Personalization of military load carriage simulations affects muscle and joint contact forces

Military overuse injuries are associated with greater internal loads during load carriage and cause decreased readiness and increased costs. Low physical fitness, including low muscle strength, is a modifiable injury risk factor, making it an important focus for injury prevention. Evaluating potential injury mechanisms from biomechanical modeling and simulation may be useful, particularly for individuals who have unique muscle strengths and body segment mass distributions. However, modeled muscle strength from average adults is likely underestimated for trained, active-duty military personnel, which may influence internal load calculations. Thus, we evaluated the effects of muscle strength and segment mass distribution personalization on joint contact impulses and peak forces, and muscle force impulses from musculoskeletal simulations. Full-body kinematics, ground reaction forces, and electromyography were collected from 16 active-duty participants under two walking conditions: 1) no-pack and 2) pack (posteriorly added load, total 46 kg). These data drove and validated simulations of models with different types of personalization. Modeled segment masses were scaled using measured regional masses from dual-energy x-ray absorptiometry. Modeled muscle strengths were scaled from eight maximum voluntary isometric contractions measured using an instrumented dynamometer including the lumbar, hip, knee, and ankle. Personalized mass-distribution scaling had 6% greater lumbar joint contact impulses in walking compared to not personalized (p = .006). Personalized muscle strength scaling had greater joint contact impulses at the lumbar (42%), hip (25%), and knee (8%) (p < .001), and smaller impulses at the ankle (-4%) (p = .048) compared to not personalized. Results were similar for peak joint contact forces. Model personalization is beneficial to quantify internal loading and evaluate training interventions.