## 2 Materials and Methods
### Balance Measurement Using Posturography  
Posturographic assessment was established with the IBS (neurodata GmbH, Vienna, Austria) (Figure 1b). The IBS consists of four independent force plates used to measure postural regulation at a sampling rate of 32 Hz. Sway intensities at different frequency ranges were determined using Fast Fourier Transformation (FFT). The raw signal (force-time signal) was subtracted from the mean value and then subjected to a FFT with a rectangular window. On the ordinate, the amplitude of the frequency components was exposed and, consequently, the ordinate was dimensionless in that the results of the FFT are proportional to the output signal [21]. Different functional frequency bands were used to delineate the postural subsystems (F1, F2-4, F5-6, F7-8) [22; 23; 24]:  
1. F1: Frequency band 1 (0.01-0.03 Hz)--visual and nigrostriatal system;
2. F2-4: Frequency band 2-4 (0.03-0.5 Hz)--peripheral-vestibular system;
3. F5-6: Frequency band 5-6 (0.5-1.0 Hz)--somatosensory system;
4. F7-8: Frequency band 7-8 (>1.0 Hz)--cerebellar system.  
Additionally, motor output was determined as:  
1. Stability indicator (ST): The root mean square of successive differences in pressure signals. Greater instability is indicated by a greater ST.
2. Weight distribution index (WDI): Standard deviation of the weight distribution score.
3. Synchronization (Synch): Six values that describe the relationship of vibration patterns between plates calculated as a scalar product: 1000--complete coactivity; \(-1000\)--complete compensation; 0--no coactivity or compensation.
4. Forefoot-hindfoot ratio (Heel): Percentage of load distribution between the forefoot and hindfoot with an emphasis on heel loading.
5. Left-right ratio (Left): Percentage of load distribution between the left and right feet with an emphasis on left side loading.  
Subjects were tested barefoot using a single trial (32 s) for each of the following test conditions:  
1. Head straight, eyes open, without foam pads (NO);
2. Head straight, eyes closed, without foam pads (NC);
3. Head straight, eyes open, on foam pads (PO);
4. Head straight, eyes closed, on foam pads (PC);
5. Head rotated 45\({}^{\circ}\) to the right, eyes closed, without foam pads (HR);
6. Head rotated 45\({}^{\circ}\) to the left, eyes closed, without foam pads (HL);
7. Head up (dorsiflexed), eyes closed, without foam pads (HB);
8. Head up (plantarflexed), eyes closed, without foam pads (HF).  
Initially, subjects were asked to stand upright, with their weight evenly distributed on the two force plates while focusing on a fixed target (placed at each subject's respective body height). In this starting position, subjects were then asked to stand freely and as still as possible. Positions, reliability, frequency bands, and parameters of motor output used in the IBS have been previously described in detail [12]. For example, the intraobserver reliability has been confirmed using both asymptomatic subjects [25] and patients [26; 27]. ICCs for every parameter and all test positions have been reported from 0.71 to 0.95 [25; 26; 27]. In support of these previous reliability investigations, which showed that the reliability averaged over eight positions were clearly higher than from a single position, we also used the mean values captured in the eight test positions for all parameters.