The wide variety of participants was recruited from a residential post-acute brain injury center. Of the 40 participants, 29 were male, and 11 were female. Twenty-seven of the participants were injured in automobile accidents, 5 in cerebral vascular accidents, 5 in assaults, and 3 in falls. Their ages ranged from 19 to 58 with the time since the injury ranging from 0.08 to 20.2 years. In order to participate, all participants needed to walk at least 0.5 miles per hour and effectively complete the exhaled gas collection requirements.