PURPOSE: The National Ambulance Resilience Unit (NARU) works on behalf of each National Health Service (NHS) Ambulance Trust in England to strengthen national resilience and improve patient outcome in challenging pre-hospital scenarios. NARU personnel are trained to deal with hazardous or difficult situations and mass casualty incidents. The aim of this study was to quantify the physically demanding tasks undertaken by NARU personnel. METHODS: The study was completed in two phases. In Phase 1, 10 subject matter experts from a range of NHS Ambulance Trusts participated in a two-day researcher led workshop to define the most physically demanding tasks performed by NARU personnel. In Phase 2, 34 participants (29 men and 5 women, stature 1.77 ± 0.08 m, body mass 84 ± 14 kg, estimated VO2max 39 ± 7 mL·kg-1·min-1) performed scenarios of the tasks defined in Phase 1, with measurements of heart rate, speed of movement, load carried, and weights and forces of objects moved. Data are expressed as mean ± standard deviation. RESULTS: Eleven criterion tasks were defined in Phase 1: Swift Water Rescue, Re-board Boat, Unload Vehicle and Set-up Decontamination Unit, Clinical Decontamination (CD), Movement in Gas Tight Suits, Marauding Terrorist Firearms Attack (MTFA) , Over Ground Rescue, Unload Incidence Response Unit Vehicle, Above Ground Rescue, Over Rubble Rescue, and Subterranean Rescue. The shortest tasks were the Swift Water Rescue and Re-board Boat at ~1 min, with resultant heart rates of 76 ± 10 and 61 ± 14 %HRmax, respectively. The longest tasks were the CD and MTFA (~120 min) resulting in heart rates of 65 ± 11 and 73 ± 11 %HRmax, respectively. The greatest forces exerted were during equipment lifting and carrying (e.g. 167 kg by 6 people) and dragging of casualties (e.g. single person 88 kg casualty and stretcher drag). CONCLUSION: All five components of fitness (aerobic endurance, anaerobic endurance, muscular strength, muscular endurance and mobility) are required to successfully perform the criterion tasks undertaken by NARU personnel. These data can be used to inform interventions to enhance physical performance and develop physical employment standards for specialist ambulance responders. ACKNOWLEDGEMENT - This work was conducted in collaboration with the National Ambulance Resilience Unit in the United Kingdom.