Musculoskeletal model predictions sensitivity to upper body mass scaling during gait

Musculoskeletal modeling based on inverse dynamics provides a cost-effective non-invasive means for calculating intersegmental joint reaction forces and moments, solely relying on kinematic data, easily obtained from smart wearables. On the other hand, the accuracy and precision of such models strongly hinge upon the selected scaling methodology tailored to subject-specific data. This study investigates the impact of upper body mass distribution on internal and external kinetics computed using a comprehensive musculoskeletal model during level walking in both normal weight and obese individuals. Human motion data was collected using seventeen body worn inertial measuring units for nineteen (19) healthy subjects. The results indicate that variations in segmental masses and centers of mass, resulting from diverse mass scaling techniques, significantly affect ground reaction force estimations in obese subjects, particularly in the vertical component, with a root mean square error (RMSE) of 54.7 ± 23.8 %BW; followed by 12.3 ± 8.0 %BW (medio-lateral); and 6.2 ± 3.2 %BW (antero-posterior). The vertical component of hip, knee, and ankle joint reaction forces also exhibit sensitivity to personalized mass distribution variations. Importantly, the degree of deviation in model predictions increases with body mass index. Statistical analysis using single sample Wilcoxon-Signed Rank test for non-normal data and t-test for normal data, revealed significant differences (p < 0.05) in the computed errors in kinetic parameters between the two scaling approaches. The body shape-based scaling approach significantly impacts musculoskeletal modeling in clinical applications where the upper body mass distribution is crucial, such as in spinal deformities, obesity, and low back pain. This approach accounts for the body shape inherent variability within the same BMI category and enhances the predicted joint kinetics.