Objectives This study investigates the causal effects of open-water swim drafting by leveraging a natural ex periment induced by staggered race starts during the COVID-19 pandemic. Furthermore, the study aims to quantify the benefits of swim drafting in real-world triathlon race settings and estimate the effect of different drafting positions within swim groups. Design A quasi-experimental design is employed, exploiting exogenous variation in swim drafting oppor tunities caused by COVID-19 restrictions. The study uses panel data from triathlon races before, during, and after the pandemic. Methods Using agglomerative hierarchical clustering, likely swim group formations are computed from swim out segment times. The impact of drafting positions on swim performance is estimated using a two way fixed-effects regression model, controlling for athlete- and event-specific factors. Robustness checks in the form of alternative regression model specifications and samples are used to partially address some potential endogeneity concerns, including reverse causality and omitted variable bias. Results Empirical findings reveal that drafting benefits were statistically insignificant in 2020 due to reduced drafting opportunities but re-emerged post-pandemic at slightly lower levels than pre-pandemic peri ods. The results indicate that drafting only becomes beneficial from the third trailing position onward, with earlier positions primarily serving to minimize fatigue, as shown by an observed inverse decay in drafting benefits (higher estimated penalties in lower-order drafting positions/stronger higher order drafting effects). Conclusions This study provides the first large-scale causal estimate of open-water swim drafting effects in real triathlon race conditions. The findings contribute to the understanding of race strategy optimization (metabolic costs), offering new insights for endurance athletes, race organizers and regulation au thorities. Additionally, the results indicate the importance of strategic positioning within swim groups to maximize performance benefits while mitigating fatigue effects.
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