In this project a spasticity model is created in OpenSim. This model is implemented by means of a controller plugin. This so-called Spastic Controller is a controller that adds a spasticity effect to any Thelen2003Muscle to which it is assigned in an OpenSim model. The controller will excite the muscle to which it is assigned when the muscle fibers of this muscle are stretched. The Spastic Controller is developed and validated using experimental data of an instrumented clinical spasticity test.