Physics-Based Guidelines for Accepting Reasonable Dynamic Simulations of Movement

Objective: During dynamic simulations, residuals are nonphysical generalized forces/moments that dynamically balance external and inertial forces/moments, accounting for data processing and modelling errors. Hicks et al. (2015) made the original residual threshold recommendations for an acceptable simulation, but these thresholds are not based on the dynamic, physics-based movement characteristics. In this study, we present three new, physics-based guidelines for accepting dynamic simulations of movement using zero moment point computations and thresholds for forces, center of pressure, and free moment. Methods: We formulate new guidelines and evaluate them alongside the original 2015 recommendations using two movements: single-leg jump-landing (SLJL) and walking gait. We also present a MATLAB function for users to test if their simulations meet these guidelines. Results: We found that on average, only 4.3% (SLJL) and 8.2% (walking gait) of the original 2015 residuals volume met all the new physics-based guidelines. The free-moment guideline was the most restrictive for reasonable simulations, especially for high-velocity movements at times with lower vertical ground reaction forces. Additionally, some of the new recommended residuals volume fell outside of the original 2015 recommendations. Moreover, accepting reasonable simulations using different thresholds leads to different joint torques as high as 24 Nm (SLJL) and 8.2 Nm (walking gait). Conclusion: The physics-based guidelines are overall more restrictive than the original 2015 recommendations and elicit different simulation kinetics. Significance: Using different guidelines may lead to different conclusions and clinical interpretations. We advocate for the physics-based guidelines as they are built upon the dynamic, physics-based characteristics of the movement.