This study aimed to characterise the underwater phase of the backstroke start. Nine highly trained backstroke swimmers performed a maximal 3x15 m of the starting variant with feet parallel and partially emerged and the highest horizontal handgrip. The best 15 m trial was selected for each swimmer. Motion capture system tracked right side markers. Each individual velocity curve was normalised from the immersion until the beginning of the upper limbs propulsion. The velocity at full immersion and at five critical instants of the 1st undulatory underwater cycle was assessed. After the full immersion, swimmers performed a downward kick with lower horizontal and resultant velocity than those displayed at full immersion (1.15±0.18 vs. 2.09±0.26 and 1.62±0.36 vs. 2.39±0.33 m/s, respectively). The transition to the 1st upward kick generated greater horizontal, vertical and resultant velocity than those noted at 1st downward kick (1.79±0.18 vs. 1.15±0.18, -1.23±0.51 vs.-1.0±0.34, 2.14±0.23 vs. 1.62±.36 m/s, respectively). Compared to the 1st upward kick, swimmers displayed lower horizontal, vertical and resultant velocity at the 1st part of the transition from the 1st up to the 2nd downward kick {1.67±0.15 vs. 1.79±0.18, -0.29±0.21 vs. -1.23±.51, 1.73±0.13 vs. 2.14±0.23 m/s, respectively). Lower horizontal and resultant velocity was observed at the 2nd downward kick compared to the 2nd part of the transition from the 1st up to 2nd downward kick (0.96±0.22 vs. 1.68±0.14, 1.14±0.30 vs. 1.70±0.15 m/s, respectively). Subsequently to the full immersion, a downward kick was performed implying the swimmers' deceleration, which was minimised by continued undulatory underwater cycles. These findings highlighted the absence of the gliding phase during the backstroke start.
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