Development of a Dynamic Biomechanical Model for Load Carriage: Phase III Part B: Characterization of Load Control During a Human Trials Circuit

Abstract : This work was undertaken in support of the research thrust "Development of a Dynamic Biomechanical Load Carriage Model". The report describes human response characteristics, both objective and subjective, to a mobility circuit. The goals of the study were to examine the relationships among aerobic demand, performance time and load as well as the relationships between posture, shoulder and lumbar reaction forces and load. Eleven subjects volunteered for three aspects of the study: a maximal aerobic capacity test and two test days for each load carriage system, Pack C and Pack D. During the testing, subjects were instrumented with a heart rate monitor, accelerometers on the person and in the pack, and strap force sensors on the lower shoulder strap and on the waist belt. The circuit was completed five times in Battle Order of 5.5 kg, followed by randomized rucksack loads of 15.7 kg, 25.5 kg and 34.3 kg, as well as a self-selected maximal load. Subjects were asked to assume a pace they could sustain all day, given the load they were asked to carry. The circuit consisted of a number of marching and mobility tasks and was timed for all components of the circuit. Statistics were done using two- and three-way repeated measures ANOVAs. There were significant differences in timed performance and load (p=0.001) and between aerobic demand and load (p=0.008) but not between packs. This relationship held true for both laps and tasks. This resulted in a decrease in performance time of 19% and an increase in aerobic demand of 21% for the heaviest load compared to Battle Order. There was a significant increase in upper body and head lean angles and shoulder and lumbar reaction forces often between packs and always between loads. Posture changed by as much as 40% of the light weight's postural lean angles with shoulder and hip reaction forces being 120% to 150% greater than the light load with the lumbar forces exceeding recommended limits.

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