Predictive and reactive co-ordination of grip and load forces in bimanual lifting in man.

We investigated the intra- and inter-manual coordination of grip force (GF) and load force (LF) during bimanual lifting and holding of a single object. In a voluntary task involving lifting a predictable load (Experiment 1), we showed scaling of GF to LF generated by either hand, similar to effects seen in previous unimanual studies. Moreover, the GF rates generated by the two hands were correlated. In part this correlation was due to the correlation between the LF rates. However, the GF rates remained correlated when the effects of the correlation in LF rates were partialled out. This novel finding suggests an additional co-ordinative constraint at the level of specification of GFs. As a contrast to the predictable loading in the first experiment, in the second experiment loading was temporally unpredictable and elicited reactive increases in GF. In Experiment 2, the intermanual correlation of GF rates was stronger than in Experiment 1. We speculate that this result reflects greater degrees of co-ordinative constraint at lower levels in the motor control hierarchy.