Firefighting is a hazardous task associated with a heavy workload where task duration may be limited by air cylinder capacity. Increased fitness may lead to better air ventilation efficiency and task duration at a given heavy work intensity. This study compared performance, air ventilation and skeletal muscle oxygen extraction during a maximal graded walking test (GWT), a 10METS (metabolic equivalent) treadmill test (T10) and a simulated work circuit (SWC). Participants (n=13) who performed the SWC in a shorter time had significantly lower air cylinder ventilation values on the T10 (r=-0.495), better peak oxygen consumption (r=-0.924) during the GWT and significantly greater skeletal muscle oxygen extraction during the SWC (HbDiff, r=0.768). These results demonstrate that the fastest participants on the SWC had better air ventilation efficiency that could prolong interventions in difficult situations requiring air cylinder use.