Objective Impedance control plays an important role in stability. This paper intends to explore such mechanism through modeling human reaching movement. Method Implemented with revised model, we apply optimal control theory to neuro-muscle-skeleton model to calculate the stiffness ellipses. Result Compared with the original model and experimental figures, the model we proposed could overcome the shortage of monotonous changing of the original one and fit the data better. Conclusions So that this paper concludes that co-contraction contributes to impedance control even during free upper limb planar movement.