Exploring magnetic resonance imaging validation of length-based scaling of musculoskeletal models using OpenSim and AddBiomechanics for walking

Scaling is an important step for achieving accurate participant-specific models when studying human motion. Scaling a generic musculoskeletal model using OpenSim is time consuming, depends on the user expertise, and requires a static pose. Recently, AddBiomechanics introduced automatic scaling of the participant-specific models independent of user expertise and static pose. However, its validation is limited to synthetic data. In this exploratory study, we compared models scaled viaAddBiomechanics and OpenSim against models scaled based on magnetic resonance images (MRI). We performed an optical motion capture experiment in which we recorded walking at 0.8 m/s, 1.2 m/s, and 1.6 m/s, followed by an MRI scan of the lower extremities for 10 participants (M/F). We scaled the model using OpenSim, AddBiomechanics, and using the MRI data. In OpenSim, it was necessary for some participants to lock certain joint coordinates before scaling to avoid unrealistic postures. To evaluate the different models, we performed inverse kinematics for the three different walking trials of each participant and analyzed the joint angle trajectories and overall average root mean square error (RMSE) between the measured markers and virtual markers of the models scaled with OpenSim, AddBiomechanics and MRI. For those participants where we did not lock coordinates in the OpenSim model, the average marker RMSE was 1.657 cm for the OpenSim model, compared to 1.585 cm with AddBiomechanics and 1.471 cm for the MRI-based model. For the participants where we locked coordinates, the RMSE was 1.588 cm for the OpenSim model, compared to 1.725 cm with AddBiomechanics and 1.439 cm for the MRI-based model. The joint angles were similar, with the largest difference for the models with locked coordinates, where the maximum difference was 9.2° (ankle angle). Our exploratory study suggests that AddBiomechanics offers a practical alternative to OpenSim, showing comparable accuracy with no meaningful differences, while requiring less time and user effort.