Effects of reserve actuators on optimization solutions: From muscle force to joint stiffness

This work presents a comparison between two optimization methods used to compute the muscle activation levels and corresponding forces that drives a set of generalized coordinates towards a set of desired trajectories. To improve the performance of musculoskeletal optimizations a supplemental set of actuators is often included in addition to the modeled muscles. Given a dynamic musculoskeletal model and five sets of reserve actuators, a series of numerical simulations have been performed using experimental data from a healthy male subject who executes a running movement at three different speeds. This is the first work to investigate the incidence of different reserve actuator sets on muscle activation-to-force optimization solutions, with respect to the estimation of the human lower limb muscle forces and corresponding joint stiffness. The results show significant differences between the obtained estimates, indicating a greater accuracy on the Computed Muscle Control solutions than pure Static Optimization solutions.