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A means to communicate by touch is established when two humans grasp a common rigid object, and such communication is thought to play a role in the superior performance two humans acting together are able to demonstrate over either agent acting alone. But the superior performance demonstrated by dyads, whether in making point-to-point movements or tracking unpredictable targets, is strictly empirical to date. Mechanistic accounts for the performance improvement and explanations relying on haptic communication have been lacking. In this paper we develop a model of haptic communication across a linkage connecting two agents that provides an explicit means for the dyad to achieve a higher loop gain than either agent acting alone and higher than the two agents acting together without haptic feedback. We show that haptic communication closes an additional feedback loop through the linkage and the sensorimotor control systems of both agents. This feedback loop contributes a new factor to the loop gain and thus a definitive mechanism for the dyad to improve performance. Our model predicts higher internal forces with haptic communication, which have previously been observed. Additional testable hypotheses emerge from the model and create a promising future means to transfer human-human dyad behaviors to human-robot teams. 

Haptic feedback provided in the axis of a motor task cannot be removed without changing the motor task itself. Haptic feedback couples the biomechanics of the backdrivable body to the dynamics of the environment and establishes a conduit for both power and information exchanges. To isolate the roles of haptic feedback in information exchange and power exchange, we devised a task without haptic feedback that preserved the motor challenge of controlling the coupled dynamics. We placed an identified model of a participant's biomechanics in the virtual environment and coupled it to the original task dynamics. Visual feedback was provided to substitute for the missing haptic feedback. We compared the performance of N=5 participants in the same motor task with and without haptic feedback and in the new task without haptic feedback. The presence of the coupled dynamics in the task predicted the match across conditions rather than the feedback modality. Our results provide support to the idea that rather than controlling their environment, humans control the coupled dynamics of their body and environment.