Width on Frontal Plane Postural Dynamics and Coordination in 2 Human Balance

23 The influence of stance width on frontal plane postural dynamics and 24 coordination in human bipedal stance was investigated. We tested the hypothesis that 25 when subjects adopt a narrow stance width, they will rely heavily on nonlinear control 26 strategies and coordinated counter-phase upper and lower body motion to limit center-of27 mass (CoM) deviations from upright; but as stance increases, the use of these strategies 28 will diminish. Freestanding frontal plane body sway was evoked through continuous 29 pseudorandom rotations of the support surface upon which subjects stood with various 30 stimulus amplitudes. Subjects were either eyes open or closed and adopted various stance 31 widths. Upper body, lower body, and CoM kinematics were summarized using root32 mean-square and peak-to-peak measures and dynamic behavior was characterized using 33 frequency-response and impulse-response functions. In narrow stance, CoM frequency34 response function gains were reduced with increasing stimulus amplitude and in EO 35 compared to EC; but in wide stance, gain reductions were much less pronounced. Results 36 show that the narrow stance postural system is nonlinear across stimulus amplitude in 37 both EO and EC conditions while the wide stance postural system is more linear. The 38 nonlinearity in narrow stance is likely due to an amplitude-dependent sensory 39 reweighting mechanism. Finally, lower body and upper body sway were approximately 40 in-phase at low frequencies (<1 Hz) and out-of-phase at high frequencies (>1 Hz) across 41 all stance widths and results were therefore inconsistent with the hypothesis that subjects 42 made greater use of coordinated counter-phase upper and lower body motion in narrow 43 compared to wide stance conditions. 44 45