Variability and Specificity in Reactive Stabilization Movements to Diverse Slip Perturbations

Recent research on slips has shown that use of perturbation training to improve specific motor skills to resist slips can vastly reduce falls. However, these interventions have only addressed slips occurring at heel-strike, and not the diverse range of slipping disturbances presented by a complex environment. This study will focus on slips that occur across the gait cycle, and the reactive stabilization movements that follow. Slips at different phases of the gait cycle have unique biomechanical contexts, and successful reactive stabilization movements are likely to be highly specific to that context. Yet nothing is known about the specificity of the repertoire of reactive stabilization movements to resist different slip conditions. Furthermore, variability in the repertoire of reactive stabilization movements is likely to affect the success/failure rate of resisting these disturbances. Thus, the objective of the proposed study is to determine the roles of variability and specificity in reactive stabilization movements to resist falling in diverse slipping conditions.

A novel wearable apparatus for slipping perturbations (WASP) will deliver lifelike, unexpected slips in early, middle and late stance in younger (19 - 35 years of age) and older adults. Biomechanical analysis based on three-dimensional motion capture data of the reactive stabilization movements will generate novel results on the specificity and variability of protective stepping and arm swinging responses. The central hypothesis is that increases in specificity and decreases in variability of reactive stabilization movements will reduce fall rates. The long-term objective of this research is to support future studies into the repertoire of reactive stabilization movements across the full range of disturbances faced in navigating the environment.

Each participant will participate in 3 visits within the span of 1 week. On the initial visit, participants will perform informed consent, have demographic and body measurement information collected and complete medical and physical activity questionnaires. Upper and lower body strength will be measured using a hand-held dynamometer during the first and last visits. On each of the three visits, subjects will perform a gait variability assessment followed by a diverse slip perturbation assessment. Before recording data, retroreflective markers will be placed on specific anatomical locations. These locations are defined to create a full-body dynamics model. For gait variability assessment, participants will be familiarized with the activity, then they will walk on a treadmill for 5 minutes at a prescribed pace of 1.3 meters per second while whole body kinematics are collected. Both linear and nonlinear measures of gait variability will be calculated to determine possible relationships between these measures and other outcome variables such as learning rate for reactive movements and fall rate.

For diverse slip perturbation assessment, participants will walk overground back and forth across a large motion capture area. Previous studies show that walking speed is highly correlated with some outcome measures, specifically variations in whole body angular momentum, gait variability measures, and loss of balance following a slip. Therefore, gait speed will be controlled to 1.3 ± 0.2 m/s, a typically comfortable speed for both younger and older adults. Participants will be informed that a slip may occur, and that if slipped, they should attempt to maintain balance and continue walking. Participants will then be slipped without warning at a randomly chosen time after 1 to 3 minutes of walking. Slips will be administered to both feet simultaneously during early, middle or late stance in a randomized order. After each slip, participants will rest seated for 5 minutes. During this rest period, motion capture data and force data from the harness will be recorded, and the WASP will be reset for the next trial. Participants will complete four slips in each of early, middle, and late stance on each visit. Measures of falls, slip severity, and reactive stabilization movements will be calculated.

Inclusion criteria

• Age 19-35 years for younger adults group, or age 65-79 years for older adults group.

Exclusion criteria

• Uncontrolled hypertension
• Peripheral arterial disease
• Vertigo
• Meniere's disease
• Chronic dizziness
• History of back or lower extremity injury
• Surgery that affects the subject's mobility
• Any neurological disease or impairment that affects limits the ability to walk, e.g. Stroke, Parkinson's disease, Multiple sclerosis