Effective rocker shapes used by able-bodied persons for walking and fore-aft swaying: Implications for design of ankle–foot prostheses
Introduction
The physiologic ankle–foot system is amazingly complex. However, its functions for particular tasks may be simple. As an example, the physiologic system has been modeled as a rocker (or rockers) by many researchers in attempts to better understand its functions during able-bodied walking. Perry [1] described this system as anatomical rockers that aid in forward progression, consisting of the heel rocker, ankle rocker, and forefoot rocker. Morawski and Wojcieszak [2] studied a walking toy with circular walking feet and found that increasing the rocker foot radius improved its stability during walking. McGeer [3] estimated from computer and physical models that the “equivalent radius” for human walking would be about 0.3 times leg length. Using a circular rocker to replace the ankle–foot system, McGeer's physical models could walk down a gentle slope without the use of external power.
Knox [4] concluded that the effective rocker shapes of prosthetic feet are important to their function for walking. He developed a method to measure these effective shapes and compared the shapes for prosthetic and physiologic ankle–foot systems during walking. The effective rocker that the ankle–foot system conforms from heel contact to opposite heel contact of walking was later termed the ankle–foot roll-over shape (ROS) [5]. The ROS may be a useful and simple tool for design and evaluation of lower limb prostheses because it has been shown to be nearly circular and resistant to changes in walking speed [5], added weight to the torso [6], and heel height [7] during able-bodied walking on level ground.
Standing and swaying within the base of support are also important functional tasks of daily living. Winter et al. [8] described the center of pressure (COP) as a response mechanism to corral a person's center of mass (COM), allowing the person to maintain balance during standing. During intentional, controlled swaying, the person's COP would also need to move ahead of the COM to prevent the body from losing balance. This behavior might contribute to a swaying effective rocker shape that is flatter (or a larger radius rocker) than that used for walking. A flatter effective rocker shape for standing and swaying may be a logical goal of the complex lower limb system, because it would offer inherent mechanical stability to the body.
Prosthesis users fall frequently and many have a lowered confidence in their balance [9]. A better understanding of functional activities in able-bodied persons could lead to improved prosthesis designs, prosthesis prescriptions, and balance of prosthesis users during standing and swaying, for example. Improved designs or prescriptions could reduce the incidence of falling and increase balance confidence in lower limb prosthesis users, possibly resulting in increased participation in social activities and improved quality of life [10].
Although previous studies have found that the walking ROS is nearly circular, it is unclear what the effective rocker shapes are for able-bodied persons during gentle swaying. Therefore, the purpose of this study was to measure the effective rocker shapes used during fore-aft swaying and walking in order to enhance our understanding of these tasks, providing insight for design of improved prostheses. We hypothesized that the effective rocker shapes of fore-aft swaying would be flatter than those found for walking, providing a more stable base for this activity. Lastly, we created a simple model to investigate ankle stiffness values that could be used in an ankle–foot prosthesis to mimic the able-bodied ankle–foot system for these two tasks.
Section snippets
Subject recruitment
We recruited 11 able-bodied subjects (six male and five female) between the ages of 24 and 35 years. Their demographic information is reported here as mean ± standard deviation: age = 28.2 ± 3.5 years, height = 175.7 ± 10.0 cm, body mass = 71.2 ± 14.2 kg, and shoe size = 8.2 ± 2.3 (US men).
After the subjects gave written consent, approved by Northwestern University's Institutional Review Board, we conducted ankle range of motion and strength tests. For ankle range of motion (non-weight bearing), the subjects
Results
We hypothesized that the effective rocker shapes for fore-aft swaying would be flatter than those for walking, i.e. the curvature would be smaller or radius larger. Visual inspections indicate that the walking ROS looks nearly circular and that the effective rocker shape of swaying is much flatter (Fig. 2A). The figure also includes the effective rocker shape of quiet standing for comparison, which is small and appears to reside at a crossing point of the walking and fore-aft swaying curves.
Discussion
A limitation of this study is the use of high-top canvas shoes, which required placement of ankle markers on the canvas of the shoe instead of the skin. It is possible that the canvas moved slightly relative to the skin surface during swaying and walking, leading to small errors in the measurement of the ankle position. We believe that these errors are small since the canvas part of the shoe fits snugly around the ankle and conforms closely to the ankle when tightly laced, which was the case in
Acknowledgements
The authors would like to acknowledge the use of the VA Chicago Motion Analysis Research Laboratory. We would like to thank Rebecca Stine, MS, for her help with data collection and analysis, Stefania Fatone, PhD, for her help with ankle strength and range of motion testing, and Sara Koehler, MS, and Brian Ruhe, PhD, for their help with statistics used in this study. This publication was made possible by grant number R03-HD050428-01A2 from the National Institutes of Health (NIH). Its contents
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