In human interaction and communication, not only verbal messages but also nonverbal behavior such as nodding and paralanguage are rhythmically related and mutually synchronized among speakers. This synchrony of embodied rhythms unconsciously enhances a sense of unification and causes an interaction-activated communication in which nonverbal behaviors such as body movements and speech activity increase, and the embodied interaction is activated. In this paper, we propose the concept of an estimation model of interaction-activated communication based on the heat conduction equation with the characteristics of precipitous speed fluctuation and develop a model that estimates the degree of interaction-activated communication by using speech input only. Further, we evaluate the developed model in estimating the period of the interaction-activated communication in an avatar-mediated communication. The results demonstrate that the developed model is effective in estimating the interaction-activated communication.