Recent developments strive for realizing robotic systems that not only interact, but closely collaborate with humans in performing everyday manipulation tasks. Successful collaboration requires the integration of the individual partner's intentions into a shared action plan, which may involve continuous negotiation of intentions. We focus on collaboration in a kinesthetic task, i.e., joint object manipulation. Here, ways must be found to integrate individual motion and force inputs from the members of the human-robot team, in order to achieve the joint task goal. Before guidelines on how robots should act in this process can be formulated, clarification on whether humans use the haptic channel for communicating their intentions is needed. This paper investigates this question in an experimental setup involving two collaborating humans. We consider physical effort as well as performance as indicators of successful intention integration. Our results strongly suggest that intention integration is enhanced via the haptic channel, i.e., that haptic communication takes place, especially in the case of shared decision situations. This provides a motivation for future investigations to model the process of intention integration itself in order to realize successful haptic human-robot collaboration.