Are you ready to jump? Predictive mechanisms in interpersonal coordination.

When two or more individuals intend to achieve a joint outcome, they often need to time their own actions carefully with respect to those of their coactors. Online perceptual feedback supports coordination by allowing coactors to entrain with and predict each other's actions. However, joint actions are still possible when no or little online feedback is available. The current study investigated the interplay between higher-level planning processes and motor simulation in a joint action task where online feedback was not available. Pairs of participants performed forward jumps (hops) next to each other with the instruction to land at the same time. They could neither see nor hear each other, but were informed about their own and the partner's jumping distance beforehand. The analysis of basic movement parameters showed that participants adjusted the temporal and spatial properties of the movement preparation and execution phase of their jumps to the specific difference in distance between themselves and their partner. However, this adaptation was made exclusively by the person with the shorter distance to jump, indicating a distribution of coactors' efforts based on task characteristics. A comparison with an individual bipedal coordination condition suggests that joint coordination might rely on similar principles as interlimb coordination. These findings are interpreted within a framework of motor simulation.