In learning/training activity of physical tasks such as sport, aerobic exercise, rehabilitation, or dance, it usually involves an instructor and one or more learners. Since it is hard for the instructor to propagate his(her) knowledge to the learners merely by oral instructions, the instructor usually has to demonstrate exemplary movements, figures out the movements which are error-prone to the learner. The learner observes the instructor’s movements, mimics them. In the whole activity, each of them observes the interaction partner’s body movements, talks to the partner and may intercept the partner’s action.