BACKGROUND: Long-distance swimming performance relies on the regulation of stroke mechanics and physiological load to sustain effective pacing. Movement smoothness, quantified by jerk cost (JC), reflects technical proficiency and has been theoretically linked to energetic cost, yet empirical evidence of this interaction in swimming remains scarce. Technological advances enable concomitant assessment of stroke smoothness and muscle oxygenation saturation (SmO*) responses during swimming. This study investigated the continuous relationship between JC and SmO* during an even-paced 3000m front-crawl swim at 90% of 400m speed (V400); METHODS: 9 local-to-regional triathletes were instrumented with a sacrum-worn inertial measurement unit and a near-infrared spectroscopy sensor on the vastus lateralis to compute lap-by-lap JC and SmO*, respectively. Linear mixed-effects models quantified population-level trends and inter-individual variability in the SmO*-JC, SmO*-lap, and JC-lap relationships, with random intercepts and slopes for swimmers; RESULTS: At the population level, higher JC (impared smoothness) was associated with lower SmO* (p<0.05), indicating a disrupted muscle oxygen balance for jerkier stroke patterns. Across the 3000m, SmO* progressively decreased while JC increased (p<0.05), reflecting concurrent physiological and technical deterioration under sustained pacing. Marked inter-individual variability was observed in baseline values and in the magnitude of both JC-SmO* coupling and temporal evolutions, revealing swimmer-specific regulation and resilience profiles; CONCLUSIONS: Stroke smoothness is linked to vastus lateralis SmO* during prolonged even-paced swimming at 90% V400. Combining wearable-derived biomechanical and physiological measures provides a first in-situ attempt to integrate individual stroke smoothness regulation patterns and associated local metabolic dynamics within a unified perspective of swimming pacing
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