Control of frontal -plane balance on a laterally-compliant raised structure.
Yang, Bing-Shiang
2004
Abstract
Falls from ladders cause injuries and fatalities in both occupational and non-occupational settings, with serious ladder injuries being more common in the elderly than in the young. In the case of a stepladder, the most common type of accident is a lateral fall. This thesis provides insights into possible causes of lateral falls from stepping onto or balancing on a laterally-compliant raised structure, such as a stepladder or chair. A forward dynamic inverted pendulum model, validated by experiment, was used to predict feasible lateral movements and tool forces for a human standing on a rigid non-fixed structure. The results show that, when transferring weight from one foot to the other, the lateral stability of the structure depends mainly on the height of the tread on which the person stands. Lateral forces exerted for more than one second at the user's shoulder height should be less than 8% of body weight (BW) when standing on a tread equal to 50% of body height. Some raised structures, especially older ones, have lateral compliance. An experimental study of 30 healthy adults (10 young females, 10 young males and 10 older males) showed that an unexpected lateral structural compliance significantly affected the duration of the step-forward-and-up movement (p < 0.01) and lateral weight transfer velocity (p < 0.001). Healthy adults adapted in the first trial to the presence of structural compliance. Older males needed more time than did young males to complete one step-up movement (p < 0.001). Older males also showed greater adaptation than did young males in repeated stepping movements onto the same compliant structure; after five practice trials they were fully adapted. An inverted-pendulum biomechanical model of an individual transferring weight from one foot to the other on a laterally-compliant raised structure was first validated using the above data. The model was then used to examine the effects of stepping strategies and system parameters on the stability of the human-compliant structure system. The lateral stability of the system was most sensitive to the lateral velocity and displacement of the whole-body center of mass at the trail-foot push-off. Based on data from older males in the previous experiment, the model-predicted value of minimum allowable lateral structural stiffness was 0.26*BW (Nm/rad); this should be increased with increasing obesity, weight of clothing and tools, and manually-exerted forces. When standing on a stepladder or chair, an older individual should use the lowest step possible, avoid hurried movements or excessive manual forces, keep the stance foot away from the lateral edge of the structure, and avoid using non-rigid, parallel-sided structures. He/she should practice stepping and balancing on the structure before actually performing a task.Subjects
Compliant Control Falls Frontal-plane Balance Ladders Laterally Postural Stability Raised Structure Weight Transfer
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