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Progress on stabilizing and controlling powered upper-limb prostheses.

INTRODUCTION While the Journal of Rehabilitation Research and Development (JRRD) is often a repository for research conducted with a large subject population to compare and validate the devices and treatments used in rehabilitation, it is also a means to keep clinicians and researchers up to date on the development of new devices and treatments. This issue, addressing the problem of upper-limb loss, emphasizes the device-development side of JRRD's mission. Compared with other forms of disability, upper-limb loss is relatively rare, and finding a large enough group of subjects is often difficult for studies that help researchers assess whether they are going in the right direction. When developing new devices are being developed, careful feasibility studies are important. Much that is reported here is preliminary and based on relatively few subjects. My objective is to quantify the state of development so that other researchers can move the field forward knowing which directions are the most promising. This issue of JRRD began with a collection of articles about any problems associated with upper-limb prostheses. However, I quickly realized that the researchers submitting articles were overwhelmingly interested in the control of powered prostheses. During the last decade, great advances have been made in acquiring more information from myoelectric signals. Many of the articles in this issue address the recognition of patterns within surface myoelectric signals to first discern the user's intent and then implement that intent in a working device. A careful look at this research shows that myoelectric control, whether of the simple two-muscle type or the more sophisticated pattern-recognition control of multiple degrees of freedom (DOFs), has a serious flaw--no inherent feedback of position, speed, or force is associated with myoelectric control. This means that the reader should pay particular attention to the articles that report on ways to use the relative motion of remaining body parts to achieve control with feedback. In the articles in this issue and in others that I will refer to, the research goal is intuitive control of prostheses. Users should have to think only about what they want to do, not about how to control their prostheses. WHAT IS THE PURPOSE OF AN UPPER-LIMB PROSTHESIS? The answer depends on the goal of the wearer. For most users, a prosthesis restores body image and cosmesis and also replaces as much function of the intact limb as possible; however, it must do so with the least possible discomfort and inconvenience. A successful prosthesis is one that can be incorporated into the user's lifestyle almost seamlessly. Just with this simple statement, many trade-offs become apparent. But what are the trade-offs between function and cosmesis? How much weight can be added to increase function without compromising comfort? How important is the stability of the socket interface? The articles in this issue show that researchers are aware of these trade-offs as they try to improve one aspect of prosthesis design at a time. In the end, no prosthesis will ever be "best." Users will always want to choose their own trade-offs. Thus, researchers must not only work to improve the individual prosthetic components and control schemes but also place their new devices on sufficient numbers of users so that the relative efficacy of the individual devices and control schemes can be determined. Such comparing of capabilities will allow the market to eliminate those devices that do not help enough users to justify their cost to society. MAKING MYOELECTRIC CONTROL INTUITIVE If a person loses the arm just above the elbow and if the muscle ends are properly attached to the distal humerus, the remaining muscles will respond to an attempt to move the missing forearm. Trying to raise the forearm will result in contraction of the biceps (and the brachialis, if still present). Likewise, an attempt to extend the forearm will result in contraction of the triceps. …