Perturbation induced step training. Perturbations will be applied by a mechanical weight drop system that has been used in previous studies. A belt with a cable will be attached to the subject's waist. The cable will be attached to metal weights that is held by an electromagnet on a vertical pole approximately 10-12ft from the subject. The weight will drop 52cm when the electromagnet is released. When the subject has attained and held a predetermined standing position for 1-3 seconds the mechanical perturbation will be activated. See training protocol below.
Pre and Post Testing Sessions: Subjects will be asked to complete pre and post testing sessions within 1 week of training. Testing sessions will include laboratory and clinical measures of balance and will last for approximately one hour.
Laboratory Measures: Participants will complete 2 standing trials, 12 voluntary stepping trials, and 12 anterior perturbation trials while standing on force platforms.
Standing trials: Subjects will be asked to stand feet hip with apart with each foot on a separate force plate that measure the pressure under their feet. Their foot placement will be traced so they can return to the same placement for each trial. Subjects will be instructed to cross their arms and stand as still as they can for 3 seconds, then march in place for 3 seconds.
Voluntary Stepping Trials: While standing with their feet in their foot tracings subjects will be asked to shift their weight until they assume a predetermined pressure under each foot. Visual feedback will be provided on a monitor in front of them. They will be instructed to hold that position until they see the "Go" cue on the screen at which point they should take 3 steps forward as fast as they can. There will be 6 stepping trials on each leg starting with the less involved leg.
Reactive Stepping Trials: Subjects will be secured in a safety harness that is mounted to the ceiling. The harness does not interfere with stepping but allows the person body weight to be supported if there is a loss of balance. A belt will be placed around the subjects' waist that will be connected by a cable to a mechanical weight drop device that will pull the subject forward to challenge their balance. The puller will provide mechanical perturbations up to 10% of subjects' body weight. The subjects will be instructed not resist the pull but to respond naturally and move their feet if needed to maintain up right posture. There will be ten reactive trials. Rest breaks will be given between voluntary and reactive trials or as needed.
Participants will complete 6 training sessions within a three-week period. Subjects will stand with their feet on a platform with a scale. Subjects will be instructed to try to use their more impaired leg if they need to step. The same mechanical perturbation that was used in the pre/post testing will be used for training with the belt attached to their waist. When they have attained and held a predetermined standing position for 1-3 seconds the mechanical perturbation will be activated. The weight bearing status of the more impaired leg will be increased by 5% after 10 out of 12 successive trials (i.e. stepping with the more impaired leg) or decreased by 5% if they always step with the less impaired leg after 6 trials. Training sessions will consist of 50 perturbation trials at 10% body weight and 3 catch trials at 2% body weight to discourage premature stepping. Training session will last about 1 hour. Subjects will be given rest breaks as needed.
