Musculoskeletal injury of the lumbar spine and lower extremity is prevalent among military service members, and results in more lost duty days than any other medical condition. Most of these musculoskeletal conditions, such as muscle strains, stress fractures, and joint pain and degradation are attributed to overuse. A key contributor to this overuse is the heavy load service members routinely carry during training and deployment, which is often in excess of recommended maximum weights. Walking with heavy backpack loads causes postural changes and increases the mechanical demand on the musculoskeletal system. In order to alleviate the effects backpack loads on the spine, backpacks are often designed with hip belts in order to redistribute some of the total load from the shoulders and the pelvis. However, it is unknown to what extent the internal forces related to injury risk in the lumbar spine, such as muscle and joint contact forces, are affected by these mitigation strategies.

Therefore, the purpose of this study was to develop a musculoskeletal model incorporating backpack attachment to the torso and pelvis in order to analyze lumbar spine and lower extremity injury risk. Joint contact forces in the lumbar spine and hip were quantified while walking using (1) a shoulder-borne only and (2) a hip-belt assisted backpack design. In addition, robustness of the model was assessed with a probabilistic sensitivity study to investigate the uncertainty in joint contact force estimates due to assumed uncertainty in model parameter values. The results from this work provide novel information regarding injury risk to the lumbar spine related to load carriage. Lumbar spine and hip joint contact forces are greater when walking with backpack loads compared to without. However, implementation of a hip-belt to distribute half of the load from the shoulders to the pelvis does not influence lumbar spine or hip joint contact forces. In addition, backpack attachment parameter values did not substantially effect joint contact force estimates. These results indicate that other factors such as, the total load carried and walking speed, have greater influence on joint contact forces than backpack design. The load carriage model developed will be useful for future analysis of various backpack designs during additional conditions such as sloped walking or running.