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 regul ation during a bimanual lifting task with two hand-held objects. Various co nditions were included during which the weight of one or both objects was c hanged in an unpredictable order every fourth trial. Results showed that fo rce control of heavy weight movements preceded by light weight movements wa s not strongly influenced across trials. Conversely, force responses of lig ht weight movements preceded by heavy weight movements were overestimated d ue to an augmented degree of grip force. However, successful updating of fo rce output occurred after one trial. Furthermore, bimanual interactions bet ween the grasping forces were observed, suggestive of a coordinative comman d that assimilated the individual response specifications. The latter also became apparent from a similar grip-load force ratio for both hands when th e objects' physical properties had become predictable, independent of the f orces that were produced according to the individual weight requirements. T hese data indicate that the grip-load force ratio is the controlled variabl e for bimanual manipulative behaviour.