The influence of isometrically derived neck muscle spatial tuning patterns on head response in dynamic conditions

The Chalmers AHBM was setup in LS-DYNA to mimic maximum voluntary contraction (MVC) experiments, and to find the optimum cervical muscle activation parameters for a setup using an optimization model in LS-OPT. The setup in LSDYNA incorporated a discrete beam element with a high stiffness, attached to the head of the AHBM, making it resistant to change in length. Pulling against this beam is similar to pushing against a rigid clamp used in the MVC experimental setups with human volunteers. The AHBM head is constrained in 3D space to have small translational and rotational displacements. Hence, it was considered an apt analogy in the numerical simulations of the AHBM, towards obtaining MVC. A linear response surface was used to obtain optimum values for muscle activation parameters in the design space, with sequential domain reduction and tolerance of convergence set to ±1%. The motion of the models was post-processed in LSPrePost. Muscle activation patterns obtained from optimizations for the two models - one with 9 muscle groups and the other with 13 muscle groups in the neck, were compared with the MVC experiment conducted by Siegmund et al. (2007), and later used for simulation of the AHBM with applied dynamic load in 5 different directions, 0, 45, 90, 135 and 180, using a previously implemented 1 DOF PID controller for the AHBM by Osth (2014) as well as a newly developed 3 DOF PID controller, which was based on functions in LS-DYNA. The dynamic spatial tuning patterns for the optimization models were compared to those obtained from the human volunteer sled experiments conducted by Olafsdottir et al. (2015). The muscle activation patterns from the optimizations showed great variation compared to those obtained from the MVC experiments, due to difference in muscle modelling, constraints on the AHBM and the optimization setup. The dynamic spatial tuning patters for the optimization setups with 1 DOF PID controller were scaled in the same directions as the muscles were activated in the MVC optimizations, however, the 3 DOF PID controller behaved differently. Larger translational and rotational head displacements were observed in the optimization models as compared to the experimental model. Models with 3 DOF PID controller resulted in significantly different kinematics as compared to the models with 1 DOF PID controller.