Mechanical and muscular determinants of metabolic cost during hand carriage of load

Background: Dismounted Soldiers routinely carry substantial loads over long distances at a fixed cadence. While much of the load is carried on the trunk, Soldiers typically carry a weapon in their hands and will likely bear additional loads on their arms in the future (e.g. communication devices, body armor). The physiologic demand of carrying load on the extremities is considerably higher than carrying the same load close to the body, but the changes in gait mechanics and muscle activity associated with this increase are not well understood. Purpose: The purpose of this study was to develop a better understanding of the changes in gait mechanics and muscle activity related to the disproportionate increase in metabolic cost associated with carrying load in the hands. This was accomplished by 1) quantifying the effects of hand carriage of load on mechanics, energetics and muscle activity, 2) evaluating the relationships between changes in mechanical and muscular variables and increases in metabolic cost during walking, and 3) investigating the effects of changes in stride frequency on gait mechanics, energetics and muscle activity during hand carriage of load. Methods: Motion capture, oxygen consumption and muscle activity data were obtained from 30 subjects walking on a treadmill at 3 mph under various combinations of load magnitude, load location and arm swing at a specific stride frequency. Spatiotemporal parameters, kinematics, coordination between trunk and pelvis rotation, metabolic rate, and muscle activity levels were evaluated for each load carriage condition. Results: Hand carrying load in front of the body using both hands is associated with significant changes in gait mechanics, muscle activity and metabolic cost. Changes in metabolic cost during walking with small trunk-borne or hand carried loads appear to primarily depend on transverse plane trunk and pelvis kinematics and anterioposterior position of the center of mass. Individuals are able to maintain a stable gait pattern with changes in stride frequency, even when hand carrying a load in front of the trunk. Conclusion: The significant biomechanical and physiologic changes associated with hand carrying a load in front of the trunk may lead to systemic and local muscle fatigue with negative consequences for Soldier task performance. Changes in gait mechanics and muscle activity associated with over striding may put individuals of shorter stature at an increased risk of fatigue. The results of these studies can aid mission planners in making better estimations of Soldier performance under different load carriage conditions and system developers in optimally designing manually carried Soldier equipment. Future studies are needed to quantify the fatigue due to biomechanical and physiologic changes associated with various modes of load carriage and evaluate its impact on Soldier performance