Richard F. ff. Weir

Mechanical design and performance specifications of anthropomorphic prosthetic hands: A review

In this article, we set forth a detailed analysis of the mechanical characteristics of anthropomorphic prosthetic hands. We report on an empirical study concerning the performance of several commercially available myoelectric prosthetic hands, including the Vincent, iLimb, iLimb Pulse, Bebionic, Bebionic v2, and Michelangelo hands. We investigated the finger design and kinematics, mechanical joint coupling, and actuation methods of these commercial prosthetic hands. The empirical findings are supplemented with a compilation of published data on both commercial and prototype research prosthetic hands. We discuss numerous mechanical design parameters by referencing examples in the literature. Crucial design trade-offs are highlighted, including number of actuators and hand complexity, hand weight, and grasp force. Finally, we offer a set of rules of thumb regarding the mechanical design of anthropomorphic prosthetic hands.

Implantable Myoelectric Sensors (IMESs) for Intramuscular Electromyogram Recording

We have developed a multichannel electrogmyography sensor system capable of receiving and processing signals from up to 32 implanted myoelectric sensors (IMES). The appeal of implanted sensors for myoelectric control is that electromyography (EMG) signals can be measured at their source providing relatively cross-talk-free signals that can be treated as independent control sites. An external telemetry controller receives telemetry sent over a transcutaneous magnetic link by the implanted electrodes. The same link provides power and commands to the implanted electrodes. Wireless telemetry of EMG signals from sensors implanted in the residual musculature eliminates the problems associated with percutaneous wires, such as infection, breakage, and marsupialization. Each implantable sensor consists of a custom-designed application-specified integrated circuit that is packaged into a biocompatible RF BION capsule from the Alfred E. Mann Foundation. Implants are designed for permanent long-term implantation with no servicing requirements. We have a fully operational system. The system has been tested in animals. Implants have been chronically implanted in the legs of three cats and are still completely operational four months after implantation.

A Comparison of the Effects of Electrode Implantation and Targeting on Pattern Classification Accuracy for Prosthesis Control

The use of surface versus intramuscular electrodes as well as the effect of electrode targeting on pattern-recognition- based multifunctional prosthesis control was explored. Surface electrodes are touted for their ability to record activity from relatively large portions of muscle tissue. Intramuscular electromyograms (EMGs) can provide focal recordings from deep muscles of the forearm and independent signals relatively free of crosstalk. However, little work has been done to compare the two. Additionally, while previous investigations have either targeted electrodes to specific muscles or used untargeted (symmetric) electrode arrays, no work has compared these approaches to determine if one is superior. The classification accuracies of pattern-recognition-based classifiers utilizing surface and intramuscular as well as targeted and untargeted electrodes were compared across 11 subjects. A repeated-measures analysis of variance revealed that when only EMG amplitude information was used from all available EMG channels, the targeted surface, targeted intramuscular, and untargeted surface electrodes produced similar classification accuracies while the untargeted intramuscular electrodes produced significantly lower accuracies. However, no statistical differences were observed between any of the electrode conditions when additional features were extracted from the EMG signal. It was concluded that the choice of electrode should be driven by clinical factors, such as signal robustness/stability, cost, etc., instead of by classification accuracy.

Muscle synergies as a predictive framework for the EMG patterns of new hand postures

Synchronous muscle synergies have been suggested as a framework for dimensionality reduction in muscle coordination. Many studies have shown that synergies form a descriptive framework for a wide variety of tasks. We examined if a muscle synergy framework could accurately predict the EMG patterns associated with untrained static hand postures, in essence, if they formed a predictive framework. Hand and forearm muscle activities were recorded while subjects statically mimed 33 postures of the American Sign Language alphabet. Synergies were extracted from a subset of training postures using non-negative matrix factorization and used to predict the EMG patterns of the remaining postures. Across the subject population, as few as 11 postures could form an eight-dimensional synergy framework that allowed for at least 90% prediction of the EMG patterns of all 33 postures, including trial-to-trial variations. Synergies were quite robust despite using different postures in the training set, and also despite using a varied number of postures. Estimated synergies were categorized into those which were subject-specific and those which were general to the population. Population synergies were sparser than the subject-specific synergies, typically being dominated by a single muscle. Subject-specific synergies were more balanced in the coactivation of multiple muscles. We suggest as a result that global muscle coordination may be a combination of higher order control of robust subject-specific muscle synergies and lower order control of individuated muscles, and that this control paradigm may be useful in the control of EMG-based technologies, such as artificial limbs and functional electrical stimulation systems.

Neural machine interfaces for controlling multifunctional powered upper-limb prostheses

This article investigates various neural machine interfaces for voluntary control of externally powered upper-limb prostheses. Epidemiology of upper limb amputation, as well as prescription and follow-up studies of externally powered upper-limb prostheses are discussed. The use of electromyographic interfaces and peripheral nerve interfaces for prosthetic control, as well as brain machine interfaces suitable for prosthetic control, are examined in detail along with available clinical results. In addition, studies on interfaces using muscle acoustic and mechanical properties and the problem of interfacing sensory information to the nervous system are discussed.

