Quantifying the thermal demands of the British Military Annual Fitness Test: an observational study and randomised control trial

Introduction Thermal modelling of the British Army’s annual physical employment test—Role Fitness Test (Soldier) (RFT(S))—indicated that the two-stage load carriage section may present a thermal risk to personnel (body core temperature >39.5°C). In response, the Army modified the RFT(S) by extending the transition time between load carriage sections from 5 to 15 min. This paper presents body core temperature (gastrointestinal temperature; TGI) data collected from a field-observational study of the modified RFT(S), and during a controlled laboratory load carriage study implementing the original 5 min transition period under three different wet bulb globe temperatures (WBGTs). Methods 70 recruits (69 men) completed their mandatory pass/fail RFT(S) during week 18 of their Combat Infantry Course. Field-observational testing was conducted on four non-consecutive days, during summer 2020 (WBGTs=4.2°C–17.5°C). In the controlled laboratory study, 10 infantry males completed the two-stage load carriage section of the RFT(S) in WBGTs of 20°C, 22°C and 25°C. The primary outcome variable, for both studies, was the number of participants reaching a TGI above 39.5°C, and time spent above this threshold. Results During the field-observational study, five recruits (7%) exceeded a critical TGI of 39.50°C. Peak TGI was always observed following load carriage stage 2 (LC2), a 2 km best-effort loaded march. Participants who exceeded 39.50°C spent between 18 s and 7 min at or above 39.50°C. During the controlled laboratory load carriage study, one soldier exceeded a TGI of 39.50°C for ~3 min in WBGT 20°C. Conclusions The most thermally demanding section of the RFT(S) is the best-effort LC2. When environmental temperatures are cool to temperate, the RFT(S) does not pose a thermal risk to serving personnel and can be safely conducted up to a WBGT of 20°C, as per current policy.