I. Introduction
Myoelectric prostheses [1]–[4] such as MyoBock (Otto Bock Corp.) are provided to help normalize life for upper-limb amputees. To achieve free control of prostheses in daily life, it is necessary to secure voluntary generation of EMG signals and conduct task training using the prosthesis under the instruction of doctors's and therapists's over a period of a few months [5]. In this context, EMG control training systems have been proposed [6]–[9] to support the recovery of muscle function and promote voluntary muscle contraction. For example, Tsuji et al. [6] proposed an EMG-based rehabilitation aid (EBRA) that supports three types of EMG-based training for voluntary muscular contraction, collaboration among multiple muscles and muscular contraction timing. However, these systems focus only on training for voluntary control of EMG signals, and do not support myoelectric prosthesis control training. In the use of prostheses, EMG signals vary with changes in arm position/posture and the generation of force to support the weight of the prosthesis. Accordingly, even if voluntary control of EMG signals can be achieved through training, it can still be difficult for user to perform target tasks using a prosthesis. For this reason, extensive training with a myoelectric unit is necessary before the user can control the prosthesis freely in daily life. However, as myoelectric prostheses are very expensive, medical facilities often neither have them nor can offer long-term training in their operation.