Steady-state <(V)over dot>O2 above MLSS: evidence that critical speed better represents maximal metabolic steady state in well-trained runners

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 <(V)over dot>O-2 was stable over time from speeds at which a steady-state <(V)over dot>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 <(V)over dot>O-2 did not change significantly between 10 and 30 min at either MLSS or MLSS + 0.5 km/h. In contrast, <(V)over dot>O-2 increased significantly over time and reached <(V)over dot>O-2 max at end-exercise at a speed similar to 0.4 km/h above CS (P < 0.05) but remained stable at a speed similar to 0.5 km/h below CS. The stability of <(V)over dot>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 <(V)over dot>O-2.