Using a new muscle activation model to improve the stability of force estimation with Hill-type models over different activation levels

Hill-type models are attractive in biomechanics simulation due to their low computational cost and commonly measured parameters. They mainly consist of two parts, the activation dynamics, and the contraction dynamics. Since Hill-type models are phenomenological models, it is meant to capture the process's main characteristics using a descriptive system that approximates the behaviour of the real physical one. This paper developed a novel activation dynamics formulation to estimate the force of the triceps surae muscles at two different contraction levels without retuning the model parameters. The new formulation simulates the cell calcium dynamics as well as the recently discovered role of store-operated calcium entry (SOCE) channels. The proposed model reflects the main characteristics required from the activation model, which are the electro-mechanical delay, the process nonlinearity, and the ability to couple the activation process with some parameters from the subsequent force-generating process. The parameters of the model are task-independent to closely approximate the muscle function and to have the ability to simulate different movements adequately. The model shows a more stable performance along with different contraction levels in comparison with two other activation models.