| Summary |
The load military service members carry while marching has increased throughout history, and the military routinely uses rucksacks that range in weight from 22-68 kilograms, depending on the type of mission and weather conditions (Van Dijk, 2009). Previous studies have examined the energy cost of load carriage under relatively short time frames, such as 30-45 minutes, but typical marching time greatly exceeds these limits during military operations. Few studies have examined marching times upwards of two hours with a fixed load during flat, incline, and decline conditions with trained military personnel. PURPOSE: The purpose of this study was to measure the energy cost of prolonged marching under heavy load. METHODS: Eleven members of East Carolina University's ROTC completed four days of testing, including a control day when they walked without load and three load carriage days that were randomized to a flat stage, -4% decline, and 5% incline. Load mass was 30 kg, and subjects attempted to walk for two hours under each treatment condition. Metabolic data was collected during the last five minutes of every twenty minute stage. RESULTS: Of the nine participants who completed four days of testing, only two were physically able to complete all of the two hour marches. In statistical analysis, Kaplan-Meier survival curves were used to measure load carriage fail time between groups (those who finished and those who did not) and by testing condition. Significance was found for VO₂ between groups of those who completed versus those who did not complete the two hour tests at p=0.0098. VCO₂ was found to be significant between groups at p=0.0084. Significance by test was found to be significant for VO₂ during the flat phase. VE and RER were not significant. Based on thresholds, the incline proved to be the most physically challenging test, followed by the flat then the decline phases. Threshold cutoffs were clinically determined to observe if the load carriage indicators (VO₂, VCO₂, VE, and RER) were associated with load carriage fail time. Those who reached the incline, decline, and flat thresholds in the first 60 minutes were more likely to fail the two hour time endpoint. The trajectory of the data was relatively stable throughout and did not exponentially increase as was hypothesized during the second hour. While there was some variation, it appeared that the metabolic measures at which participants started each test did not vary tremendously over time, suggesting that if participants started around the threshold cutoffs, they were more likely to fatigue quicker. CONCLUSION: Overall, these findings suggest that the measured variables do not increase over ruck marches lasting two hours in duration. Initial metabolic values may be indicative of how long an individual may be able to exercise beyond 60 minutes until fatigue. This information can help military commanders identify soldiers at high risk of fatigue and monitor their load, pace, and fueling strategies to limit loss of manpower and injury. |
