## 4 Discussion
### Trend Change Index (TCI)  
An increase in the mean value of the COP velocity or movement range may indicate problems with maintaining balance. There is evidence, based on the analyses of COP movement in the frequency domain, that the lines appearing in FFT diagrams may be used for the assessment of the balance-keeping strategy. For instance, the movement of the body following the cyclically oscillating virtual scenery, which may be observed in the FFT or short-time Fourier transform (STFT) diagrams in the form of lines, can be used for this purpose [1]. Studies performed by [PERSON] et al. [23] and [PERSON] et al. [24] indicate that there might be a connection between the frequency of COP oscillation and the strategy of maintaining balance. They claim that low frequencies (0-0.5 Hz) mostly account for visual-vestibular regulation, medium frequencies (0.5-2 Hz) for cerebellar regulation, and high frequencies (>2 Hz) for proprioceptive regulation. However, the above-described frequency analyses enable only the detection of the signals of a cyclic character, whereas the real COP movement may include both cyclic and non-cyclic components [16]. The developed TCI, calculated using the detection of trend changes based on stock exchange analysis methods, makes it possible to extend the analyses and supplement them with a non-cyclic component of the signal [19; 25]. The TCI value shows how often the COP was subjected to trend changes during the movement performed during the whole measurement (Figure 4A,B). The conducted investigations demonstrated that in most cases, the value of the TCI, which was calculated for the whole interval from 0 to 1 s, did not change significantly, irrespective of the measurement conditions (Table 1). Such constancy, independent of the measurement conditions, may signify that to maintain a stable body posture, even in a situation of visual stimulation, a certain number of trend changes is required, which enables the correct functioning of the proprioceptive and visual-vestibular systems [13; 16]. An important exception occurs in the case of the measurement of the scenery oscillating at a frequency of 0.7 Hz, which took place after preceding oscillations at a frequency of 1.4 Hz. There was a clear decrease in the TCI values in relation to other measurements, i.e., the number of discovered changes in the trend, which may indicate a calmer movement at a frequency of 0.7 Hz when it occurs after a frequency of 1.4 Hz. Additional interpretation of this fact is supported by the TCI values determined individually for three time periods equal to the time between subsequent trend changes. The diagrams showed a significant decrease in the TCI values in the 0.2-0.5 s period and an increase in the interval of 0.5-1 s (Figure 5A-D), denoting considerable calming of COP movement. There appeared to be fewer 'leaps' of the COP lasting shorter than 0.5 s in favor of a greater number of leaps lasting longer than 0.5 s. This interval included COP movements performed at a frequency of 0.7 Hz. Therefore, from a statistical perspective, the tested individuals were more prone to disturbances at a frequency of 0.7 Hz if they had been previously exposed to disturbances at a higher frequency. Notably, such differences were undetectable based on the analysis of the COP mean velocity or the COP movement range, where a statistically significant difference appeared only between the OE and 0.7_after tests but did not occur in other cases (Figure 4A,B). The analysis of measurements conducted at a frequency of 1.4 Hz revealed a significant increase in the TCI values in relation to other measurements in the 0.2-0.5 s period and a decrease in the 0.5-1 s period. The increase in the 0.2-0.5 s period was related to the fact that the body follows the moving scenery. This interval includes the 1.4 Hz frequency, which is also visible in the analyses in the frequency domain. This difference was noticeable, particularly in relation to the OE and CE measurements.