Although Parkinson disease (PD) causes profound balance impairments, we know very little about how PD impacts the sensorimotor networks we rely on for automatically maintaining balance control. In young healthy people and animals, muscles are activated in a precise temporal and spatial organization when the center of body mass (CoM) is unexpectedly moved.This organization is largely automatic and determined by feedback of CoM motion. Here, we show that PD alters the sensitivity of the sensorimotor feedback transformation. Importantly, sensorimotor feedback transformations for balance in PD remain temporally precise, but become spatially diffuse by recruiting additional muscle activity in antagonist muscles during balance responses. The abnormal antagonist muscle activity remains precisely time-locked to sensorimotor feedback signals encoding undesirable motion of the body in space. Further, among people with PD, the sensitivity of abnormal antagonist muscle activity to CoM motion varies directly with the number of recent falls. Our work shows that in people with PD, sensorimotor feedback transformations for balance are intact but disinhibited in antagonist muscles, likely contributing to balance deficits and falls.

31Background 32Many individuals with Parkinson's disease exhibit narrow stance width during balance and gait. Because 33 of this, stance width is an important biomechanical variable in many studies. Measuring stance width 34 accurately using kinematic markers in parkinsonian patients can be problematic due to occlusions by 35 research staff who must closely guard patients to prevent falls. 36 Methods 37We investigated whether a measure of stance width based on the mediolateral distance between the center 38 of pressure under each foot could approximate stance width measured with kinematic data. We assessed 39 the agreement between estimates of stance width obtained from simultaneous kinematic and center of 40 pressure measures during quiet standing in 15 individuals (n=9 parkinsonian, n=6 age-similar 41 neurotypical). The source data (1363 unique trials) contained observations of stance width varying 42 between 75-384 mm (≈ 25-150% of hip width). 43 Findings 44Stance width estimates using the two measures were strongly correlated (r = 0.98). Center of pressure 45 estimates of stance width were 48 mm wider on average than kinematic measures, and did not vary across 46 study groups (F 2,12 =1.81, P<0.21). The expected range of differences between the center of pressure and 47 kinematic methods was 14-83 mm. Agreement increased as stance width increased (P<0.02). 48 Interpretation 49It is appropriate to define stance width based on center of pressure when it is convenient to do so in 50 studies of individuals with and without Parkinson's disease. When comparing results across studies with 51 the two methodologies, it is reasonable to assume a bias of 48 mm. 52 Keywords 53Postural control; Center of pressure location; Measurement; Methodology; Foot position 54 55 . CC-BY 4.0 International license peer-reviewed) is the author/funder. It is made available under a