Negative accumulated oxygen deficit during heavy and very heavy intensity cycle ergometry in humans

Abstract

The concept of the accumulated O₂ deficit (AOD) assumes that the O₂ deficit increases monotonically with increasing work rate (WR), to plateau at the maximum AOD, and is based on linear extrapolation of the relationship between measured steady-state oxygen uptake (V̇O₂) and WR for moderate exercise. However, for high WRs, the measured V̇O₂ increases above that expected from such linear extrapolation, reflecting the superimposition of a "slow component" on the fundamental V̇O₂ mono-exponential kinetics. We were therefore interested in determining the effect of the V̇O₂ slow component on the computed AOD. Ten subjects [31 (12) years] performed square-wave cycle ergometry of moderate (40%, 60%, 80% and 90% ${\hat{\theta }_{{\rm{L}}} }$), heavy (40%Δ), very heavy (80%Δ) and severe (110% V̇O_{2 peak}) intensities for 10–15 min, where ${\hat{\theta }_{{\rm{L}}} }$ is the estimated lactate threshold and Δ is the WR difference between ${\hat{\theta }_{{\rm{L}}} }$ and V̇O_{2 peak}. V̇O₂ was determined breath-by-breath. Projected "steady-state" V̇O₂ values were determined from sub-${\hat{\theta }_{{\rm{L}}} }$ tests. The measured V̇O₂ exceeded the projected value after ~3 min for both heavy and very heavy intensity exercise. This led to the AOD actually becoming negative. Thus, for heavy exercise, while the AOD was positive [0.63 (0.41) l] at 5 min, it was negative by 10 min [−0.61 (1.05) l], and more so by 15 min [−1.70 (1.64) l]. For the very heavy WRs, the AOD was [0.42 (0.67) l] by 5 min and reached −2.68 (2.09) l at exhaustion. For severe exercise, however, the AOD at exhaustion was positive in each case: +1.69 (0.39) l. We therefore conclude that the assumptions underlying the computation of the AOD are invalid for heavy and very heavy cycle ergometry (at least). Physiological inferences, such as the "anaerobic work capacity", are therefore prone to misinterpretation.