1. Department of Physical Medicine and Rehabilitation, Harvard Medical School, Boston, Massachusetts, USA.
2. Spaulding Rehabilitation Hospital, Charlestown, Massachusetts, USA.
3. Department of Biostatistics, College of Medicine, University of Florida, Gainesville, Florida, USA.
4. Breathing Research and Therapeutics Center, Department of Physical Therapy, College of Medicine, University of Florida, Gainesville, Florida, USA.