Background: Open-water 400m swimming imposes additional biomechanical constraints compared to pool settings, requiring adaptive stroke kinematics strategies. Aim: To analyze how pool-to-open-water transition and lake current direction affect stroke kinematics and efficiency in recreational swimmers. Methods: Three female recreational open-water swimmers (27.6 ±16.6 years; 70.3 ±22.1 kg; 1.71 ±0.03 m) completed 400m trials in a pool and a lake. A marked 25m lane with buoy turns simulated pool conditions in open water. A GoPro Hero 4 (120 Hz) in the sagittal plane recorded all trials (CAAE: 95096625.3.0000.5153). SR, SL, MV, SI, and ** were analyzed every 25m using Kinovea, excluding 5m turn segments. Wilcoxon signed-rank tests (*<0.05) assessed environment and current effects per swimmer. Results: Pool-to-open-water comparisons revealed individual responses: Subject 2 showed no differences, while Subjects 1 and 3 reduced SR (29.8*26.8 and 28.3*25.9 cycles/min). Subject 1 increased SL, SI, and ** in open water, compensating for navigation demands and absent wall push-offs. Swimming against the current raised SR while reducing SL and SI. All subjects showed significant improvements swimming with the current: SL increased from 1.07*1.36, 1.07*1.50, and 1.07*1.43 m/stroke; MV from 0.96*1.16, 0.85*1.17, and 0.93*1.20 m/s; and ** from 38*49%, 41*58%, and 38*51%, for Subjects 1, 2, and 3, respectively. Subject 3 also reduced SR (26.08*25.74 cycles/min), while Subjects 1 and 3 improved SI (2.06*3.18 and 1.98*3.43 m²/s). Conclusions: Preliminary results suggest heterogeneous biomechanical strategies in response to open-water constraints and current conditions, underscoring the need for individualized technical training and further research with larger samples.
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