## 3 Results and Discussion
### Ecological Experimental Validation  
The horizontal force-time curve exerted by hands and produced during a backstroke start data acquisition is similar to the one peak force profile previously presented (\(\sim\)0.5 BW; [24]). Most of the force-time curves displayed in backstroke start studies have analysed the horizontal component exerted on feet (e.g., [24; 29]), which is similar to the dual peak force profile found in the current study (Figure 7a), with swimmers performing \(\sim\)1.6 BW in the both peak forces. Vertical backstroke start forces exerted by feet have shown similar values to those found in [PERSON] et al. (i.e., \(\sim\)1.0 BW; [10; 29]). Lateral force exerted by feet has depicted a peak force instant before the hands-off. Horizontal and vertical force-time curves exerted on feet and hands obtained in the new track start technique performed with the back plate (Figure 8) have registered similar rear (\(\sim\)800 N and \(\sim\)1000 N) and front foot (\(\sim\)800 N and 600 N; cf. [1; 8]) as well as hand force profiles (\(\sim\)150 N and \(\sim\)800 N; cf. [8]). Previous studies have evidenced that the test-retest reliability of the kinetic gait parameters in the aquatic environment presented poor (medial lateral force component) to excellent reliability (vertical and antero-posterior force component; [30]). Figure 6: Force plates calibration results: 600 mm \(\times\) 300 mm force plate vertical force. Figure 7: Cont. In relay start techniques, the horizontal force-time curve exerted on the feet and obtained during one step start technique (Figure 9) revealed a similar finding as obtained by [PERSON] et al. [31], although these authors had not measured right and left forces exerted on each foot. Figure 8: Individual antero-posterior (x) and vertical (y) force—time curves representing forces exerted on left and right hands (LH and RH), as well as rear and front foot (ReF and RF) in the new track start technique synchronised with the trigger (trg). Figure 7: Backstroke start horizontal (**a** panel), vertical (**b** panel) and lateral (**c** panel) forces from seven trials of one swimmer (**c** panel). Tri-axial forces measured in one underwater force plate during the open backstroke to breaststroke changeover have evidenced two distinct turn phases, the hand contact (\(\sim\)4.60 to 5.50 s) followed by swimmers' rotation (\(\sim\)4.80 s to 7.30 s) and the push-off (\(\sim\)7.30 to 7.80 s; Figure 10). Previous studies have already mentioned that the push-off phase depicted explosive lower limb movements since the first feet contact (\(\sim\)7.20 s) to the end of push-off (\(\sim\)7.80 s), which has been mainly observed in the horizontal force component [13; 28]. [PERSON] and co-authors [31] have found \(\sim\)193 N for the peak horizontal ground reaction force in female swimmers performing the open turn technique. The current and the previously developed instrumented starting block configurations have shown restrictions regarding independent hand force and momentum measures with natural swimmers' hands and feet placement during all ventral start techniques. (e.g., [2; 6; 8]). The solution found in the current project was the replication of four more tri-axial force plates of 300 mm \(\times\) 125 mm designed and numerically simulated (Figure 11a-d) to be placed in the frontal start block edge. An 800 N vertical, and 200 N medium-lateral and horizontal load were applied and registered 280.11, 109.89 and 117.34 \(\upmu\)c maximal deformation and a maximal 286.45 Hz frequency. The four-force plates solution  
Figure 10: Individual backstroke to breaststroke open medley turning technique force–time curve with horizontal, vertical and medium-lateral components (purple, yellow and red line, respectively). Figure 9: Horizontal force–time curve profiles from eight swimmers at the one step relay start technique: Rear foot (red line), front left and right foot (green and blue line, respectively).