Critical swimming speed does not represent the speed at maximal lactate steady state.

Critical power and critical swimming speed (CSS) are mathematically defined as intensities that could theoretically be maintained indefinitely without exhaustion. Several investigations have been conducted to attribute a physiological meaning to these variables, but results in swimming remain equivocal. Thus, the purpose of this study was to compare CSS with direct determination of the speed at maximal lactate steady state (S (MLSS)). Eight well-trained swimmers (aged 18.6 +/- 1.9 years) performed four tests to exhaustion (95, 100, 105, and 110 % of maximal aerobic speed [MAS]) in order to determine CSS from the distance-time relationship. S (MLSS) was determined from four sub-maximal 30-min constant intensity tests (ranging from 75 % to 90 % MAS). CSS (92.7 +/- 2.6 % MAS) was significantly higher than S (MLSS) (88.3 +/- 2.9 % of MAS) and the bias +/- 95 % limits of agreement for comparisons between CSS and S (MLSS) (0.07 +/- 0.13 m x s(-1)) indicated that the extent of disagreement was too great to use these two variables interchangeably. However, CSS and S (MLSS) were strongly correlated (r = 0.87; SEE = 0.033 m x s(-1); p < 0.01). Results from the present study demonstrate that in swimming, CSS does not represent the maximal speed that can be maintained without a continuous rise of blood lactate concentration and direct determination of S (MLSS) is necessary if precision is required in experimental studies.