Limited Effects of Age on the Use of the Ankle and Counter-Rotation Mechanism in the Sagittal Plane

Abstract

During standing the center of mass (CoM) can be controlled relative to the base of support by applying ankle moments to shift the center of pressure (CoP mechanism). An additional mechanism is the counter-rotation mechanism, i.e., changing the angular momentum of segments around the CoM to change the direction of the ground reaction force. In this study, we assessed anteroposterior balance performance and the related use of these postural control mechanisms in children, younger and older adults. Sixteen children (6-9y), 17 younger 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 sagittal plane, varying in height (15-19 cm). Full body kinematics were measured. Performance related outcome measures, i.e., the number of trials with balance loss and the Root Mean Square of the time series of the CoM acceleration (due to the CoP and counter-rotation mechanism) and the contributions of the CoP and counter-rotation mechanism to the CoM acceleration (in %) were calculated. Furthermore, selected kinematic measures, i.e., the orientation of the board and head and the Mean Power Frequency of board orientation and of CoM acceleration were calculated. Compared to younger adults, children and older adults showed a poorer balance performance. Across age groups and conditions, the contribution of the CoP mechanism to the total CoM acceleration was dominant, i.e., 95%-108%. The contribution of the counter-rotation mechanism was limited, i.e., 19%-31% (with totals higher than 100% indicating opposite effects of both mechanisms), since the counter-rotation mechanism would conflict with stabilizing the head in space. Furthermore, children did use the counter-rotation mechanism relatively more to accelerate the CoM compared to younger adults as they are possibly still learning to limit the contribution of the counter-rotation mechanism.