Non-linear dynamical features of center of pressure extracted by recurrence quantification analysis in people with unilateral anterior cruciate ligament injury.

Knowledge about the non-linear dynamical pattern of postural sway may provide important insights into the adaptability (flexibility) of human postural control in response to everyday stresses imposed on the body. A commonly used non-linear tool, i.e. recurrence quantification analysis, was chosen to investigate the effect of prior anterior cruciate ligament injury on the deterministic pattern of postural sway under different conditions of postural and cognitive difficulty. In double leg stance, as postural difficulty increased from open-eyes to closed-eyes and rigid-surface to foam-surface, the centre of pressure regularity (%determinism) increased as well. In comparison to healthy counterparts, subjects with prior anterior cruciate ligament injury produced more regularity when maintaining balance on their injured leg. Also, for both the double and single leg stance balance conditions, the performance of a secondary cognitive task (a backward digit span task) caused less center of pressure regularity than the single postural task, which suggests that both study populations required the same amount of cognitive involvement for maintaining balance. Center of pressure dynamic patterns exhibited by the anterior cruciate ligament deficient patients were more regular than those of the healthy controls indicating "complexity loss" and may be indicative of the reduced adaptability (flexibility) of a balance system to sudden perturbations.