## 4 Discussion
Similarly, as for physiological cost, a higher statistically significant growth (\(p<0.05\)) was recorded for the mean value of \(\Delta\)Ec, expressed in kJ-min\({}^{-1}\) between technical and threshold speeds (21% difference) than between threshold and racing speeds (8% difference) (NS). Also, statistically significant differences were observed between the mean values of total external work of muscles expressed in kJ-min\({}^{-1}\) at 31% for speeds vt and vs (\(p<0.001\)). Total external work increased the cost by approx. 4.2 kJ-min\({}^{-1}\) when the speed was increased from technical to racing. Similar observations were made from comparisons of total external work standardized by meter of covered distance and body mass of the athletes examined. Comparing changes of \(\Delta\)Ep values calculated per minute of work, it should be stated that no significant contrasts were found between mean values of pairs of variables for individual gait speeds. In the case of kinetic energy (\(\Delta\)Ek) changes, significant differences were recorded in post hoc testing between vt and vp (\(p<0.005\)) and vt and vs (\(p<0.001\)) as well as vp and vs (\(p<0.05\)). When speed value was increased from technical to racing speed, a mean growth of \(\Delta\)Ep of 5% was observed and of \(\Delta\)Ek approx. 46%. This proves a much higher influence of the changes in \(\Delta\)Ek than in \(\Delta\)Ep on the level of total external work in body acceleration. From a biomechanical perspective, key changes include reducing oscillations of the center of mass and improving energy transfer between body segments,which limits unnecessary energy losses during the deceleration and acceleration phases of gait [23]. This subsequently indicates the key importance of changes in speed of CoM displacement in the phase of foot contact with the ground. Side and vertical components of movement speed in athletes reach much lower values than the front-back component. Efficient work by a competitor during the contact phase after achieving racing speed should rely on minimizing a decrease of the speed component in the absorption phase, when the negative (opposite to the movement direction) phase of the front-back component of the ground reaction force takes place. A slight reduction in speed in the absorption phase will result in saving energy in the propulsion, bounce phase. The phase of foot contact with the ground should be smooth to ensure the lowest changes of kinetic energy. Effective utilization of kinetic and potential energy during the foot-ground contact phase reduces total external work, leading to more economical movement and less muscle fatigue [20]. The variance analysis of reproducible measurements showed statistically significant differences for \(\Delta\)Ec calculated only for racing speed. Mean values of \(\Delta\)Ec differed statistically in both tests by approx. 3.6 kJ-min\({}^{-1}\) (\(p<0.01\)). This reflected a lowering of the cost in the second test by approx. 20% in relation to the work results [7]. Mean values of \(\Delta\)Ec for technical speed were very close in both tests, whereas analogous mean values for threshold speed were lower in the second test by approx. 10%, but the differences were not significant (\(p<0.05\)). Similar dependencies were observed for variables standardized by meter of covered distance and athletes' body mass. Also, mean values of \(\Delta\)Ep and \(\Delta\)Ek during walking at racing speed were significantly lower in the second test, respectively by approx. 0.6 kJ-min\({}^{-1}\) and 2.8 kJ-min\({}^{-1}\) (\(p<0.05\)). This reflected a lowering of \(\Delta\)Ep by approx. 14% and of \(\Delta\)Ek by approx. 20% compared with the first test [7]. The statistical analysis performed also indicates that the physiological cost EE and total external work \(\Delta\)Ec, expressed in kJ-m\({}^{-1}\), differed significantly in mean values for all gait speeds at the level of \(p<0.001\).