## 3 Results  
One of the most important factors affecting the quality of the presented tests is the sampling frequency. This is particularly noticeable when measuring physical quantities characterized by large changes in a short time. The tests in which the sampling frequency affects the final result of the analysis include the measurement of speed [40,41], measurement of force in dynamic conditions [42,43], and measurement of the change in position [44,45]. The results of the experiment checking the effect of sampling frequency on changes in dimensions of the ellipses are presented using the example of a set of points illustrating the variability of the human body's gravity center position after performing five complete propelling cycles [46] (Figure 7) on a manual wheelchair. It should be noted that the presented results refer to the same measurement sample recorded simultaneously with eight different sampling frequencies. Changes in parameters describing the shape of the ellipse, the percentage of points described by the ellipse, the area and standard deviations in the dimensions of the semiaxes a and b were checked by evaluating the effect of sampling frequency (Table 2). Figure 5: Photograph of the used engine and PM 2.5 sensor (**A**) and a view of the test area (**B**), where 1—engine, 2—particle sensor, and WA—area of the analysis. Figure 6: A hybrid cart used in muscle activity tests, where 1—drive wheel, 2—push rings, and 3—electric motor built into the hub of the drive wheel. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \(f\) & \(n\) & \(a\) & \(b\) & \(\alpha\) & \(x_{COG}\) & \(y_{COG}\) & \(\Delta_{P}\) & \(k\) & \(A\) \\ \hline Hz & - & mm & mm & \({}^{\circ}\) & mm & mm & - & \% & mm\({}^{2}\) \\
25 & 230 & 61.46 & 16.00 & \(-\)6.80 & 117.35 & 290.60 & 0.58 & 83.9 & 3066.19 \\
29 & 263 & 61.41 & 15.74 & \(-\)6.75 & 117.34 & 290.63 & 0.58 & 83.3 & 2874.56 \\
33 & 306 & 61.43 & 16.05 & \(-\)6.75 & 117.30 & 290.62 & 0.58 & 84.0 & 3066.19 \\
40 & 367 & 61.46 & 16.09 & \(-\)6.78 & 117.29 & 290.56 & 0.57 & 84.5 & 3066.19 \\
50 & 459 & 61.40 & 15.86 & \(-\)6.81 & 117.28 & 290.58 & 0.57 & 84.3 & 2874.56 \\
67 & 612 & 61.39 & 15.86 & \(-\)6.83 & 117.27 & 290.56 & 0.58 & 84.0 & 2874.56 \\
100 & 918 & 61.37 & 15.84 & \(-\)6.84 & 117.26 & 290.56 & 0.58 & 84.5 & 2874.56 \\
200 & 1835 & 61.37 & 15.83 & \(-\)6.84 & 117.24 & 290.55 & 0.61 & 84.1 & 2874.56 \\ \hline \(\mu\) & - & 61.41 & 15.86 & \(-\)6.80 & 117.29 & 290.58 & 0.58 & 84.08 & 2946.42 \\ \(\sigma\) & - & 0.04 & 0.08 & 0.04 & 0.04 & 0.03 & 0.01 & 0.39 & 99.18 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: List of ellipse parameters and assessed criteria for one measurement test of the gravity center position made with different frequencies, where \(f\)—sampling frequency, \(n\)—number of points in the set of analyzed points, \(a\)—semiaxis of the ellipse parallel to the direction line, \(b\)—semiaxis of the ellipse perpendicular to direction line, \(\alpha\)—angle of inclination of the direction line, \(x_{COG}\)—position of the center of the ellipse on the horizontal axis, \(y_{COG}\)—position of the center of the ellipse on the vertical axis, \(\Delta_{P}\)—uniformity ratio of points distribution in relation to the mean value of the radius vector for the points of the analyzed set, \(k\)—percentage of points located inside the area defined by the ellipse, \(A\)—ellipse area, \(\mu\)—mean, and \(\sigma\)—standard deviation. Figure 7: Plots showing variation in the human body’s gravity center during manual wheelchair propulsion measured with different sampling frequencies \(f\), with marked ellipses approximating the area of presence for the set of the analyzed points. The performed operational tests do not always end with measuring a set of measurement points characterized by normal distribution. Motion capture tests are such a test type, in which the analyzed markers and their spatial position depend on the testing scenario, and in their case, the distribution of measured points is random.