This work presents the macroscopic behavior of skeletal muscles seen from a system theoretic standpoint. Then, according to the data available in the literature, we propose a first Evaluation model for the muscle isometric (constant muscle length) force generation depending on the muscle length and neuronal excitation frequency. Such a model is instrumental in providing a more physics-inspired model of such an isometric force by exploiting a nonlinear spring description with controllable characteristics, such as stiffness and rest length. To conclude, we propose a hybrid dynamical filter model to describe the components of the sensory system. This is in charge of feeding back to the Central Nervous System the measurements of muscle length and its rate of change.