The higher the fraction of maximal oxygen uptake is during interval training, the greater is the cycling performance gain.

It has been suggested that time at a high fraction (%) of maximal oxygen uptake (VO) plays a decisive role for adaptations to interval training. Yet, no study has, to date, measured the % of VO during all interval sessions throughout a prolonged training intervention and subsequently related it to the magnitude of training adaptations. Thus, the present study aimed to investigate the relationship between % of VO achieved during an interval training intervention and changes in endurance performance and its physiological determinants in well-trained cyclists. Twenty-two cyclists (VO 67.1 (6.4) mL·min ·kg; males, n = 19; females, n = 3) underwent a 9-week interval training intervention, consisting 21 sessions of 5 × 8-min intervals conducted at their 40-min highest sustainable mean power output (PO). Oxygen uptake was measured during all interval sessions, and the relationship between % of VO during work intervals and training adaptations were investigated using linear regression. A performance index was calculated from several performance measures. With higher % of VO during work intervals, greater improvements were observed for maximal PO during the VO test (R  = 0.44, p = 0.009), PO at 4 mmol·L [blood lactate] (R  = 0.25, p = 0.035), the performance index (R  = 0.36, p = 0.013), and VO (R  = 0.54, p = 0.029). Other measures, such as % of maximal heart rate, were related to fewer outcome variables and exhibited poorer session-to-session repeatability compared to % of VO. In conclusion, improvements in endurance measures were positively related to the % of VO achieved during interval training. Percentage of VO was the measure that best reflected the magnitude of training adaptations.