The present study investigated the effects of initial levels of cycling performance, peak oxygen uptake (O_2peak) and gross efficiency (GE) on the subsequent adaptations of these variables and their relationship following high-intensity training (HIT) designed to increase O_2peak in competitive cyclists. Sixty cyclists (O_2peak = 61 ± 6 mL kg^-1 min^-1) were assigned a 12-week training program consisting of twenty-four supervised high-intensity interval training sessions and ad libitum low intensity training. GE was calculated at 125, 175, and 225 W and performance was determined by mean power during a 40-min time-trial (Power_{40 min}). In addition to correlation analyses between initial level and pre- to post-intervention changes of the different variables, we compared these changes between four groups where participants were categorized with either low and/or high initial levels of O_2peak and GE. Average volume of high- and low-intensity training during the 12-week intervention was 1.5 ± 0.3 and 8.3 ± 2.7 h·week^-1, respectively. Following the 12-week training period, there was a significant increase in absolute and body mass normalized O_2peak and Power_{40 min} (p < 0.05) and a significant decrease in GE (p < 0.05) for all athletes pooled. There was no change in body mass following the 12-week training period. We found a moderate negative correlation between initial level of O_2peak and the change in O_2peak following the training period (r = -0.32; p < 0.05). A small negative correlation was also found between initial Power_{40 min} and its change following training both when expressed in absolute power and power normalized for body mass (r = -0.27 and -0.28; both p < 0.05). A moderate negative correlation was also found between initial levels for GE and its change following training (r = -0.44; p < 0.01). There were no differences between the four groups based on initial levels of O_2peak and GE in the response to training on O_2peak, GE, or Power_{40 min} (all p > 0.12). In conclusion, the present findings suggest that there are statistically significant effects of initial levels of cycling performance and O_2peak and on the subsequent adaptations following a 12-week HIT program, but the small and moderate effects indicate limited influence on training practice.

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