Impact of Upper Body Mass Scaling on Musculoskeletal Model Predictions during Gait

Utilizing musculoskeletal modeling through an inverse dynamics approach for gait assessment offers a non-invasive method to compute internal joint kinetics and ground reaction forces and moments solely from kinematic data, reducing reliance on cumbersome equipment. The effectiveness of these models relies on the scaling approach adopted to tailor the model to individual subject data. While constant percentage-based, also called uniform scaling-based, has traditionally been used, recently developed upper body shape-based mass distribution approach which accounts for inter-subject inherent mass distribution variation within the same body mass index category, has demonstrated sensitivity of muscle forces and joint kinetics during static posture to segmental masses and centers of mass variation. This study investigates the influence of upper body mass distribution on internal and external kinetics computed using a full body musculoskeletal model during level walking in normal-weight healthy individuals. The findings reveal that variations in segmental masses and centers of mass resulting from different mass scaling approaches significantly alters ground reaction force prediction, especially the vertical component, followed by the medio-lateral and antero-posterior components. Joint reaction forces also show sensitivity to variations in personalized mass distribution, particularly the vertical component at the hip, knee, and ankle joints, followed by the medio-lateral and antero-posterior components. These results emphasize the importance of caution when employing subject-specific upper body musculoskeletal models with uniform mass scaling for gait kinetics assessment.