1. Baseline parameters are treated as normally distributed, first POST parameters as uniformly distributed samples, with variance [2]: Variations in the kinematics along the z axis were identified from the ratio of maximum and average velocity (C-ratio) [3]. 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 In a 0 g environment, objects have mass but no weight. How do we perform when moving weightless objects with different masses? We measured the arm kinematics of subjects performing movements while holding objects of different masses, and analysed the effect of adaptation to microgravity. 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. Conclusions & Discussion The 2D model, with a constant joint stiffness profile, reproduces the experimental results adequately. A progressive stiffness increase could take place during adaptation and explain the kinematic differences between the first movement with the LIGHT MASS and the re-adapted baseline. A parameter optimization could shed light on the stiffness variations during the adaptation process. There are kinematic variations between adapted states with different hand inertia (C-ratio). Support NASA NVJ04HJ14G NIH AR84546-01A1 Materials and Methods Data Analysis Data were calibrated for magnetic distortion [8]. Movement duration across all subjects averaged 0.58±0.15s Kinematic data were low pass filtered at 3Hz. Results The first movement with the LIGHT MASS is significantly faster than both the adapted baselines with HEAVY MASS and LIGHT MASS. An aiming error [5,6] is present in the experimental as well as simulated data The numerical simulation suggests that an adjustment of joint stiffness is not necessary to explain these variations of the tangential hand velocity induced by the change of inertia. Although the maximum tangential velocity is the same for LIGHT MASS and HEAVY MASS baseline movements, the velocity profile along the vertical axis is different within each inertial condition, as seen for primarily vertical movements by Papaxanthis et al (2005). Simulation 861.3 References 1.Gomi H, Kawato M (1997) Biological Cybernetics 76: 163-171. 2.Kadis RL (2000) Measurement Techniques 43: 403-404 3.Papaxanthis C, Pozzo T, McIntyre J (2005) Neuroscience 135: 371-383. 4.Risher DW, et.al. (1997) Journal of Biomechanical Engineering-Transactions of the Asme 119: 417-422. 5.Sanguineti V, et.al. (2003) Human Movement Science 22: 189-205. 6.Smith MA, Shadmehr R (2005) Journal of Neurophysiology 93: 2809-2821. 7.Tsuji T, Morasso PG, Goto K, Ito K (1995) Biological Cybernetics 72: 475-485. 8.Zachmann G (1997) Proceedings of the Computer Graphic International 0-8186- 7825-9/97 © 1997 IEEE 9.Zatsiorsky and Seluyanov, (1985) Biomechanics IX-B: 233-239 Model was fed a PER baseline movement computed from time normalized and averaged trajectories of the PER baseline set. Deviation of tangential velocity from baseline PER to the first POST movement is compared in the simulated and the experimental data. Joint torques from an inverse 2D dynamics model of the arm were used in a direct dynamic simulation [4]. The dynamic model employs inertial [9], damping [7] and joint stiffness [1,7] data available from literature. Stiffness profile is constant for the first ~200ms of movement, and then it increases [1,6]. Baseline PERFirst POSTBaseline POST Last 20 movements with the HEAVY MASS for each subject First movement with the LIGHT MASS for each subject Last 10 movements with the LIGHT MASS for each subject Parabolic flight maneuvers on NASA C-9B. 20-25s period of microgravity per parabola. Eight subjects (2 female, 6 male; age 45±16 years), quasi- planar reaching movements. Forward target or Leftward target. 30g (LIGHT MASS), or 580g (HEAVY MASS), same geometry and texture. Alternated forward/leftward reaches, 3-5 reaches per target during each 0g period. 20 parabolas per subject: 2 parabolas while holding the LIGHT MASS - 15 baseline PER parabolas while holding the HEAVY MASS - 3 POST parabolas with LIGHT MASS. Polhemus® Liberty magnetic motion tracking system. One position sensor mounted on the wrist brace. Forward Target (18cm) Left Target (32 cm) Start Position x y z 55° V tg ∆V tg Z ∆Z Quartile distribution of tangential velocity Quartile distribution of vertical displacement C-V z C-V tg [m/s] * * * * * * v xy [m/s] * ANOVA p < 0.05 * * * V tg [m/s] * * [m] [s]