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During transport of an object using the precision grip with thumb and index finger, a modulation of the grip force is needed in response to the forces evoked by the movement. We measured the grip force (GF) and the load force (LF) in 10 healthy participants moving a 640-g object forward and upward. The task was repeated with various speeds. There were considerable changes with speed of the LF trajectory but not of the GF trajectory. A loss of synergy between GF and LF appeared in fast lifts. This is in contrast to the close coupling between load force and grip force repeatedly demonstrated during simple lifts. We suggest that (a) speed should be considered as an input parameter for movement planning, and (b) regulation of GF and of LF are independent under certain conditions. We discuss whether the grip-load force synergy should be considered a special case rather than a more general principle.
B. Hejduková, N. Hosseini, B. Johnels. P.E. Ingvarsson, and G. Steg are with the Department of Neurology, University of Goteborg, SE41345 Gothenburg, Sweden. T. Olsson and Hosseini are with the Department of Signals and Systems at me Chalmers University of Technology, SE 41296. Goteborg. Sweden. Additionally, Hejduková is with the Department of Neurology in the First Faculty of Medicine at Charles University. Prague, Czech Republic.