Breath-by-breath O₂ uptake ( [(V)\dot]_\textO2 \dot{V}_{{{\text{O}}_{2} }} , L min⁻¹) and blood lactate concentration were measured before, during exercise, and recovery in six kata and six kumite karate Word Champions performing a simulated competition. [(V)\dot]_{\textO 2 \textmax} , \dot{V}_{{{\text{O}}_{{ 2 {\text{max}}}} }} , maximal anaerobic alactic, and lactic power were also assessed. The total energy cost ( V_{\textO 2 \textTOT} , V_{{{\text{O}}_{{ 2 {\text{TOT}}}} }} , mL kg⁻¹ above resting) of each simulated competition was calculated and subdivided into aerobic, lactic, and alactic fractions. Results showed that (a) no differences between kata and kumite groups in [(V)\dot]_{\textO 2 \textmax} , \dot{V}_{{{\text{O}}_{{ 2 {\text{max}}}} }} , height of vertical jump, and Wingate test were found; (b) V_{\textO 2 \textTOT} V_{{{\text{O}}_{{ 2 {\text{TOT}}}} }} were 87.8 ± 6.6 and 82.3 ± 12.3 mL kg⁻¹ in kata male and female with a performance time of 138 ± 4 and 158 ± 14 s, respectively; 189.0 ± 14.6 mL kg⁻¹ in kumite male and 155.8 ± 38.4 mL kg⁻¹ in kumite female with a predetermined performance time of 240 ± 0 and 180 ± 0 s, respectively; (c) the metabolic power was significantly higher in kumite than in kata athletes (p ≤ 0.05 in both gender); (d) aerobic and anaerobic alactic sources, in percentage of the total, were significantly different between gender and disciplines (p < 0.05), while the lactic source was similar; (e) HR ranged between 174 and 187 b min⁻¹ during simulated competition. In conclusion, kumite appears to require a much higher metabolic power than kata, being the energy source with the aerobic contribution predominant.