Introduction: The weight that soldiers are required to carry in training and in combat has continually increased over the years. Changes in load carried or pace of activity will alter the physiological and biomechanical stress associated with the activity. Whether it is part of the soldier's training or an actual operation, managing the proper load and speed to minimize fatigue can be integral to the soldier's success. Without a proper understanding of the multitude of factors that may affect load carriage performance, mission success may be jeopardized. The purpose of this review is to summarize and clarify the findings of load carriage research and to propose a new method for analyzing the intensity of load carriage tasks, the Load-Speed Index.
Materials and methods: We reviewed studies that examined military load carriage at walking speeds and included articles that featured non-military participants as deemed necessary.
Results: Major factors that can affect load carriage performance, such as speed of movement, load carried, load placement, body armor, and environmental extremes all influence the soldier's energy expenditure. A critical aspect of load carriage performance is determining the appropriate combination of speed and load that will maximize efficiency of the activity. At the higher end of walking speeds, the walk-to-run transition represents a potential problem of efficiency, as it may vary on an individual or population basis.
Conclusions: This review provides a comprehensive overview of these factors and suggests a new Load-Speed Index, which can be utilized to define thresholds for load and speed combinations and contribute to the understanding of the physiological and biomechanical demands of load carriage marches. The literature recommends that load and speed should be managed in order to maintain an exercise intensity ~45% VO2 max to delay time to fatigue during prolonged marches, and the Load-Speed Index corroborated this finding, identifying 47% VO2 max as a threshold above which intensity increases at a greater rate with increases in load and speed. The Load-Speed Index requires validation as a predictive tool. There are no definitive findings as to how load affects the speed at which the walk-to-run transition occurs, as no investigations have specifically examined this interaction. Additional research is clearly needed by examining a wide range of loads that will facilitate a clearer understanding of speed and load combinations that optimize marching pace and reduce energy expenditure.