## 2 Subjects and Methods
The magnitude of metabolic power for each stage of the pool test was determined using the following equation:  
\[P_{ai}=E_{ai}\times t^{-1} \tag{3}\]  
where, \(P_{ai}\) is metabolic power (W) and \(t\) is the swimming time of the 100 m lap in test s.  
To verify the \(E_{ai}\) values obtained using the above-described approach at the last stage of the test, as well as to compare them with published data, a second approach was used, which is most often used in studies of the energy cost of human swimming at maximum or near maximum velocity [1,20,21, 22]. This approach, commonly referred as the Wilkie's approach [23], allows determining the amount of the total metabolic energy input (\(E_{it}\)) (\(E_{it}=E_{ai}+E_{bi}\); \(E_{bi}\)--energy of basal metabolism, for supramaximal efforts). The total metabolic rate for the last step of the test was also calculated based on Equation (3): \(P_{ii}=E_{ti}\times t^{-1}\). To experimentally determine the total external mechanical power (\(P_{to}\)), active hydrodynamic resistance force (\(F_{r(ad)}\)), or active drag force, and the dimensionless coefficient of active drag force (\(C_{x(\text{ad})}\)) were assessed using the method of small perturbations, usually known as the velocity perturbation method--VPM (for more methodological details, see [24,25]). A 30 m swimming test was performed twice at maximum velocity (\(v_{0\text{max}}\)) by each swimmer using the swimming technique of his/her specialty. The first trial was performed in free swimming and, in the second trial, the swimmer towed a hydrodynamic body of known drag. The key point of the VPM (affecting the measurement accuracy) is the optimal value of the perturbation factor. Therefore, in all cases, the magnitude of the resistance of the hydrodynamic body was selected individually, so that the differences in \(v_{0\text{max}}\) between the first and second trials were in the required range: \(\Delta v_{0\text{max}}=\) 4-6%. VPM is usually considered limited by the possibility of each subject not being able to produce the same power in both consecutive repetitions, as assumed as a precondition, but this seems to be controlled in these velocity reduction conditions. The VPM, or its modifications, are often used in similar studies [26, 27, 28, 29, 30], since it allows to obtain, in all swimming techniques, quantitative values of \(F_{r(\text{add})}\) and \(P_{to}\) and calculating \(C_{\tau(\text{add})}\), taking into consideration the characteristic hydrodynamic size of the subject's body. Further, for each experimental velocity of the step-test (\(v_{\text{0 exp}}\)), the \(P_{to}\) value was calculated using the equation: \(P_{to}=P_{tom\text{max}}/v_{\text{0 max}}{}^{3}\times v_{\text{0 exp}}{}^{3}\). The basis for this approach are the results of a special study in which the relationship between the swimming velocity and the main hydrodynamic characteristics was studied [31]. In a special research pool with a length of 500 m filled with seawater, five elite male swimmers who specialized in front crawl performed repeated swimming trials of various lengths (30, 75, 120, 200, 350 m). The VPM was used to determine the main hydrodynamic characteristics of each swim for each swimmer. It was found that with an increased test distance and, accordingly, the duration of the swim, the \(v_{\text{0 exp}}\) of the subjects naturally decreases. The analysis of the results, carried out based on the general fluid force equation, showed that there is a quadratic dependence between \(v_{\text{0 exp}}\) and \(F_{r(\text{add})}\), a cubic dependence between \(v_{\text{0 exp}}\) and \(P_{to}\) and that the values of \(C_{\tau(\text{add})}\) remained constant.