Human mobility assist technologies currently rely on complex actuation system or on passive elastic elements to restitute mobility to individuals with motion impairments. These mobile technologies require actuators with a very high power-to-weight ratio such not to hinder their efficiency. One such actuator is the pneumatic artificial muscle (PAM), which consists of a soft elastic bladder, filled with air surrounded by a wire meshing. Whereas PAMs are generally used and modeled as active actuators, their use and model development in passive applications have been limited. The development and validation of a stiffness model would bridge the gap towards applying PAMs as passive components on assistive technologies. This paper presents the development of a stiffness model taking into account internal muscle pressure, muscle geometric parameters and friction in the muscle wall.