Background This study used a novel protocol to test the hypothesis that a plateau in oxygen consumption (VO_2max) during incremental exercise testing to exhaustion represents the maximal capacity of the cardiovascular system to transport oxygen.

Methods Twenty-six subjects were randomly divided into two groups matched by their initial VO_2max. On separate days, the reverse group performed (i) an incremental uphill running test on a treadmill (INC₁) plus verification test (VER) at a constant workload 1 km h⁻¹ higher than the last completed stage in INC₁; (ii) a decremental test (DEC) in which speed started as same as the VER but was reduced progressively and (iii) a final incremental test (INC_F). The control group performed only INC on the same days that the reverse group was tested.

Results VO_2max remained within 0.6 ml kg⁻¹ min⁻¹ across the three trials for the control group (p=0.93) but was 4.4% higher during DEC compared with INC₁ (63.9±3.8 vs 61.2±4.8 ml kg⁻¹ min⁻¹, respectively, p=0.004) in the reverse group, even though speed at VO_2max was lower (14.3±1.1 vs 16.2±0.7 km h⁻¹ for DEC and INC₁, respectively, p=0.0001). VO_2max remained significantly higher during INC_F (63.6±3.68 ml kg⁻¹ min⁻¹, p=0.01), despite an unchanged exercise time between INC₁ and INC_F.

Conclusion These findings go against the concept that a plateau in oxygen consumption measured during the classically described INC and VER represents a systemic limitation to oxygen use. The reasons for a higher VO₂ during INC_F following the DEC test are unclear.