We investigated the quality of predictive grip force control during gravity changes induced by parabolic flight maneuvers. During these maneuvers gravity varied: There were 2 periods of hypergravity, in which terrestrial gravity nearly doubled, and a 20-s period of microgravity, during which a manipulated object was virtually weightless. We determined grip and load forces during vertical point-to-point movements of an instrumented object. Point-to-point movements were a combination of static (stationary holding) and dynamic (continuous movements) task conditions, which were separately analyzed in our previous studies. Analysis of the produced grip forces revealed that grip adjustments were closely linked to load force fluctuations under each gravity condition. In particular, grip force maxima coincided closely in time with load force peaks, although these occurred at different phases of the movement depending on the gravity level. However, quantitative analysis of the ratio of maximum grip force to the corresponding load force peak revealed an increased force ratio during microgravity when compared to that during normal and hypergravity, We hypothesize that the impaired precision of force coupling with respect to force magnitude during microgravity results from reduced feedback information about the object's mass during the stationary holding of the object in between each movement. The results indicate that the temporal grip force regulation is highly automatized and stable, whereas economical planning of force magnitude is more flexible and might reflect changes of the external loading condition.
D.A. Nowak, J. Philipp, and S. Glasauer are with the Department of Neurology at Ludwig-Maximilians University, Klinikum Großhadern. Marchioninistraße 23. D-81377 Munich, Germany. J. Hermsdörfer and C. Marquardt are with the Clinical Neuropsychology Research Group in the Department of Neuropsychology at Krankenhaus München-Bogenhausen, Dachauerstraße 164, D-80992 Munich, Germany. N. Mai. deceased, was with Ludwig-Maximilians University at the time the research was conducted.