MOVING OBJECTS IN MICROGRAVITY. A. Pierobon*, D. Piovesan, P. DiZio, J.R. Lackner. Ashton Graybiel Spatial Orientation Laboratory and Volen Center for Complex Systems, Brandeis University, Waltham, MA. Introduction Carrying an object in a normal 1 g environment requires motor attunement to its weight and inertia. In a 0 g environment, objects have mass but no weight. How do we accommodate when moving weightless objects with different masses? We measured the arm kinematics of the same movements performed while holding a 30g versus a 580g object with the same geometry and texture, and analysed the effect of adaptation to microgravity on the hand trajectory. We also computed torques with an inverse 2D dynamics model of the arm and used them in a direct dynamic simulation, studying the mechanical effect of changing the held inertia while applying the same joint torques, and comparing the simulated trajectories to the experimental results. The inertia of the held object affects maximum tangential hand velocity, and the shape of the vertical component of the velocity, but not the maximum vertical displacement. The results of the mechanical simulation suggest that an adjustment of joint stiffness is not necessary to explain the effects of the variation of inertia on the tangential hand velocity. Conclusions & Discussion Support NASA NVJ04HJ14G, NIH AR A1 Materials and Methods Experiments took place on board NASA C-9B during parabolic flight maneuvers, which simulated 20-25s periods of microgravity per parabola. Eight subjects (2 female, 6 male; age 45±16 years) took part in the experiment. Each subject, while strapped on the seat with a portable table in front of him/her, performed planar reaching movements. Starting from a common position the movements were directed towards either a forward target (approximately 20cm from the start position, along a trajectory parallel to the subject’s body midline) or a leftward target (approximately 32cm from the start position, along a trajectory deviated 55° to the left of the subject’s body midline). Movements were performed while either holding a 30g, 5.5” long by 1 diameter, hollow plastic cylinder (LIGHT MASS), or an equivalent cylinder with the same geometry and texture, but filled with 550g of tungsten powder (HEAVY MASS). Subjects would be wearing a wrist brace, impeding any movements of the wrist relative to the forearm, and were instructed to reach alternatively the forward and the leftward targets with their pointing finger, at a comfortable, but fast pace, while trying to be as accurate as possible. This resulted in 3-5 reaches per target during each 0g period. Subjects were instructed to remain as still as possible during the 1.8g and 1g portions of the flight path. Subjects kept their eyes open during the task and during the rest periods between parabolas. Data Analysis -Metal objects around the workspace can distort the Polhemus® magnetic field and therefore affect the measures. A magnetic distorsion map was used to calibrate the measured position data. Results - Variation of tangential speed not completely explained by inertial variation: feedforward strategy is kinematic, not dynamic Simulation References 1. Gomi H, Kawato M (1997) Human arm stiffness and equilibrium-point trajectory during multi-joint movement. Biological Cybernetics 76: Ref ID: 53 2.Papaxanthis C, Pozzo T, McIntyre J (2005) Kinematic and dynamic processes for the control of pointing movements in humans revealed by short-term exposure to microgravity. 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During the first 2 parabolas subjects performed reaching movements while holding the light mass for familiarization with the task. During the following 15 parabolas subjects performed the reaching movements while holding the HEAVY MASS, and for the last 3 parabolas the object was changed to LIGHT MASS. The position of the subject’s hand was measured with a Polhemus® Liberty magnetic motion tracking system. Position data were sampled at 240Hz. One position sensor was mounted to the subject’s wrist brace, and a second one was mounted to the subject’s lateral epicodyle, although only data from the sensor on the wrist are considered in this work. Forward Target Left Target Start Position x y z