Continuous rise in oxygen consumption during prolonged military loaded march in the heat with and without fluid replacement: a pilot study

Introduction V˙O 2 drift, the rise in oxygen consumption during continuous exercise, has not been adequately reported during prolonged military marches. The purpose of this study was to analyse V˙O 2 and energy expenditure (EE) during a loaded march with and without rehydration efforts. Second, the study aimed to compare EE throughout the march with predicted values using a validated model. Methods Seven healthy men (23±2 years; V˙O 2max: 50.8±5.3 mL/kg/min) completed four 60 min loaded marches (20.4 kg at 50% V˙O 2max) in a warm environment (30°C and 50% relative humidity). Three were preceded by hypohydration via a 4-hour cold water immersion (18°C). The control (CON) visit was a non-immersed euhydrated march. After water immersion, subjects were rehydrated with 0% (NO), 50% (HALF) or 100% (FULL) of total body mass lost. During exercise, V˙O 2 and EE were collected and core temperature change was calculated. To determine if EE could be accurately predicted, values were compared with a calculated estimate using the US Army Load Carry Decision Aid (LCDA). Results At the start of exercise, there was no difference between conditions in V˙O 2 (ALL: 24.3±0.3 mL/kg/min; p=0.50) or EE (ALL: 8.6±1.0 W/kg; p=0.68). V˙O 2 (p=0.02) and EE (p<0.01) increased during exercise and were 12.3±10.0% and 12.8±9.5% greater, respectively, at 60 min across all trials and were not mitigated by rehydration amount. There was an effect of core temperature change on V˙O 2 for each condition (CON: r=0.62; NO: r=0.47; HALF: r=0.70; FULL: r=0.55). LCDA-predicted values were different from measured EE during exercise. Conclusion V˙O 2 drift occurred during loaded military marches and was associated with increases in EE and core temperature change. Pre-exercise hypohydration with water immersion followed by rehydration did not influence the degree of drift. LCDA prediction of EE may not agree with measured values during prolonged loaded marches where V˙O 2 drift occurs.