Biomechanics of Load Carriage--Historical Perspectives and Recent Insights

Loads carried by the warfighter have increased substantially throughout recorded history, with the typical U.S. ground soldier carrying external loads averaging 45 kg during operations in Afghanistan. Incidence of disability in the U.S. Army has also increased sixfold since the 1980s, predominantly driven by increases in musculoskeletal injuries, with load carriage implicated as a possible mechanism. This article will provide a brief overview of the biomechanics of load carriage and will provide some recent insights into how the stress of the loads carried by military personnel can affect the musculoskeletal system. Studies into the biomechanics of load carriage have documented motion-related differences such as increased step rate, decreased stride length, and more trunk lean with increases in pack-borne loads. However, there is a paucity of literature on the relationship between load carriage and biomechanical mechanisms of overuse injury. Findings of recent studies will be presented, which add mechanistic information to increased stresses on the lower extremity. This was particularly true at the knee, where in one study, peak knee extension moment increased 115% when carrying a 55 kg load (0.87 ± 0.16 Nm·kg⁻¹) vs. no external load (0.40 ± 0.13 Nm·kg⁻¹). Efforts to model injury mechanisms require continued biomechanical measurements in humans while carrying occupationally relevant loads to be validated. Specifically, imaging technologies (e.g., bone geometry scans) should be incorporated to produce higher fidelity model of the stresses and strains experienced by the load carrier. In addition to laboratory-based biomechanics, data are needed to further explore the mechanistic relationship between load magnitude and injury; to this end, wearable sensors should continue to be exploited to accurately quantify biomechanical stresses related to load carriage in the field.