The targeted reinnervation technique creates new electromyographic control signals for more complex prosthetic systems. With little training, this subject and others have demonstrated an ability to operate a prosthesis using these additional electromyographic control signals added through the nerve transfers. 3–7
This study provides an exciting demonstration of the multiple DOF control that is possible by combining targeted muscle reinnervation electromyographic control signals, residual limb electromyographic control signals and conventional shoulder switches and force sensing resistors. With this new device the subject was able to easily control 2 DOF simultaneously in many different combinations. Control of 3 DOFs was not quantified but clinically observed. Three DOFs seemed harder to control concurrently, but were possible for gross movements such as reaching out with the arm (elbow and shoulder) while opening the hand. Additionally, simultaneous actuation of 4 DOFs was theoretically possible but rarely done because the device does not provide any proprioceptive feedback and positioning this many DOFs required significant cognitive demands. It is acknowledged that this is not an ideal control system and that the cognitive demand is still great. This is confirmed by the fact that the subject rarely used more than 2 motors at a time, even though it was theoretically possible for him to operate 4 and he preferred to use the limbs at relatively slow speeds. This demand will undoubtedly remain great for high level amputees due to the lack of proprioceptive feedback and the need to use visual feedback to place the device. However, this may not negate the benefits of additional control signals with more physiologically appropriate control. Although the subject might not consistently exhibit simultaneous control of the larger number of motors, the ease of physiologically appropriate myoelectric control was likely beneficial in the fluidity of seamless sequential movements that could be observed with use. This work clearly demonstrated that patients are able to control many more than the 3 DOFs that are available on current devices. While the current system is too complex for broad clinical application, it is clear the patients could robustly control a subset of the 6 DOFs such as adding just a wrist flexion/extension component or a powered shoulder flexion/extension component. This work also motivates the development of more advanced prostheses for commercial use.
The subject preferred the speed of the motors to be relatively slow. This gave him greater accuracy. In many tasks, especially larger movement tasks, the subject appeared to quickly hit the maximum speed of the functions being controlled. This is particularly apparent in and . Much of the time the relative speed of the 2 movements are constant as depicted with a straight slope; this is likely due to both DOFs operating at their maximum speed. Clearly the subject can operate 2 functions simultaneously at less then maximal speed. This is depicted well in where the subject was able to draw a very good circle using the elbow and humeral rotator in combination.
The increased cognitive demands in part came from the need to control 3 of the DOFs using 1 dual-action rocker switch and 2 bidirectional force sensing resistors. Considerable effort was required on the part of the subject to properly aim the shoulder at 4 separate switches with the correct pressure in order to allow the arm to function in the intended way. Further research is being explored on how to better use the motion of the shoulder to control movements, since the current design clearly has too many functions controlled with the residual shoulder. Ideally, only shoulder functions would be controlled with shoulder movement. This would be more physiologically appropriate and intuitive.
The 6-DOF arm allowed the subject to perform many tasks that were not possible with the 3-DOF system such as high reaching, pushing up his glasses, and donning a hat. The powered shoulder facilitated the increased work space in the environment, but the wrist flexion/extension function was critical for prepositioning the hand and being able to functionally use the increased workspace. The humeral rotator allowed the workspace to include the subject’s body; this is very important for activities such as dressing, hygiene and simple, but important, tasks like scratching ones own nose.
In functional testing, times were not consistently less for the 6-DOF system over the 3-DOF system. A few possible explanations exist. The increase in cognitive demands required for controlling these additional functions likely slowed operation. Another issue is that the subject was often able to accomplish the same gross positioning movements of the powered shoulder through trunk movement and knee flexion. For example, during cup stacking experiments while using the 3-DOF arm, he would manually move the shoulder up, and leave it in that locked position; when he needed the arm lower, he would then bend at the waist and bend his knees. Although this movement was faster, it required greater physical effort and made use of the device appear less natural. For activities where such trunk and knee movement would not be possible (eg, if sitting or otherwise constrained), the 3-DOF device would be less functional. The subject had much less practice with the 6-DOF arm since his use of this device was limited to time spent in the laboratory. Conversely, the 3-DOF system was used on a regular basis at home. Finally, for tasks that only required use the hand, wrist rotation, and elbow, the extra functions of the 6 function arm allowed joints to be mistakenly activated and slow performance. For example, the blocks and box test only required the use of the hand and elbow, thus inadvertent wrist or shoulder movement slowed performance.
The powered shoulder and humeral rotator clearly had functional value to this subject by increasing his access to high objects. Similarly the humeral rotator allowed him to work the midline space of his body with great ease. His appreciation for these joints may be due to the fact that he has bilateral upper-limb loss and cannot reach high objects or work in his midline well with a sound limb. The relative value of these functions which add weight and complexity remains to be determined with unilateral amputees that can compensate with a sound limb to a large extent.
The amount of control demonstrated by our subject in this study was remarkable. Because of the concentration and practice needed to operate 2 force sensing resistors and a rocker with the shoulder to control 3 different DOFs, he is clearly an exceptional user, and we would not expect a typical user to be so adept with such limited wear time. For a 6 DOF upper-limb prosthesis to be clinically viable an easier, a more intuitive control system must be developed and move away from using of the shoulder to control wrist movements. One way to potentially improve control is to use advanced electromyographic signal processing techniques such as pattern recognition algorithms.13
The targeted muscle reinnervation electromyographic contains much more information than just open and close hand and flex and extend elbow. Motoneurons of wrist, thumb, and finger muscles reinnervate the target muscle and this information is embedded in the surface electromyographic. Using pattern recognition techniques we have been able to extract control information related to wrist, finger, and thumb movements.14
This will hopefully provide an easy and intuitive method for users to control a prosthetic wrist with 2 or 3 DOFs and multifunction hands that provide the user a choice of many different hand-grasp patterns.
Better prostheses are also needed. This 6-DOF prosthesis is only an experimental device and not described as a potentially clinically viable product: it is too heavy (≈5.75kg), it is not durable and the battery life is a 2 to 3 hours when used during these tests, and new socket/electrode systems are an area of current research since self-adhesive electrodes are not a clinically viable solution. The purpose of this work was to demonstrate that more functions can be controlled in a useful way. Since only 3 powered functions are commercially available—elbows, wrist rotators, and terminal devices, hopefully this work gives a greater impetus for developing commercial devices with greater dexterity for individuals with upper-limb amputation.