# Kinematic Determinants of the Swimming Push Start in Competitive Swimmers
## 4 Discussion
Therefore, underwater versus surface velocity ratio in the 25 m trials probably constrained swimmers to emerge closer to the starting wall as compared to a race pace trial. Gender differences in the overall start distance were shorter than previously reported in competitive dive starts or turns of national level swimmers [11], probably related to the relative shorter distances travelled by swimmers in the push start. In relation to start sub-phases contribution, average glide distances (between 0.86 and 1.67 m) were shorter than those reported during dive starts [36], but also shorter than recommended glide distances \(\approx\)2 m [24] on the turn. In relative terms, glide distances represented from 12% to 26% of the overall start, with backstroke glides being shorter than in the ventral positions. Considering that no differences in drag forces have been reported between ventral and dorsal body positions [32], the shorter glide distances in backstroke could be related to lower forces applied on the wall push-off or inefficient angles of attack of the swimmers' body segments after leaving the starting wall [37]. However, no evidence in the literature has reported differences in both aspects between the dorsal and ventral body position. During the leg kicking phase, distances represented around two thirds of the overall start distances with each stroke presenting specific distances. Compared to previous research, the duration of the leg kicking phase in front-crawl (an average of 2.08 s) was longer than the previously reported of 1.23 \(\pm\) 0.47 in the tumble turns [10] or 1.27 \(\pm\) 0.37 in the dive start [38], which highlights the specificity of the push start compared to the dive start or the turn segments. This confirms the need of individualizing the leg kicking distances according to the underwater to surface swimming velocity ratio in each stroke and/or type of start, as previously described in competition [39]. Finally, the transition phases of the butterfly stroke were shorter than those of the alternative techniques (front-crawl and backstroke) with maximal distance contributions of 15%. The distance travelled in the front-crawl transition was shorter than that reported during the first arm stroke of national level swimmers [4]. The linear regression analysis of the push start performance indicated that times to 10 m could be explained at 50% and 58%, respectively, for the butterfly and backstroke techniques, by the distances and average velocities of each start sub-phase. The average velocity attained in the kicking leg phase seemed to be a key parameter for both the ventral and dorsal position. Indeed, large correlations were detected between faster leg kicking velocities and shorter times to 10 m in both the backstroke and butterfly strokes (Figure 1). According to the beta coefficients of regression equations (Table 2), small changes on leg kicking velocity would have a huge impact on times to 10 m (for example, 0.5 s improvement in backstroke by increasing leg kicking velocity 0.1 m/s). This was previously reported for the whole underwater sections of competitive starts [38] and turns [40], but no specific evidence has been provided for the leg kicking phase isolated from the glide and transition part. It should be noted that average velocity, and not distance attained in this sub-phase, had an influence on start performance. This is in line with underwater velocity rather than distance having an impact on the overall performance of 100 m events on World Swimming Championships [6; 39] and suggests the importance of achieving fast (rather than long) underwater segments in the sprint events. Apart from the underwater swimming, the glide and transition sub-phases seem to play an important role on some of the push starts. Indeed, faster glides and shorter transitions were related to faster times to 10 m in backstroke (Figure 1 and Table 2). For the glide phase, previous research has emphasized the role of the hip depth during gliding to reduce drag forces [12; 41].