Resolving Differences between MLSS and CP by Considering Rates of Change of Blood Lactate during Endurance Exercise.

The identification of the power output that separates the heavy intensity exercise domain, wherein steady states in blood [lactate] and pulmonary V̇O2 are achievable, from the severe intensity domain, wherein they are not, has conceptual and practical value in exercise physiology. Purpose: This study aimed to develop a new method that more closely represents the heavy to severe exercise domain boundary by evaluating the rates of blood lactate accumulation during the constant power output exercise bouts that are used in the assessment of the maximal lactate steady state (MLSS). Methods: Eight well-trained male cyclists completed five exercise tests of up to 30 min for the determination of the traditional MLSS (MLSSTRAD) and a further four maximal tests for the determination of critical power (CP). The rates of change of blood [lactate] between 10 min and the end of exercise in the MLSS tests were plotted against the corresponding power outputs, and a two-segment linear regression model was used to identify individualized break points in lactate accumulation versus power output (modified MLSS [MLSSMOD]). Results: MLSSMOD was significantly higher than MLSSTRAD (297 ± 41 vs 278 ± 41 W, P < 0.001) but was not significantly different from CP (297 ± 41 W, P > 0.05); MLSSMOD and CP were closely aligned (r = 0.97, bias = −0.52 W, SEE = 10 W, limits of agreement = −20 to 19 W). The rates of change of both blood [lactate] and V̇O2 were significantly greater, and exercise intolerance occurred before 30 min, at a power output slightly above MLSSMOD. Conclusions: A novel method for evaluating blood lactate kinetics during MLSSTRAD protocol produces MLSSMOD that is not different from CP and better represents the heavy to severe exercise domain boundary. [ABSTRACT FROM AUTHOR]