This paper presents the design of an admittance-based assistive controller and preliminary experimental results for a high performance parallel robot used for ankle rehabilitation. The goal of this work was to design a suitable control algorithm for diagnosis, training and rehabilitation of the ankle in presence of musculoskeletal injuries. An admittance control technique is used to perform patient-active exercises with and without motion assistance. Electromyographic (EMG) signals are used to evaluate patient's effort during training/exercising. The results indicate the great potential of the rehabilitation device as a tool to fasten and improve the ankle therapies outcome.