Bimanual organization of manipulative forces: evidence from erroneous feedforward programming of precision grip.

The objective of the present study was to investigate grip-load force regulation during a bimanual lifting task with two hand-held objects. Various conditions were included during which the weight of one or both objects was changed in an unpredictable order every fourth trial. Results showed that force control of heavy weight movements preceded by light weight movements was not strongly influenced across trials. Conversely, force responses of light weight movements preceded by heavy weight movements were overestimated due to an augmented degree of grip force. However, successful updating of force output occurred after one trial. Furthermore, bimanual interactions between the grasping forces were observed, suggestive of a coordinative command that assimilated the individual response specifications. The latter also became apparent from a similar grip-load force ratio for both hands when the objects' physical properties had become predictable, independent of the forces that were produced according to the individual weight requirements. These data indicate that the grip-load force ratio is the controlled variable for bimanual manipulative behaviour.