## 4 Discussion
### Postural Regulation and Subsystems  
[PERSON] et al. (2019) showed a larger amount of instability in their moderate-to-severe primary OA group than the mild or control groups except for the somatosensory and cerebellar systems. They deduced that moderate-to-severe patients with knee OA depend on their eyesight in order to compensate for their postural instability. The calculated values for all frequency bands were much higher comparable with this study and [PERSON] et al. (2016).  
Following ACL rupture and surgical reconstruction and consistent with our study, [PERSON] et al. (2016) observed the largest reductions of postural subsystems for the somatosensory (consistently below the healthy median reference) and cerebellar systems. However, in contrast to [PERSON] et al. (2016) we detected a hyperactivity in both systems (consistently above the healthy interquartile range; Figure 2C,D) and not a suppression, especially at six-months post-operation. Obviously, these types of lower limb surgeries would have completely different effects concerning postural subsystems. The change of somatosensory information, potentially caused by mechanoreceptor damage (e.g., HTO surgery), may subsequently lead to a reduction of postural stability and regulation ([PERSON] et al., 2019). Both investigations are examples and proofs for the neuroplasticity of biological systems and the model of selective compensatory optimization. For example, the relation between the somatosensory system and the spinocereblum system, which is responsible for processing the afferent (somatosensory) information is very close. The relationship between the peripheral-vestibular system and the vestibulocenebellum system is comparable. In this context, the hyperactive effects are caused by reduced activity of other postural systems, in this case particularly the peripheral-vestibular system (consistently below the healthy percentile 75; Figure 2B). [PERSON] et al. (2016) demonstrated that knee osteoarthritis (OA) causes changes not only in the tissues within the articular cavity, but also the ligaments, tendons, and periarticular tissues including the muscles. Furthermore, patients with knee OA have deficiencies in the number of sensory receptors and therefore their proprioception compared to similar age controls [PERSON] et al. (2016).  
At six-months post-HTO, postural regulation returned to an asymptomatic level for mediolateral weight distribution. The cross-sectional comparison to asymptomatic subjects at all examinations showed that OA and HTO led to hyperactivity of the cerebellar and somatosensory systems and postural instability. Obviously, the subsequent rehabilitation was not able to improve postural regulation and stability. This investigation provides further insight into the underlying mechanisms of postural regulation and in the understanding of the interaction of postural subsystems. Future research examining postural stability after one- and two-year postoperative periods will provide a subsequent midterm and long-term evaluation of surgery and rehabilitation. It is possible that improvements in postural stability and regulation need longer than six-months to occur. In relation to other studies ([PERSON] et al., 2019; [PERSON] et al., 2019), we wanted to establish a more holistic approach instead of the frequently used isolated orthopedic view (flexibility, strength, pain).