Control of a Six Degree-of-Freedom Prosthetic Arm after Targeted Muscle Reinnervation Surgery

OBJECTIVES To fit and evaluate the control of a complex prosthesis for a shoulder disarticulation-level amputee with targeted muscle reinnervation. DESIGN One participant who had targeted muscle reinnervation surgery was fitted with an advanced prosthesis and his use of this device was compared with the device that he used in the home setting. SETTING The experiments were completed within a laboratory setting. PARTICIPANT The first recipient of targeted muscle reinnervation: a bilateral shoulder disarticulation-level amputee. INTERVENTIONS Two years after surgery, the subject was fitted with a 6 degree of freedom (DOF) prosthesis (shoulder flexion, humeral rotation, elbow flexion, wrist rotation, wrist flexion, and hand control). Control of this device was compared with that of his commercially available 3-DOF system (elbow, wrist rotation, and powered hook terminal device). MAIN OUTCOME MEASURE In order to assess performance, movement analysis and timed movement tasks were executed. RESULTS The subject was able to independently operate all 6 arm functions with good control. He could simultaneously operate 2 DOF of several different joint combinations with relative ease. He operated up to 4 DOF simultaneously, but with poor control. Work space was markedly increased and some timed tasks were faster with the 6-DOF system. CONCLUSIONS This proof-of-concept study shows that advances in control of shoulder disarticulation-level prostheses can improve the quality of movement. Additional control sources may spur the development of more advanced and complex componentry for these amputees.

User-Modulated Impedance Control of a Prosthetic Elbow in Unconstrained, Perturbed Motion

Humans use the agonist-antagonist structure of their muscles to simultaneously determine both the motion and the stiffness of their joints. Designing this feature into an artificial limb may prove advantageous. To evaluate the performance of an artificial limb capable of modulating its impedance, we have created a compact series elastic actuator that has the same size and similar weight as commercially available electric prosthetic elbows. The inherent compliance in series elastic actuators ensure their safety to the user, even at high speeds, while creating a high-fidelity force actuator ideally suited for impedance control. This paper describes three serial studies that build on each other. The first study presents modeling of the actuator to ensure stability in the range of impedance modulation and empirically tests the actuator to validate its ability to modulate impedance. The actuator is found to be stable and accurate over a wide range of impedances. In the second study, four subjects are tested in a preliminary experiment to answer basic questions necessary to implement user-modulated impedance control. Findings include the superiority of velocity control over position control as the underlying motion paradigm and the preference for high stiffness and non-negative inertia. Based on the findings of the second study, the third study evaluates the performance of 15 able-bodied subjects for two tasks, using five different impedance paradigms. Impedance modulation, speed, and error were compared across paradigms. The results indicate that subjects do not actively modulate impedance if it is near a preferred baseline. Fixed impedance and viscosity modulation provide the most accurate control.

Improvements to Series Elastic Actuators

Actuators must be safe when interacting with humans, even in unexpected situations. Safety requires low impedance (low forces for a given perturbation) at all frequencies, not only in the actuators stable bandwidth. Series elastic actuators (SEAs) are capable of achieving low impedance across all frequencies, though their force-frequency saturation envelope is decreased as a result. This paper examines ways to increase the force fidelity of SEAs by introducing inner control feedback loops and using appropriate sensor location. Findings include the superiority of a low fidelity sensor distal to stiction sources over a high fidelity sensor proximal to stiction and the superiority of an inner position feedback loop over an inner velocity feedback loop

Implantable myoelectric sensors (IMES) for upper-extremity prosthesis control- preliminary work

We are developing a multichannel/multifunction prosthetic hand/arm controller system capable of receiving and processing signals from up to sixteen Implanted MyoElectric Sensors (IMES). A BION/spl reg/ II package will house the implantable electrode electronics and associated circuitry. An external prosthesis controller will decipher user intent from telemetry sent over a transcutaneous magnetic link by the implanted electrodes. The same link will provide power for the implanted electrodes. Development of such a system will greatly increase the number of control sources available to amputees for control of their prostheses. This will encourage the design and fitting of more functional prostheses than are currently available.

Development of an implantable myoelectric sensor for advanced prosthesis control.

Modern hand and wrist prostheses afford a high level of mechanical sophistication, but the ability to control them in an intuitive and repeatable manner lags. Commercially available systems using surface electromyographic (EMG) or myoelectric control can supply at best two degrees of freedom (DOF), most often sequentially controlled. This limitation is partially due to the nature of surface-recorded EMG, for which the signal contains components from multiple muscle sources. We report here on the development of an implantable myoelectric sensor using EMG sensors that can be chronically implanted into an amputees residual muscles. Because sensing occurs at the source of muscle contraction, a single principal component of EMG is detected by each sensor, corresponding to intent to move a particular effector. This system can potentially provide independent signal sources for control of individual effectors within a limb prosthesis. The use of implanted devices supports inter-day signal repeatability. We report on efforts in preparation for human clinical trials, including animal testing, and a first-in-human proof of principle demonstration where the subject was able to intuitively and simultaneously control two DOF in a hand and wrist prosthesis.