Abstract: The purposes of this study were to ascertain how physiological adaptations, as reflected by critical speed and distance above critical speed ( D'), impact the competitive performance of a world-class female long-distance swimmer; and to determine whether a model including the expenditure and recovery of D' could be used to understand pacing in swimming. From August 2011 to August 2021, we retrieved 800-m performance and splits data from races in which she improved her time, and also the 400-m and 200-m freestyle performances from the same competitions. Performances from the 200, 400, and 800 m were used to estimate critical speed and D'. The 800-m splits were used to calculate the usage of D' during the race and to investigate pacing. The differential W' balance model ( W' BAL ) was used to calculate its analogous in swimming, the D' BAL . Critical speed increased from 1.516 to 1.616 ms -1 while D' fluctuated ~15 m from 2011 to 2016. D' BAL approached 0 m at the end of the races and may be useful to understand pacing. Critical speed and 800-m speed presented a nearly perfect correlation (0.99, p < 0.001) suggesting that critical speed is of paramount importance for long-distance swimming performance. While critical speed is clearly important, D' did not directly correlate with 800-m performance. This result suggests that the work capacity above critical speed is not a primary determinant of 800-m swimming performance. However, we cannot say it is unimportant, as minor improvements represent an opportunity to set world records. Outstanding long-distance performance seems to depend more on aerobic fitness than on the capacity to work above critical speed Ver menos