Bullet points • Performance enhancement in 800-m freestyle paralleled improvements in critical speed (CS). • D' is not a strong determinant factor in her 800-m freestyle performance. • The differential W'BAL model can be used to analyze the pacing patterns. 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.
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