This paper develops a Hill model based technique to estimate human elbow torque from sEMG measurements. Some new parameters are included in the optimization process in order to improve the resulting estimated torque. These parameters correspond to activation levels of muscles involved in motion generation. They have not previously been used in other works dealing with this kind of model. Results from experiments with several subjects in different movement conditions and using the new optimized parameters lead to some conclusions about the generality of the optimized models and the influence of the new parameters on the improvement of the estimation.