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

Two mechanisms can be used to accelerate the center of mass (CoM) to control the CoM in relation to the base of support during standing. The first is applying ankle moments to shift the center of pressure (CoP), which has been coined the 'CoP mechanism'. The second is changing the angular momentum around the CoM to change the direction of the ground reaction force, i.e., the 'counter-rotation mechanism'. At both the beginning and the end of the lifespan, problems with postural control are common. In this study, we asessed anteroposterior balance performance and the related use of these postural control mechanisms in children, younger adults, and older adults. Sixteen pre-pubertal 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) of the board above the point of contact with the floor. Full body kinematics were measured. 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 were calculated. Additionally, the RMS of the time series of the CoM acceleration due to the CoP and counter-rotation mechanism and the contributions of the CoP and the counter-rotation mechanism to the CoM acceleration (in %) in the sagittal plane were calculated. Furthermore, selected kinematic measures, i.e., the orientation of the board and the head and the Mean Power Frequency of balance board orientation and of CoM acceleration were calculated. Compared to younger adults, children and older adults showed a poorer balance performance, reflected by a greater RMS of CoM accelerations and more balance loss in older adults. 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 ranged between 19%-31% (with totals higher than 100% indicating opposite effects of both mechanisms). We suggest that the contribution of the counter-rotation mechanism is limited, since the counter-rotation mechanism would conflict with stabilizing the orientation of the head in space. Furthermore, children did use the counter-rotation mechanism relatively more to accelerate the CoM compared to younger adults. Possibly this reflects that they are still learning to limit the contribution of the counter-rotation mechanism to the same extent as adults. ### Competing Interest Statement The authors have declared no competing interest.