Fig. 2.

Rate of pulmonary oxygen uptake for a representative subject in the O2+ group before (top panel) and after (bottom panel) an eight-week training intervention comprising HIIT performed twice per week. The data were collected during unloaded cycling (time = − 60 to 0 s) followed by a square-wave increase to 30% of the subject’s peak work rate. Closed circles depict 5-s averages of breath-by-breath V̇O₂ data while solid line depicts modelled fit. Notice the marked reduction in the V̇O₂ MRT (i.e., the time taken for V̇O₂ to reach ~ 63% of the steady-state amplitude) and consequent reduction in the O₂ deficit indicated by the dashed line (O₂ deficit = MRT x Amplitude)