# The Effects of Concussions on Static Postural Stability
## 4 Discussion
### Effects of Concussion History on Postural Stability  
In the analyses of postural sway variables between the participants with a concussion history (CONC) and without a concussion history (NO CONC), the results revealed that the CONC group had higher postural sway than the NO CONC group, indicating greater postural decrements. More specifically, the CONC group demonstrated greater COP-X max, avg. displacement along X, and 95 EA in the foam BL EO condition of CTSIB. The CONC group also demonstrated greater COP-X max, COP-Y max, avg. displacement along X, avg. displacement along Y, and 95 EA in DOM EC condition, as well as COP-Y max, COP-Y min, avg. displacement along Y, and 95 EA in ND EC condition of unilateral stability test. In essence, the CONC group showed a greater postural sway in both anterior-posterior and medial-lateral directions compared to the NO CONC group. These findings agree with the researchers' original hypothesis that concussions would have a negative effect on postural stability, as well as with the previous studies [3,15,18]. Postural stability decrements following concussions can occur due to multiple reasons. Postural stability in humans is mainly maintained by three main systems: the afferent system, the CNS, and the efferent system. The afferent system consists of the three visual, vestibular, and somatosensory systems, with healthy individuals' somatosensory system contributing to 70% of the postural control in a well-lit room on a firm surface [19]. The CNS  
Figure 4: Significant postural sway variables between the individuals with a history of concussion (CONC) and those without concussion (NO CONC) in eyes closed (EC) non-dominant condition of the unilateral stability test. (**Top left**) COP-Y max; maximum center of pressure (COP) excursion in the medial-lateral direction. (**Top right**) COP-Y min; minimum COP excursion in the medial-lateral direction. (**Bottom left**) Avg. Displacement along Y; average COP displacement in the medial-lateral direction. (**Bottom right**) 95 EA; 95% ellipsoid area. Bars represent standard errors. consists of higher brain centers, while the efferent system consists of the musculoskeletal system [20]. Coordination between these systems is required for balance, and any effect on these three systems can affect someone's postural stability. Concussions (and the majority of other traumatic brain injuries) are well-known to affect the vestibular system [21,22]. The vestibular organ located in the inner ear is mainly comprised of three semi-circular canals that detect angular acceleration of the head and the two maculae (saccule and utricle) that detect linear movement as well as the effects of gravitational force on the body. Moreover, endolymph and hair cells in semicircular canals, along with the hair cells in maculae, detect the direction of body movements [23]. Thus, any disruption to the anatomy of this highly sensitive vestibular system (especially endolymph, hair cells, and natural orientation of the structures) affects the person's postural stability. As concussions result following a severe collision of the head against another surface, it interferes with the vestibular system, which explains the headache, dizziness, lightheadedness, nausea/vomiting, and tinnitus following concussions. Furthermore, the symptoms of photophobia and blurred vision could suggest a disturbance to the individual's visual system, affecting the contribution of their visual system in postural control. The visual system, consisting of eyes, optical tracks, and optic nerves gathers information on the surrounding environment to maintain balance and is considered the fastest sensory system out of the three and hence capable of reacting to acute perturbations [24]. Thus, interruptions to the function of visual systems affect someone's ability in postural stability [25]. In the present study, there were no significant differences in balance between CONC vs. NO CONC groups during the baseline stability test.