It will be a two-session training with each session lasting for approximately an hour. The same training duration and format will be used as the PBT group, except that no perturbations will be induced during the treadmill walking.

The number of laboratory-falls following perturbation during assessment sessions will be recorded. A fall will be defined if there is a body-weight support of 30% or more from the full-body safety harness.

The number of real-life falls in 12 months after randomization will be recorded. Fall circumstances and related injuries will be enquired via phone contact. A fall is defined as an event resulting in a person unintentionally coming to rest on the ground or other lower level, not as the result of a major intrinsic event e.g., syncope, stroke, seizure, or overwhelming hazard such as an earthquake.

Step time of the recovery steps after the perturbation will be traced by the motion capture system synchronized with the force plates embedded underneath the slip-treadmill belts.

Step length of the recovery steps after the perturbation will be traced by the motion capture system synchronized with the force plates embedded underneath the slip-treadmill belts.

The trunk angle at the recovery steps after the perturbation will be calculated as the angle between the trunk segment with the vertical line. The position of the trunk will be traced by the motion capture system synchronized with the force plates embedded underneath the slip-treadmill belts.

Hip height at the recovery steps after the perturbation will be calculated as the vertical distance from the ground to the midpoint of bilateral hips and normalized by the subjects' height. The positions of bilateral hips will be traced by the motion capture system synchronized with the force plates embedded underneath the slip-treadmill belts.

Responsive postural adjustment upon translation will be assessed through Motor Control Test on a computerized dynamic posturography (Bertec® Balance Advantage® Dynamic System, Bertec® Corporation, Columbus, USA). During the Motor Control Test, participants will be requested to maintain their standing balance in response to 18 translations of the standing platform in either forward or backward direction with small, medium or large amplitudes. Weight symmetry, latency of reaction, and amplitude of sway will be recorded. Full-body safety harness will be worn throughout the tests to protect the participants from any potential loss of balance.

Responsive postural adjustment upon tilting will be assessed through Adaptation Test on a computerized dynamic posturography (Bertec® Balance Advantage® Dynamic System, Bertec® Corporation, Columbus, USA). During the Adaptation Test, the force platform will either tilt up or down for a total of 10 repetitions. Sway energy will be recorded. Full-body safety harness will be worn throughout the tests to protect the participants from any potential loss of balance.

Muscle latency will be collected from trunk and leg muscles using electromyography.

Balance confidence will be measured with the Chinese version of the shortened version of Activities-Specific Balance Confidence Scale (ABC-6). The scale value is from 0 to 100, with higher values representing higher balance confidence.

Cognitive flexibility and executive function assessed by Trial Making Tests A and B (in the unit of seconds)

Functional lower body strength will be assessed by Five-time Sit to Stand Test (in the unit of seconds)

Dynamic balance will be assessed by Alternate Step Test (in the unit of seconds)

Mobility will be assessed by Timed Up and Go test (in the unit of seconds).

White matter density at cortical and subcortical regions will be taken by structural MRI. The regions of interest include the cerebellum, basal ganglia, thalamus, hippocampus, inferior parietal cortex, occipital and frontal lobe.

Gray matter volume at cortical and subcortical regions will be measured by Structural MRI. The regions of interest include the cerebellum, basal ganglia, thalamus, hippocampus, inferior parietal cortex, occipital and frontal lobe.

White matter volume at cortical and subcortical regions will be measured by Structural MRI. The regions of interest include the cerebellum, basal ganglia, thalamus, hippocampus, inferior parietal cortex, occipital and frontal lobe.

Cortical thickness in mm measured by structural MRI. The regions of interest include the cerebellar, inferior parietal, occipital, and frontal cortices.

The degree of anisotropy of water diffusion in white matter tracts will be derived from the diffusion tensor. The regions of interest include the cerebellum, basal ganglia, thalamus, hippocampus, inferior parietal cortex, occipital and frontal lobe.

Mean diffusivity will be derived from the diffusion tensor. The regions of interest include the cerebellum, basal ganglia, thalamus, hippocampus, inferior parietal cortex, occipital and frontal lobe.