Title: Balance failure in single limb stance due to ankle sprain injury: an analysis of center of pressure using the fractal dimension method.
Abstract.Dynamic stability is ubiquitous to fulfilling daily living activities. Instrumented postural control analysis plays an important role in evaluating the effects of injury on dynamic stability during balance tasks, and is often characterized with measures based on the displacement of the center-of-pressure (COP) assessed with a force platform. However, the desired outcome of the task is frequently characterized by a loss of dynamic stability, secondary to injury. Typically, failed trials of balance tasks are discarded during research investigations. The novelty of the present study is that COP characteristics of failed trials in injured participants are compared to successful trial data in another injured group, and a control group of participants, using the fractal dimension (FD) method. Twentynine participants with acute ankle sprain attempted and succeeded a task of eyes closed single limb stance (SLS) on their non-injured limb (successful injury group). A separate group of twenty-eight participants with acute ankle sprain attempted and failed the task on their injured limb (failed injury group). Finally a control group of sixteen participants successfully completed the task on their nondominant limb (successful non-injured group). Between trial analyses of these groups revealed significant differences in COP trajectory FD (successful injury group:1.26 ± 0.15; failed injury group:1.17 ± 0.14; successful non-injured group:1.38 ± 0.11) with a large effect size. The present findings demonstrate that successful eyes-closed SLS is characterized by a larger FD of the COP path trajectory when compared to failed trials, and that injury causes a decrease in COP path trajectory FD.