While the number of people who need rehabilitation has been increasing because of the aging population, there are only a limited number of physical therapists engaged in rehabilitation, making it difficult to perform rehabilitation at a sufficient level. In this situation, various devices have been developed to replace physical therapists. However, no rehabilitation devices that can respond to the complicated degrees of freedom of an ankle joint complex (AJC) are commercially available. In the present study, we developed an AJC rehabilitation device using a Stewart platform parallel link mechanism. Using the device, we aim to measure and control the AJC with six degrees of freedom so that complicated composite motions of the AJC can be realized. To evaluate the device’s usefulness, we investigated how the composite motion generated by moving the AJC along the trajectory the device reproduced could influence a crural muscle. Muscular activities of the anterior tibial, soleus, and gastrocnemius muscles, generated by a composite motion of plantar flexion and inversion, had a similar feature to those generated by plantar flexion. However, the muscular activity of the peroneus longus muscle generated in the composite motion was significantly different from that generated only in plantar flexion. In the composite motion of plantar flexion and inversion, based on the knowledge that activity to develop only back muscles while suppressing muscular activities of the anterior tibial and peroneus longus muscles is possible. Based on the knowledge, the device was used to perform isokinetic contraction for evaluating the device’s usefulness for muscular training. We found a difference between the combination of active muscles during the composite motion and that during plantar flexion. A load can be applied to different muscles depending on the composite motion, which indicates that the device can be suitable for rehabilitation or training with high degrees of freedom.

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