The literature addressing the glycolytic contribution to overall energy expenditure has primary been utilized during resistance training and cycling. There is a paucity of data examining the glycolytic contribution to overall energy expenditure (EE) in the form of high-intensity intermittent sprint work. Therefore, the purpose of this study was to examine the variation in oxidative and glycolytic contribution during two HIIT protocols using a 1:1 work-to-rest ratio and a 2:1 work-to-rest ratio. Seventeen physically active participants performed three exercise testing sessions. The first session involved an incremental maximal exertion treadmill test along with a sprint familiarization. Sessions two and three involved a 30:30 and 30:15 HIIT protocol performed in a counterbalanced order. The HIIT sessions involved four sets, of four sprints with three minutes of recovery between each set. During both HIIT sessions, oxygen consumption accumulation, carbon dioxide production, and respiratory exchange ratio were measured to calculate overall oxidative and EPOC contribution. Blood lactate concentration was measured to calculate overall glycolytic contribution. Total EE was determined by summating oxidative, glycolytic and EPOC measurements. Relative contribution of energy system involvement was analyzed using a 2 x 4 repeated measures ANOVA. Paired t-test determined significant differences in total EE between sessions. Comparing total session EE, independent t-test revealed no significant difference between HIIT sessions (p = .947). There was a significant difference between the two sessions with respect to overall kCal expenditure from the oxidative system (p = .037), glycolytic system (p < 0.01), and EPOC (p <0.01). Independent t-test revealed a significant difference in glycolytic (p < 0.01), oxidative (p < 0.01), and EPOC contribution (p < 0.01) between both HIIT sessions. Repeated-measures ANOVA revealed a significant difference between sets and the two HIIT session (open full item for complete abstract)