In physical and occupational therapy two people interact through force and motion. Other common examples of this interaction include lifting and moving a bulky object, teaching manual skills, dancing, and handing off a baton or a drinking glass. These tasks involve kinesthetic interaction, a communication channel distinct from spoken language and gestures. Understanding kinesthetic interaction should be important in designing robots to assist with physical and occupational therapy. In this paper we describe our experiments on kinesthetic interaction between two people cooperating on a 1 degree of freedom task. We characterize the interaction forces between the two people, dividing them into a productive "net force" and an orthogonal "difference force." Our results suggest three effects (1) an emergent specialization of the two participants into different roles, (2) an oscillation of forces at about 8 Hz, and (3) a steady force in opposition to one another that could be analogous to co-contraction in an individual.