The application of a force platform measuring the ground reaction forces, based on which it is possible to determine the periodical positions of the COP, appears to be a useful tool for the evaluation of the postural balance of patients with PD [32, 33]. Changes in postural swinging may be described with ‘classic’ values, which are determined during the COP course. A more advanced analysis of the posturographic signal using the analyses in the frequency domain finds its application especially in the measurements making use of a cyclic factor upsetting the postural balance.
The application of trend change analyses to the COP movement based on algorithms used in the analysis of stock exchange rates constitutes an entirely new approach to the assessment of the ability to maintain balance. The results of computations make it possible to determine and evaluate not only the number of momentary corrections of posture but also indicate the changes affecting the distance covered by the COP during each correction of the posture and the duration of such corrections.
Analysis Of The Trend Change Index
Balance disorders are one of the most frequent symptoms of motor disorders in PD. Similarly, the progression of the disease affects the characteristics of postural sway [34, 35]. In accordance with the adopted Hoehn and Yahr classification, problems maintaining balance are characteristic of the third stage of the disease. However, more and more studies indicate that postural sway is subject to changes already taking place in the early phases of the disease, before the occurrence of clinical symptoms. The above-mentioned findings justify the search for a diagnostic method based on COP displacements [36]. One of the fundamental parameters used in balance tests is the mean velocity of the COP displacement. The value of this variable often differentiates patients with PD from the CG [32]. The conducted investigations (Fig. 4A and Table 2) confirm these results only when healthy individuals (both age groups) and stage III PD patients are compared, where an increase in the mean COP velocity was observed in the case of ill individuals in both the EO and EC tests. As for the stage II PD patients, statistically significant differences were observed only in comparison with the CGy group in the EO test. This highlights that the differentiation between PD patients and the CG group occurs only in patients with later stages of PD (stage III). In the studies by Tadayoshi et al. [33] and Ferrazzoli et al. [37], the root mean square parameter determined for the mean COP velocity was indicated as the most significant factor, which directly translates into the standard deviation calculated for these values. Figure 4A also demonstrates that in the case of the calculated velocity, there is an increase in the interquartile distribution value for stage III PD patients compared to CG, both for the EO and EC tests. These results are also confirmed by the research done by Barbosa et al. [38] and Ickenstein et al. [39], who noticed the increase in the interquartile distribution of the COP velocity without taking into consideration the stages of the disease.
The TCI index is a parameter that enables the extension of the conducted analyses and, at the same time, the improvement of the interpretation of the results using posturographic examinations. The computational results obtained based on the conducted tests indicate a statistically higher value of the proposed TCI index for PD stages II and III than for CG (Table 2). The results were confirmed in the EC and EO tests. In the case of stages II and III of PD, the number of postural corrections during the test does not statistically differ from the value obtained in the case of CGy (Fig. 3). The determination of the TCI enabled better differentiation of individuals with PD in stages II and III in relation to CG and CGy.
Analysis Of Momentary Velocity, Displacements And Time Between Subsequent Trend Changes Of The Cop Signal
The deterioration of balance-keeping ability in patients with PD may be observed in the changes of parameters, such as the mean COP velocity or an increase in the range of the COP movements [40, 41]. The analysis of these types of parameters indicates only global changes in relation to the total time of the measurement. Analyzing the TCI, TCI_dV, TCI_dT, and TCI_dS enables one to determine changes connected with momentary ‘leaps’ of the COP, defined as trend changes in the COP movement.
The analysis of the median values of the TCI_dV index determined for subsequent groups of tested individuals indicates that they are subject to trend changes similar to the mean COP velocity. However, in terms of numerical values, TCI_dV reaches lower values than the COP velocity, which may result from the fact that the algorithm specifying the TCI filters out noises from the signal, i.e., omittable tiny changes in the COP position (Fig. 4B). Statistically significant differences occurred everywhere where there were differences in the mean COP velocity in the comparison of the tested groups, with one exception. The TCI_dV index also exhibited differences between the CG and CGy test groups in the EO conditions. Taking this into consideration, the TCI_dV index provides similar information to the analysis of the mean COP velocity for the entire examination.
The application of the TCI, TCI_dT, and TCI_dS indices constitutes a completely novel approach enabling the observation of changes taking place in the strategy of maintaining balance. The comparison of the CGy group with the CG group clearly demonstrates that in the CG group, there was a statistically significant prolongation of time (TCI_dT) in the EO and EC tests as well as the distance (TCI_dS) in the EO tests. This prolongation of time, i.e., the decrease in subsequent postural corrections, reflects the changing balance-keeping strategy and may indicate the declining role of the proprioceptive system in maintaining balance in favor of the organ of sight [42, 43].
In the case of patients with PD, an inverse trend in the TCI_dT changes is observed compared to the CG. The time periods between subsequent trend changes reach values that are observed in young people. In addition, patients with stage III PD revealed a much greater prolongation of TCI_dS and, in comparison with the CG, an increase in the TCI values. The observed changes in the TCI demonstrate that patients with PD reveal not only much more frequent postural corrections but also much shorter time periods of leaps between the subsequent COP positions, identified as trend changes. Statistically significant differences between the CG and patients with PD (stages II and III) make it possible to differentiate between healthy and ill individuals, which was previously not possible in the case of stage II PD patients only based on the mean COP velocity. In addition, the TCI_dS index, which was calculated for the CE test, distinguishes between stage II and stage III PD groups in an unambiguous way, indicating that the disease progression is related to the extension of distance between the subsequent COP positions, identified as trend changes. Such an extension of the TCI_dS distance entailing an increase in the velocity of leaps of the COP to subsequent positions (TCI_dV) may also be an indication of a greater probability of the fall occurrence. Moreover, an unpredictable destabilizing factor combined with an increased value of the COP velocity may lead to such an increase in the velocity vector value that the deceleration of the body movement becomes impossible and results in the crossing of the stability boundary [41, 42].