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

Abstract

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 young adults when standing on different (unstable) surfaces. Sixteen pre-pubertal children (6-9y), 17 young adults (18-24y) and eight older adults (65-80y) performed bipedal upright standing trials of 16 seconds 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 young 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%-113% vs 23%-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 hypothesize that the CoP mechanism is dominant, since the counter rotation mechanism would conflict with stabilizing the orientation of the head in space.

Limited effects of age on the use of the ankle and counter-rotation mechanism in the sagittal plane

“…We found that children used the counter-rotation mechanism relatively more to accelerate the CoM compared to younger adults. This is in contrast to our previous study using balance boards that could freely move in the frontal plane, in which we did not find an effect of age on the relative use of the counter-rotation mechanism (van den Bogaart et al, 2022). The increased contribution of the counter-rotation mechanism in children cannot be explained by differences in body height between children and younger adults, as accelerating the body center of mass by the counter-rotation mechanism is less efficient at lower height (A.1.1.…”

“…Decreased pertinence of proprioceptive information from ankle muscles and a reduction of the effectiveness of ankle moments to accelerate the CoM when standing on a balance board could be an explanation for this (Horak et al, 2001; van Dieen et al, 2015). The number of balance losses (23), with balance boards that could freely move in the sagittal plane, was much larger than in our previous study (van den Bogaart et al, 2022), with uniaxial balance boards that could freely move in the frontal plane (3). Surface instability in the sagittal plane thus seems more challenging compared to surface instability in the frontal plane.…”

“…The methods and participants of this study were identical to that of our previous study (van den Bogaart et al, 2022). In the current experimental setup, however, the direction of the movement of the balance boards was in the anteroposterior direction (i.e., in the sagittal plane).…”

“…We also expected poorer balance performance and increased use of the counter-rotation mechanism during standing on the balance boards compared to standing on a rigid surface. Additionally, we hypothesized that the CoP mechanism is dominant, based on our findings when assessing the use of the postural control mechanisms in the frontal plane (van den Bogaart et al, 2022).…”

“…The use of these postural control mechanisms has been suggested to be direction-specific (Winter et al, 1998; Winter et al, 1996). Therefore, in a previous paper, we focused on the use of the postural control mechanism in the frontal plane (van den Bogaart et al, 2022). The current manuscript focusses on the use of the postural control mechanisms in the sagittal plane in the same population.…”