Effects of age and surface instability on the control of the center of mass

During standing, posture can be controlled by accelerating the Center of Mass (CoM) through shifting the center of pressure (CoP) within the base of support by applying ankle moments ("CoP mechanism"), or through the "counter-rotation mechanism", i.e., changing the angular momentum of segments around the CoM to change the direction of the ground reaction force. Postural control develops over the lifespan; at both the beginning and the end of the lifespan adequate postural control appears more challenging. In this study, we aimed to assess mediolateral balance performance and the related use of the postural control mechanisms in children, older adults and younger adults when standing on different (unstable) surfaces. Sixteen pre-pubertal children (6-9y), 17 younger adults (18-24y) and eight older adults (65-80y) performed bipedal upright standing trials of 16 s on a rigid surface and on three balance boards that could freely move in the frontal plane, varying in height (15-19 cm) of the surface of the board above the point of contact with the floor. Full body kinematics (16 segments, 48 markers, using SIMI 3D-motion analysis system (GmbH) and DeepLabCut and Anipose) were retrieved. Performance related outcome measures, i.e., the number of trials with balance loss and the Root Mean Square (RMS) of the time series of the CoM acceleration, the contributions of the CoP mechanism and the counter-rotation mechanism to the CoM acceleration in the frontal plane and selected kinematic measures, i.e. the orientation of the board and the head and the Mean Power Frequency (MPF) of the balance board orientation and the CoM acceleration were determined. Balance loss only occurred when standing on the highest balance board, twice in one older adult once in one younger adult. In children and older adults, the RMS of the CoM accelerations were larger, corresponding to poorer balance performance. Across age groups and conditions, the contribution of the CoP mechanism to the total CoM acceleration was much larger than that of the counter-rotation mechanisms, ranging from 94% to 113% vs 23% to 38% (with totals higher than 100% indicating opposite effects of both mechanisms). Deviations in head orientation were small compared to deviations in balance board orientation. We suggest that the CoP mechanism is dominant, since the counter-rotation mechanism would conflict with stabilizing the orientation of the head in space.