Implementing Active Properties of Musculature to Obtain Posture Maintaining Human Body Models for Crash Simulations

BACKGROUND: Today, human body models (HBM) are recognized as important tools within traffic safety research at universities and in the automotive industry. In pre-crash simulations, the occupant’s muscle tonus, bracing, and muscle reflexes have a large impact on the occupant kinematics. Therefore, it is necessary that HBM have biofidelic representations of both the active and passive properties of the musculature to perform reliable simulations of the combined pre-crash and crash phases. Several HBM with active muscles for impact simulations have been developed, however in these models the muscle activation levels are given as a function of time for each load case. Recently, researchers have developed multi body models where the muscle activation levels are governed by closed loop control. However, this has not been done for the finite element method, which is preferable in traffic safety and crash simulations.AIM: The aim of this project is to implement closed loop control of active human musculature in an FE environment suitable for impact simulations to enable posture maintaining and bracing HBM.METHOD: The object version of the LS-DYNA FE code was used for this study. As a first step, a muscle model for the upper extremity was developed with single beam element musculotendon units with non-linear contractile and parallel elastic components. The muscle element was used as an actuator in a closed loop system where the output force was controlled by changing the muscle activation level. A PID-controller for the muscle elements was implemented with a subroutine for user control functions, written in FORTRAN. The upper extremity of the THUMS v3.0 HBM provided the skeletal geometry. Then, the developed technique was applied to the full trunk, back, abdominal, and neck musculature of a sitting HBM, the THUMS v3.0. RESULTS: The developed musculoskeletal FE model of the upper extremity was numerically robust and could simulate simple arm movements in flexion and extension. The model could maintain its original posture when gravity was applied. The material properties of the upper extremity musculature were successfully tuned towards experimental data. The implemented PID-controller was sucessfully tuned to counteract other external disturbances such as impact like perturbations and to simulate bracing. Validation was performed using isometric data from the literature, dynamic data from volunteer experiments using continous perturbations, and new experiments with brief impact like loading. Applying the devloped method with user defined subroutines to control muscle activation in the material models to a fullbody HBM to maintain a sitting posture under gravity loading illustrates that this is a very promising technique for active HBM in FE impact simulations.CONCLUSION: Closed loop control of material models was implemented in the FE code LS-DYNA and applied to muscle activation. The developed method was successfully applied to a model of the upper extremity. A full body HBM was posture maintaing under gravity loading using PID control of the activation level in the muscls material models. The technique is now ready to be applied to have posture maintaing HBM in combined pre-crash and crash FE simulations