Patients suffering from affections of the lower limb bearing joints account for a significant number of the total of post-traumatic conditions. It is the task of rehabilitation medicine to expedite the recovery of such patients, one of the recovery techniques being Continuous Passive Motion (CPM). Applying recovery-aimed motions to the lower limb joints – known as passive motions - requires the development of new high-performance equipment. The paper presents and discusses the kinematics, construction and actuation of a novel rehabilitation device actuated by a pneumatic muscle developed for the recovery of the ankle by continuous passive motion. Due to the used energy source - compressed air - the end-of-stroke shocks are completely absorbed and user discomfort is thus minimized.