Cost of locomotion during loaded marching in the heat
Authors: Tingelstad Hans Christian , Kehoe Brian , Verdon Eric , Seminuk Kevin , Reilly Tara , Haman Francois 
Published: November 2023

Although exercise in the heat and its effect on thermoregulatory responses has been well studied, limited information is available concerning the effect of high environmental temperatures on thermoregulatory responses, cardiovascular strain and cost of locomotion during loaded marching in the heat. Members of the armed forces are wearing protective equipment like bulletproof vests, helmets and tactical vests while performing a loaded march, often in high environmental temperatures, which could potential expose the individual uncompensable heat stress. Therefore, the purpose of this study was to investigate the effect of elevated environmental temperature on core and body temperature, heart rate and cost of locomotion in military service members during a 5 km loaded march. Participants were seven service members of the Canadian Armed Forces Light Infantry (age 24±6 years, height 178±5 cm, weight 78.8±14.7 kg, VO2max 49.2±5.6 ml·min−1·kg−1, lean body mass 66.3±8.3 kg, body fat % 14.9±6.5), who performed a loaded march carrying 35 kg of equipment, under two environmental conditions (NORMAL 21°C and 50 relative humidity (RH), and HOT 30°C and 50% RH). Treadmill speed was set to 5.17km/h and the incline was set to 1% to simulate level ground walking. The 35 kg external load consisted of military boots, uniform, helmet, fragmentation vest, tactical vest, a Colt 7 replica rubber rifle and a loaded day pack. Participants walked on the treadmill for 60 min, or until voluntary termination. Heart rate was measured using a Garmin 310xt heart rate monitor, while energy expenditure was calculated from O2 consumption and CO2 production, using an open circuit flow through metabolic system. Cost of locomotion was calculated as energy expended per kg weight (body weight and external load) per kilometer walked (J·kg−1·km−1). Core temperature was measured using a telemetric pill (Jonah Temperature Capsule). All participants completed the loaded march in the normal condition, however one participant was unable to complete the full hour in the HOT condition due to gastric distress. Heart rate was significantly higher from 25 min until the end of the loaded march when participants performed the loaded march in the HOT condition compared to NORMAL. Heart rate during the last 5 min in the HOT condition was 152±11 bpm, compared to 136.9±13.2 bpm in the NORMAL condition. Core temperature was significantly higher in the HOT condition compared to NORMAL, and was 38.2±0.5°C in the HOT condition compared to 37.7±0.3°C in the NORMAL condition after 60 min of loaded march. Cost of locomotion (J·kg−1·km−1) was also affected by environmental temperature, and was significantly higher in the HOT condition compared to NORMAL (AUC hot 165.5±10.4 vs normal 152. ±8.9). The results from this study shows that performing a loaded march at 30°C and 50% RH wearing military protective equipment lead to uncompensable heat stress, causing an increase in core temperature and heart rate (cardiovascular strain). We also observed that the uncompensable heat stress caused an increase in the cost of locomotion without changing any factors other than environmental temperature.