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Musculoskeletal injuries of the lumbar spine occur frequently among military service members and are associated with heavy backpack loads. Musculoskeletal modeling and simulation facilitate biomechanical evaluation to compare different backpack designs. We developed a backpack attachment model that can be tuned to represent various load distributions between the torso and pelvis. We generated walking simulations to estimate muscle and joint contact forces of unloaded walking and while carrying 38 kg using shoulder-borne backpacks and hip belt-assisted backpacks for six U.S. Marines. Three-dimensional peak and average lumbar (L4-L5) and hip joint contact forces over the stance phase were compared between each load condition. Axial L4-L5 and axial and anterior hip joint contact forces were greater during both backpack conditions compared to the unloaded condition. Joint contact forces were similar between backpack conditions. Future studies incorporating additional participants, walking conditions, and backpack load distributions are suggested for further model development and backpack design evaluation. [ABSTRACT FROM AUTHOR]