Residual deficits from concussion as revealed by virtual time-to-contact measures of postural stability

Objective: Traumatic brain injury (TBI), including mild traumatic brain injury (MTBI), commonly known as a "concussion", is still one of the most puzzling neurological disorders and least understood injuries facing sport medicine and the scientific community [Cantu R. Concussion assessment Ongoing controversy. In: Slobounov S, Sebastianelli W, editors. Foundations of sport-related brain injuries. New York: Springer Press; 2006. p. 87-111.]. It was our primary objective to assess the dynamic properties of postural control in subjects prior to and after sport-related MTBI using the traditional center of pressure (COP) and virtual time-to-contact (VTC) measures. Methods: We assessed 12 student-athletes prior to and 30 days after they suffered sport-related MTBI on a number of standing still and dynamic postural tasks. All subjects were clinically asymptomatic at day 30 of testing and were cleared for full sport participation based upon neurological and neuropsychological assessments, as well as clinical symptoms resolution. Results: The findings showed: (1) no significant differences for any of the standard COP-based measures of postural control (i.e., 90% ellipse COP area, COP velocity and Stability Index) as a function of testing day (prior to and 30 days post-injury, p > .05); (2) no differences in terms of VTC shape, distribution and nominal values before and after concussion during standing still postural tasks regardless of vision conditions, p > .05; and (3) there were significant alterations in VTC in terms of absolute values, range of VTC at the deflection points and mode at the day 30 post-injury. Conclusions: The deficits in VTC control are indicative of residual postural abnormality in subjects suffering from mild traumatic brain injuries and provide further evidence that VTC is used to regulate dynamic postural movement. Significance: The results show that residual postural abnormalities in concussed individuals may be undetected using conventional research methods and the implications of this for clinical practice are discussed.