Hill-type models are frequently used in biomechanical simulations. They are attractive for their low computational cost and close relation to commonly measured musculotendon parameters. Still, more attention is needed to improve the activation dynamics of the model specifically because of the nonlinearity observed in the EMG-Force relation. Moreover, one of the important and practical questions regarding the assessment of the model's performance is how adequately can the model simulate any fundamental type of human movement without modifying model parameters for different tasks? This paper tries to answer this question by proposing a simple physiologically based activation dynamics model. The model describes the ?kinetics of the calcium dynamics while activating and deactivating the muscle contraction process. Hence, it allowed simulating the recently discovered role of store-operated calcium entry (SOCE) channels as immediate counter-flux to calcium loss across the tubular system during excitation-contraction coupling. By comparing the ability to fit experimental data without readjusting the parameters, the proposed model has proven to have more steady performance than phenomenologically based models through different submaximal isometric contraction levels. This model indicates that more physiological insights is key for improving Hill-type model performance.

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.