The purpose of this study was to evaluate energy system contribution during heavy (above anaerobic threshold (AT)) cycling when preceded by either moderate (<AT) or heavy intensity warm-up. The slow component (SC) of VO2 was defined as the increase in VO2 beyond three minutes of exercise. The total adjusted oxygen cost of activity (i.e. VO2, excluding the oxygen cost of breathing, but adding an oxygen equivalent of anaerobic energy contribution) was calculated for heavy intensity cycling at 60% ∆ for 8 females. The total adjusted oxygen cost was not affected by intensity of prior exercise and remained constant after 3 min of exercise. Total anaerobic contribution during the second exercise bout decreased over time regardless of prior exercise condition, but was significantly lower following prior heavy exercise. These results are consistent with previous findings that the SC represents a shift in energy system utilization rather than an increasing energy cost.