forename: Lynn
authors.initials: L
authors.lastname: Gerber