# Recurrence Quantification Analysis as a Form of Postural Control Assessment: A Systematic Review
## 4 Discussion
### RQA Results across the Sensory Conditions (Visual Control, Surface)  
Many of the papers included in this review followed similar testing conditions. These consisted of combinations of both legs standing on firm or foam surfaces with eyes open and closed [44]. [PERSON] et al. [44] showed that as the sensory conditions became more severe (eye closing and/or head extension), the amount and variability of postural sway increased while its intermittency decreased. The same conclusions appear in the study by [PERSON] et al. [15], who were the first to apply RQA analysis to CoP data to investigate the effects of head position and visual feedback on postural sway dynamics. They indicated that vision affects the deterministic structure (in degree and complexity) of CoP motions and that spontaneous swaying in different head orientations affects CoP non-stationarity.  
[PERSON] et al. [50] showed that for the elderly group, removing visual feedback decreased ENT values significantly in both directions and %DET in the ML direction. [PERSON] et al. [56] showed that as postural conditions became more challenging (changing from both legs standing on a rigid surface with visual control to standing on a foam surface without visual inspection), the postural sway in both AP and ML directions became more regular (higher %REC and %DET), more complex (higher ENT), and more stationary (lower TREND) in healthy adults and people with nonspecific low back pain (LBP). Moreover, they showed that the increase in cognitive difficulty was associated with less regularity (lower %REC and %DET), less complexity (lower ENT), and more stationary (lower TREND) in both directions. Additionally, LBP patients did not reduce their %REC, %DET, and TREND in the AP direction with increasing cognitive difficulty to the same extent as healthy people. For the corresponding measurement conditions, similar results were obtained by [PERSON] et al. [59]. Unfortunately, of the RQA measures, they only considered %DET, which was higher in the medial-lateral direction in patients with knee osteoarthritis compared to healthy subjects. Moreover, in both groups, %DET increased with increasing postural difficulty, while it decreased when moving from single- to dual-task conditions. [PERSON] et al. [46] also investigated the effect of a prior anterior cruciate ligament injury on the deterministic pattern of postural sway under the above-mentioned conditions. They showed that as postural difficulty increased from open-eyes to closed-eyes and rigid-surface to foam-surface, the center of pressure regularity (%determinism) and ENT increased as well. As previously mentioned, the performance of a secondary cognitive task (a backward digit span task) caused less center of pressure regularity than the single postural task. In all studies, the following relationship appears: an increase in variability is associated with a decrease in randomness (i.e., an increase in determinism), and vice versa. [PERSON] et al. [55] found that higher levels of postural variability in people with [PERSON]'s disease compared to a matched control group accompanied greater determinism, % recurrence, and max line. [PERSON] and [PERSON] [73] found that when the sensory organization test conditions became difficult, CoP variability increased, while the %DET decreased. Those findings indicate that the CoP time series in the previously mentioned groups are not noisier than the postural sway of controls but exhibit strongly deterministic dynamic patterns.