To validate our proposed measures of stability against existing stability measures, we conducted a set of experiments using a CAREN. The same experimental data were used as in23. 15 non-disabled individuals and three individuals with a disability participated in this study. All participants gave informed consent to perform the experiments approved by the research ethics board of the University of Alberta (protocol number: Pro00066076), and all experiments were performed in accordance with relevant guidelines and regulations. Each participant walked on a platform-mounted treadmill in 60-m trials under a “low-perturbation” and a “high-perturbation” walking condition. During both low- and high-perturbation trials, the BOS perturbations occurred in the form of continuous vertical and horizontal displacements and sagittal rotations of the CAREN platform. The dominant frequencies and amplitudes of the low (high) perturbation were 1 Hz (3 Hz) and 1 cm (6 cm) in the vertical and horizontal directions, respectively, and of 1 Hz (3 Hz) and 0.6 deg (3 deg) in the rotational direction, respectively. The perturbation profiles were randomized for every participant and every trial and did not have an established pattern. All three types of perturbations were applied during the same trial by super-positioning these three types. The participants were notified of perturbation occurrence 3 s prior to each the start of perturbation through a visual message appearing on the CAREN screen. Treadmill speed was adapted to the participant’s preferred walking speed to mimic the natural walking condition. The perturbation trials were performed after an initial familiarization where the participants were asked to walk on the treadmill with their preferred pace to adapt to the self-paced condition. Four reflective markers were used to track the motion of the platform. We tracked the BOS motion using four markers mounted on a rigid plate attached to each foot that obtained the trajectory of the heels and toes. Four markers were mounted on another plate attached over the sacrum, and were used to obtain the body COM motion based on the location of the COM with respect to the sacrum suggested by Yang and Pai 36. Normalized COM position and velocity with respect to the BOS at each instant were used to calculate the COM state trajectory. The ExFSR for the duration of consecutive steps were obtained using (i) the corresponding FSRs for each perturbation profile computed using the equations proposed by Bahari et al. 23 and (ii) the distance between the toe tip of the posterior foot (Foot1) and heel of the anterior foot (Foot2) during the double-support phase (Fig. 2).

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