BACKGROUND: Load-velocity profiling (LVP) using semi-tethered devices is a validated methodology for quantifying force and velocity capabilities in surface swimming, yet its application to underwater undulatory swimming (UUS) remains unexplored. *This study aimed to characterize the LV profiled during UUS, investigate the relationship between the LV profile and UUS performance and anthropometrics, and examine sex-based differences. METHODS: Thirty-three competitive swimmers (21 males, 12 females) performed a LV profile test. Individual LV profiles were constructed via linear regression of each trial and *the theoretical maximum velocity (V0), the theoretical maximum load (L0), and the slope of the regression line (Slope) were calculated. Anthropometric measurements included height, arm span, body mass, sitting height, body length, biacromial and biiliocristal breadths, and derived indices. RESULTS: High linearity was confirmed for the LV profile (R² = 0.987 ± 0.015, males; 0.981 ± 0.023, females). Except for Slope in males, the LV profile-derived variables were largely to nearly perfectly correlated with UUS performance in both sexes (p<0.05). Body mass showed moderate association with L0 (r = 0.39) and Slope (r = 0.42) in males and no other significant association between anthropometrics and performance was observed (p>0.05). Males exhibited higher V* (1.58 ± 0.17 vs. 1.39 ± 0.14 m·s-1, p<0.01) and L* (7.99 ± 2.38 vs. 5.76 ± 1.02 kg, p<0.01). Females showed steeper load-velocity Slopes (-0.21 ± 0.05 vs. -0.25 ± 0.03, p<0.05). CONCLUSIONS: LV profile methodology is applicable to UUS assessment, with limited direct influence of anthropometric factors and clear differences between sexes
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