LAUSR

We compared voluntary oscillatory sway for eight subjects tested in 1.8g and 1g gravito-inertial force (GIF) levels of parabolic flight. Subjects performed voluntary forward-backward (FB) and lateral left-right (LR) swaying as the forces and moments under the soles of each foot were measured. We calculated the experimental values of three parameters - two ankle stiffness parameters KSAP and KSML acting in orthogonal FB and LR directions, and one parameter KED related to leg pivot shifting. Simulations of the Engaged Leg Model (Bakshi et al. 2019a; b) correctly predicted the experimentally determined stability bounds of upright balance and also the scaling of the postural parameters as a function of GIF magnitude. The effective stiffness, KSAP, at the ankles played the primary role to prevent falling in FB swaying and both model predictions and experimental data showed KSAP to scale up in proportion to GIF magnitude. For LR swaying, the model predicted a 3:4 scaling of AP stiffness to change in GIF magnitude, which was borne out by the experimental data. Simulations predict stability (non-falling) not to depend on lateral stiffness, KSML, which was experimentally found not to depend on the GIF magnitude. Both model and experiment showed that the geometry dependent pivot shift parameter KED was invariant to a change in GIF magnitude. Thus the ELM explains voluntary sway and balance in altered GIF magnitude conditions, rotating environments with Coriolis perturbations of sway, as well as normal terrestrial conditions.