The Load-Velocity Slope Is an Indicator of the Active Drag in all Competitive Swimming Strokes

Introduction Active drag in swimming is a critical variable that affects swimmers' performance, as well as the physiological load, but it is challenging for practitioners to assess this variable. This study aimed to assess if the load-velocity profiling method can be used as an indicator of active drag. Methods A total of 419 swimmers performed three semitethered swimming trials in their speciality among the four competitive strokes with different external loads. Linear regression between external load and swimming velocity, as well as the external load relative to the body mass and swimming velocity, were established. The active drag and drag coefficient of each swimmer were calculated using a velocity perturbation method. Results There were significant correlations of the active drag with the absolute slope (correlation coefficient ≥ 0.696, P < 0.001) and relative slope (correlation coefficient ≥ 0.538, P < 0.001) in all four strokes and both sexes. A multiple regression analysis exhibited that the primary determinant of these relationships was the drag coefficient (semipartial correlation ≥0.422, P < 0.001). The effects of the height and body mass index (BMI) on the relationship between the drag and the absolute slope were small (0.195 ≤ semipartial correlation ≤0.249, P < 0.001), which became either nonsignificant (height: P ≥ 0.282) or trivial (BMI: -0.099 ≤ semipartial correlation ≤ -0.081, P ≤ 0.011) when focusing on the relative slope. Conclusions These results indicate that the absolute load-velocity slope is a strong indicator of the active drag, and the relative slope is useful when indirectly assessing the drag coefficient.