Understanding and predicting the behavior of the powered wheelchair users play an important role in developing the driver assistance systems for the intelligent wheelchair. This article presents a model of wheelchair users, from the point of view of control engineering, based on the interaction between the human hand-joystick and the lumped-parameter model of the human hand muscle system. The interaction between hand and joystick is represented by a robot arm with a four-bar closed-chain mechanism. This mechanism is operated by four musculotendon units based on Hill's muscle model. This configuration allows simulating whole motions of the joystick as well as the evolutions of muscle activities during maneuvering the powered wheelchair. The advantage of this model is the integration of the biomechanical parameters of the muscular system of the human hand, which is very useful for simulating the defects related to the degree of physical impairment of wheelchair users. To validate the proposed model, we compare the simulation outputs of the user model with the previously published experimental results in the framework of the Wheelchair Skills Test (v. 4.1). The simulation results show that the proposed model can reflect the difficulties at the biomechanical level of users in driving the wheelchair.