Steady-state $$\dot{V}{\text{O}}_{2}$$ above MLSS: evidence that critical speed better represents maximal metabolic steady state in well-trained runners

Abstract

The metabolic boundary separating the heavy-intensity and severe-intensity exercise domains is of scientific and practical interest but there is controversy concerning whether the maximal lactate steady state (MLSS) or critical power (synonymous with critical speed, CS) better represents this boundary. We measured the running speeds at MLSS and CS and investigated their ability to discriminate speeds at which $$\dot{V}{\text{O}}_{2}$$ V ˙ O 2 was stable over time from speeds at which a steady-state $$\dot{V}{\text{O}}_{2}$$ V ˙ O 2 could not be established. Ten well-trained male distance runners completed 9–12 constant-speed treadmill tests, including 3–5 runs of up to 30-min duration for the assessment of MLSS and at least 4 runs performed to the limit of tolerance for assessment of CS. The running speeds at CS and MLSS were significantly different (16.4 ± 1.3 vs. 15.2 ± 0.9 km/h, respectively; P < 0.001). Blood lactate concentration was higher and increased with time at a speed 0.5 km/h higher than MLSS compared to MLSS (P < 0.01); however, pulmonary $$\dot{V}{\text{O}}_{2}$$ V ˙ O 2 did not change significantly between 10 and 30 min at either MLSS or MLSS + 0.5 km/h. In contrast, $$\dot{V}{\text{O}}_{2}$$ V ˙ O 2 increased significantly over time and reached $$\dot{V}{\text{O}}_{2\,\,\max }$$ V ˙ O 2 max at end-exercise at a speed ~ 0.4 km/h above CS (P < 0.05) but remained stable at a speed ~ 0.5 km/h below CS. The stability of $$\dot{V}{\text{O}}_{2}$$ V ˙ O 2 at a speed exceeding MLSS suggests that MLSS underestimates the maximal metabolic steady state. These results indicate that CS more closely represents the maximal metabolic steady state when the latter is appropriately defined according to the ability to stabilise pulmonary $$\dot{V}{\text{O}}_{2}$$ V ˙ O 2 .

Power profiling and the power-duration relationship in cycling: a narrative review

“…For a long time, the maximum lactate steady state (MLSS) was considered as the gold standard for this boundary (Billat et al 2003 ; Keir et al 2015 ; Kilding and Jones 2005 ). However, recent work (Galán-Rioja et al 2020 ; Jamnick et al 2020 ; Jones et al 2019 ; Nixon et al 2021 ) has suggested that CP better estimates the maximal metabolic steady state, the highest power output where a steady state in the oxygen uptake ( O 2 ) response can still be observed, despite increasing blood lactate values (Bräuer and Smekal 2020 ). There is still some debate as to which method (if any) is superior for differentiating between metabolic steady state and non-steady state exercise, and whether both MLSS and CP can actually be used interchangeably (Jones et al 2019 ; Keir et al 2015 ; Nixon et al 2021 ; Poole et al 1988 ).…”

Impairment in maximal lactate steady state after carbon monoxide inhalation is related to training status

“…It has been suggested that the assumption of the MLSS as an indicator of a steady state whole-body metabolism may be incorrect since steady-state VO 2 behaviour can be observed at intensities slightly above MLSS (Hill et al, 2021;Iannetta et al, 2021;Nixon et al, 2021).…”

Commentary on “Over 55 years of Critical Power: Fact or Artifact”

“…As a consequence, the authors conclude that the choice of trial length for CP estimation is arbitrary. However, we argue that when the length of the trial is between 2 and 15 min, CP has repeatedly shown to differentiate intensity domains and steady-state exercise from nonsteady-state exercise 4,5,[7][8][9] . Trials shorter than 2 min or longer than 15 min may not elicit maximal oxygen uptake 10,11 , which is necessary as the CP model applies to the severe-intensity domain.…”