1 1 2 Movement Stability under Uncertain Internal Models of Dynamics 3 4

30 Sensory noise and feedback delay are potential sources of instability and variability for the 31 online control of movement. It is commonly assumed that predictions based on internal 32 models allow the central nervous system (CNS) to anticipate the consequences of motor 33 actions and protect the movements from uncertainty and instability. However, during motor 34 learning and exposure to unknown dynamics, these predictions can be inaccurate. Therefore, 35 a distinct strategy is necessary to preserve movement stability. This study tests the hypothesis 36 that in such situations, subjects adapt the speed and accuracy constraints on the movement, 37 yielding a control policy that is less prone to undesirable variability in the outcome. This 38 hypothesis was tested by asking subjects to hold a manipulandum in precision grip and to 39 perform single-joint, discrete arm rotations during short-term exposure to weightlessness (0 40 g), where the internal models of the limb dynamics must be updated. Measurements of grip 41 force adjustments indicated that the internal predictions were altered during the early 42 exposure to the 0 g condition. Indeed, the grip force/load force coupling reflected that the grip 43 force was less finely tuned to the load-force variations at the beginning of the exposure to the 44 novel gravitational condition. During this learning period, movements were slower with 45 asymmetric velocity profiles and target undershooting. This effect was compared to 46 theoretical results obtained in the context of optimal feedback control, where changing the 47 movement objective can be directly tested by adjusting the cost parameters. The effect on the 48 simulated movements supported quantitatively the hypothesis of a change in cost-function 49 during the early exposure to a novel environment. The modified optimization criterion 50 reduces the trial-to-trial variability despite that noise affects the internal prediction. These 51 observations support the idea that the CNS adjusts the movement objective to stabilize the 52 movement when internal models are uncertain.