Mechanisms of musculoskeletal injury risk during military load carriage

Abstract

Walking slope and carrying heavy loads are physically demanding tasks that each require different postures, joint mechanics (kinematics, moments, and powers), and muscle activity from level, unloaded walking. In addition, hiking with heavy military loads, which often occurs on varied terrain, is linked to higher incidence of musculoskeletal injury. A large share of these overuse injuries is to the lumbar spine; however, how the combined effects of walking slopes and backpack loads change posture, joint mechanics, and muscle activity, and thus contribute to injury, remains unclear. Further, while backpacks often incorporate a hip-belt to relieve pressure on the shoulders, the extent that hip-belt load sharing alters torso muscle activity and lumbar forces is unknown. This body of Ph.D. work determined biomechanical responses to the combined effects of -10° (Down), 0° (Level), and +10° (Up) walking slopes and three different backpack load conditions: 1) unloaded (No-Pack), 2) hip-belt assisted (Hip-Belt), and 3) shoulder-borne only (Shoulder). Torso muscular demand was quantified using electromyography signals. Lumbar joint contact forces and hip muscle work were quantified using musculoskeletal modeling and simulation approaches. Walking slope influenced all outcome metrics, but changes were not always progressive from Down to Up. Muscle activity from rectus abdominus and iliocostalis, and peak compressive lumbar joint contact forces were greater during Down and Up compared with Level; however, external oblique activity decreased and compressive lumbar impulses increased from Down to Up. In addition, concentric and eccentric hip muscle work generally increased from Down to Up, except for rectus femoris and semimembranosus. Hip-Belt and Shoulder backpacks both caused less muscle activity from longissimus, but rectus abdominus activity during Shoulder was greater than both No-Pack and Hip-Belt. Both backpacks generally caused greater lumbar joint contact forces, however, the compressive lumbar impulse during Down was greater in Shoulder compared with Hip-Belt. Muscle work was often greater with both Hip-Belt and Shoulder compared with No-Pack, but the magnitude of differences in several muscle work metrics depended on walking slope and were greatest during Up. In addition, eccentric muscle work from psoas was greater during Hip-Belt compared with Shoulder. These studies collectively provide insight into how body-borne load carriage systems, like backpacks, should be evaluated for reducing overuse injuries. Uphill walking with either backpack caused the greatest risk of injury relative to other walking conditions; however, there were a few key differences between Hip-Belt and Shoulder that suggest benefit from proper use of a hip-belt attachment and illustrate the importance of considering hip joint demands in future equipment designs.