BACKGROUND: Arm stroke trajectory plays a central role in propulsion and efficiency in front crawl swimming. The vertical displacement of the hand is associated with propulsive phases and technical adjustments across swimming intensities. However, most biomechanical studies rely on discrete variables, which may overlook changes occurring throughout the stroke cycle. Functional data analysis enables the investigation of continuous movement patterns and may improve detection of intensity-dependent changes in stroke mechanics. This study investigated whether different intensities affect the vertical displacement profile of the arm stroke in front crawl. METHODS: Hand trajectories were obtained using a system of 12 cameras positioned above and below the water around a calibrated acquisition volume. Competitive swimmers performed front crawl trials at three intensities: 70%, 85%, and 100% of the average speed from a maximal 200-m test. Three-dimensional hand trajectories from two stroke cycles were reconstructed, and vertical displacement along the Z-axis was analyzed over the normalized stroke cycle. FANOVA compared displacement curves and identified time intervals with significant differences. RESULTS: Significant differences were observed between intensities. The Z-axis trajectory at 100% differed from 70% and 85%, while no difference occurred between 70% and 85%. Differences appeared mainly during propulsive phases, between 23-38% and 45-62% of the stroke cycle, corresponding to pull and push phases. CONCLUSIONS: Swimming intensity alters vertical displacement during propulsive phases. Functional analysis was more effective than discrete approaches for identifying when these changes occur, providing objective information for technique adjustments and supporting the applicability of FANOVA in swimming biomechanics.
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