Assessment of postural control

Three primary tests were performed: quiet sitting, multi-directional leaning (“octagon”), and self-initiated perturbations. During each test, participants were instructed to sit unsupported on the force plate and attempt to actively maintain their trunk as upright as possible, maintain balance, and minimize the use of their arms and heads. One researcher was standing behind each participant to ensure the safety of the participants and to provide assistance to prevent falls if needed. During the quiet sitting test, participants were instructed to sit as quietly as possible for 60 sec with eyes open. After 2 min of rest, the task was repeated with the participant's eyes closed. During the multi-directional leaning test, the ability to voluntarily displace the center of pressure (COP) to a maximum distance without losing balance was assessed.³⁹ Participants were instructed to lean from the center position as far as possible in the indicated direction without losing balance, while keeping their backs extended. The COP position registered by a force plate was visualized on the monitor to provide feedback on body displacement. Eight targets were presented at 45 degree angle increments around the center, starting in the forward right direction. The participant was asked to move the COP indicator to each target, which was present for 5 sec, hold the position during that time, and return to the starting position.

Then, a self-initiated perturbation test was performed in seven participants (P1–4, P6–8) to assess the efficiency of postural corrective responses. The data from participant P5 were omitted from the group analysis because of incomplete video and three-dimensional (3D) kinematic recordings during the self-initiated perturbation test. In agreement with previous literature,⁴⁰ the participants were instructed to rapidly raise their extended right arm forward to a horizontal position and then return the arm back to the initial position based on auditory cues.