Predictability influences finger force control when catching a free-falling object.

We studied grip force control when catching a free-falling object with the dominant hand. An instrumented object was dropped either from the subjects' opposite hand or expectedly from the experimenter's hand. Following digit-object contact, triggered responses were observed in the load and grip force profiles. The peak rates of load force increase and the peak load forces produced at the time the catching fingers made contact with the object were of similar magnitude for the experimenter- and self-release conditions. However, the peak rates of grip force development and the peak grip forces were more pronounced when the object was dropped by the experimenter. These findings suggest that the prediction of the load magnitude was less precise when the object was dropped from the experimenter's hand. In addition, a correlation analysis between maximum grip and load force rates revealed a less precise coupling between the force rates in the experimenter-release condition. The time lags between maximum force rates and maximum forces were longer for the experimenter-release than for the self-release condition. These observations may indicate a less precise temporal coupling between grip and load force profiles in the experimenter-release condition. As observed during other manipulative tasks, the co-ordination between grip and load forces is a prerequisite to cope with collision forces when catching free-falling objects. Grip force control during catching is both highly automatic and flexible depending on the predictability of the task.