Cardiovascular and thermal strain during 3-4 days of a metabolically demanding cold-weather military operation

Background: Cardiovascular (CV) and thermal responses to metabolically demanding multi-day military operations in extreme cold-weather environments are not well described. Characterization of these operations will provide greater insights into possible performance capabilities and cold injury risk.

Methods: Soldiers from two cold-weather field training exercises (FTX) were studied during 3-day (study 1, n = 18, age: 20 ± 1 year, height: 182 ± 7 cm, mass: 82 ± 9 kg) and 4-day (study 2, n = 10, age: 20 ± 1 year, height: 182 ± 6 cm, mass: 80.7 ± 8.3 kg) ski marches in the Arctic. Ambient temperature ranged from -18 to -4 °C during both studies. Total daily energy expenditure (TDEE, from doubly labeled water), heart rate (HR), deep body (T_pill), and torso (T_torso) skin temperature (obtained in studies 1 and 2) as well as finger (T_fing), toe (T_toe), wrist, and calf temperatures (study 2) were measured.

Results: TDEE was 6821 ± 578 kcal day^-1 and 6394 ± 544 for study 1 and study 2, respectively. Mean HR ranged from 120 to 140 bpm and mean T_pill ranged between 37.5 and 38.0 °C during skiing in both studies. At rest, mean T_pill ranged from 36.0 to 36.5 °C, (lowest value recorded was 35.5 °C). Mean T_fing ranged from 32 to 35 °C during exercise and dropped to 15 °C during rest, with some T_fing values as low as 6-10 °C. T_toe was above 30 °C during skiing but dropped to 15-20 °C during rest.

Conclusions: Daily energy expenditures were among the highest observed for a military training exercise, with moderate exercise intensity levels (~65% age-predicted maximal HR) observed. The short-term cold-weather training did not elicit high CV and T_pill strain. T_fing and T_toe were also well maintained while skiing, but decreased to values associated with thermal discomfort at rest.